Intestinal carbonic anhydrase, bicarbonate, and proton carriers play a role in the acclimation of rainbow trout to seawater

The physiological significance of plasma-accessible carbonic anhydrase in the respiratory systems of fishes

Carbonic anhydrase (CA) activity is ubiquitously found in all vertebrate species, tissues and cellular compartments. Most species have plasma-accessible CA (paCA) isoforms at the respiratory surfaces, where the enzyme catalyzes the conversion of plasma bicarbonate to carbon dioxide (CO2) that can be excreted by diffusion. A notable exception are the teleost fishes that appear to lack paCA at their gills. The present review: (i) recapitulates the significance of CA activity and distribution in vertebrates; (ii) summarizes the current evidence for the presence or absence of paCA at the gills of fishes, from the basal cyclostomes to the derived teleosts and extremophiles such as the Antarctic icefishes; (iii) explores the contribution of paCA to organismal CO2 excretion in fishes; and (iv) the functional significance of its absence at the gills, for the specialized system of O2 transport in most teleosts; (v) outlines the multiplicity and isoform distribution of membrane-associated CAs in fishes and methodologies to determine their plasma-accessible orientation; and (vi) sketches a tentative time line for the evolutionary dynamics of branchial paCA distribution in the major groups of fishes. Finally, this review highlights current gaps in the knowledge on branchial paCA function and provides recommendations for future work.

Transcriptional regulation of esophageal, intestinal, and branchial solute transporters by salinity, growth hormone, and cortisol in Atlantic salmon

In marine habitats, Atlantic salmon (Salmo salar) imbibe seawater (SW) to replace body water that is passively lost to the ambient environment. By desalinating consumed SW, the esophagus enables solute-linked water absorption across the intestinal epithelium. The processes underlying esophageal desalination in salmon and their hormonal regulation during smoltification and following SW exposure are unresolved. To address this, we considered whether two Na+ /H+ exchangers (Nhe2 and -3) expressed in the esophagus contribute to the uptake of Na+ from lumenal SW. There were no seasonal changes in esophageal nhe2 or -3 expression during smoltification; however, nhe3 increased following 48 h of SW exposure in May. Esophageal nhe2, -3, and growth hormone receptor b1 were elevated in smolts acclimated to SW for 2.5 weeks. Treatment with cortisol stimulated branchial Na+ /K+ -ATPase (Nka) activity, and Na+ /K+ /2Cl- cotransporter 1 (nkcc1), cystic fibrosis transmembrane regulator 1 (cftr1), and nka-α1b expression. Esophageal nhe2, but not nhe3 expression, was stimulated by cortisol. In anterior intestine, cortisol stimulated nkcc2, cftr2, and nka-α1b. Our findings indicate that salinity stimulates esophageal nhe2 and -3, and that cortisol coordinates the expression of esophageal, intestinal, and branchial solute transporters to support the SW adaptability of Atlantic salmon.

Dietary LC-PUFA and environmental salinity modulate the fatty acid biosynthesis capacity of the euryhaline teleost thicklip grey mullet (Chelon labrosus)

The capacity to biosynthesise long-chain (≥C20) polyunsaturated fatty acids (LC-PUFA) depends upon the complement and function of key enzymes commonly known as fatty acyl desaturases and elongases. The presence of a Δ5/Δ6 desaturase enabling the biosynthesis of docosahexaenoic acid (22:6n-3, DHA) through the "Sprecher pathway" has been reported in Chelon labrosus. Research in other teleosts have demonstrated that LC-PUFA biosynthesis can be modulated by diet and ambient salinity. The present study aimed to assess the combined effects of partial dietary replacement of fish oil (FO) by vegetable oil (VO) and reduced ambient salinity (35 ppt vs 20 ppt) on the fatty acid composition of muscle, enterocytes and hepatocytes of C. labrosus juveniles. Moreover, the enzymatic activity over radiolabelled [1-14C] 18:3n-3 (α-linolenic acid, ALA) and [1-14C] 20:5n-3 (eicosapentaenoic acid, EPA) to biosynthesise n-3 LC-PUFA in hepatocytes and enterocytes, and the gene regulation of the C. labrosus fatty acid desaturase-2 (fads2) and elongation of very long chain fatty acids protein 5 (elovl5) in liver and intestine was also investigated. Recovery of radiolabelled products including stearidonic acid (18:4n-3, SDA), 20:5n-3, tetracosahexaenoic acid (24:6n-3, THA) and 22:6n-3 in all treatments except FO35-fish, provided compelling evidence that a complete pathway enabling the biosynthesis of EPA and DHA from ALA is present and active in C. labrosus. Low salinity conditions upregulated fads2 in hepatocytes and elovl5 in both cell types, regardless of dietary composition. Interestingly, FO20-fish showed the highest amount of n-3 LC-PUFA in muscle, while no differences in VO-fish reared at both salinities were found. These results demonstrate a compensatory capacity of C. labrosus to biosynthesise n-3 LC-PUFA under reduced dietary supply, and emphasise the potential of low salinity conditions to stimulate this pathway in euryhaline fish.

An atlas of plasma-accessible carbonic anhydrase availability in the model teleost, the rainbow trout

The unique teleost oxygenation system that permits enhanced oxygen unloading during stress comprises three main characteristics: pH-sensitive haemoglobin, red blood cell (RBC) intracellular pH (pHi) protection, and a heterogeneous distribution of plasma-accessible carbonic anhydrase (paCA). A heterogeneous distribution of paCA is essential; its presence permits enhanced oxygen unloading during stress, while its absence at the gills maintains conditions for oxygen uptake by pH-sensitive haemoglobins. We hypothesised that paCA would be absent in all four gill arches, as has been previously indicated for arch two, and that paCA would be present in all other tissues. Through a suite of biochemical and molecular methods, we confirmed the absence of paCA from all four arches. We also found evidence for paCA in nine other tissues and a lack of paCA availability in the stomach. Expression was highly variable between tissues and suggests these differences may be associated with their respective metabolic activities. Additionally, we analysed the specific CA-IV isoform expressed within each tissue and showed almost complete separation of expression between tissues; CA-IVa was detected in the heart, brain, anterior intestine, and liver, whereas CA-IVb was detected in all intestine sections, pyloric caeca, kidney, and white muscle. This adds to a growing collection of work suggesting CA-IVa and b play divergent roles in gas exchange and ion/acid-base balance, respectively. The current study represents the first comprehensive atlas of paCA availability within the circulatory system of the model teleost, rainbow trout, and fills important gaps in our knowledge of this unique oxygenation system.

Functional divergence of teleost carbonic anhydrase 4

The functional role of membrane-bound carbonic anhydrases (CAs) has been of keen interest in the past decade, and in particular, studies have linked CA in red muscle, heart, and eye to enhanced tissue oxygen extraction in bony fishes (teleosts). However, the number of purported membrane-bound CA isoforms in teleosts, combined with the imperfect system of CA isoform nomenclature, present roadblocks for ascribing physiological functions to particular CA isoforms across different teleost lineages. Here we developed an organizational framework for membrane-bound CAs in teleosts, providing the latest phylogenetic analysis of extant CA4 and CA4-like isoforms. Our data confirm that there are three distinct isoforms of CA4 (a, b, and c) that are conserved across major teleost lineages, with the exception of CA4c gene being lost in salmonids. Tissue distribution analyses suggest CA4a functions in oxygen delivery across teleost lineages, while CA4b may be specialized for renal acid-base balance and ion regulation. This work provides an important foundation for researchers to elucidate the functional significance of CA4 isoforms in fishes.

Do extreme postprandial levels of oxygen, carbon dioxide, and ammonia in the digestive tract equilibrate with the bloodstream in the freshwater rainbow trout (Oncorhynchus mykiss)?

The gastrointestinal tract (GIT) lumen of teleosts harbors extreme conditions, especially after feeding: high PCO₂ (20-115 Torr), total ammonia (415-3710 μM), PNH₃ (79-1760 μTorr in the intestine), and virtual anoxia (PO₂ < 1 Torr). These levels could be dangerous if they were to equilibrate with the bloodstream. Thus, we investigated the potential equilibration of O₂, CO₂, and ammonia across the GIT epithelia in freshwater rainbow trout by monitoring postprandial arterial and venous blood gases in vivo and in situ. In vivo blood was sampled from the indwelling catheters in the dorsal aorta (DA) and subintestinal vein (SIV) draining the posterior intestine in the fasting state and at 4 to 48 h following catheter-feeding. To investigate possible ammonia absorption in the anterior part of the GIT, blood was sampled from the DA, SIV and hepatic portal vein (HPV) from anaesthetized fish in situ following voluntary feeding. We found minimal equilibration of all three gases between the GIT lumen and the SIV blood, with the latter maintaining pre-feeding levels (PO₂ = 25-49 Torr, PCO₂ = 6-8 Torr, and total ammonia = 117-134 μM and PNH₃ = 13-30 μTorr at 48 h post-feeding). In contrast to the SIV, we found that the HPV total ammonia more than doubled 24 h after feeding (128 to 297 μM), indicative of absorption in the anterior GIT. Overall, the GIT epithelia of trout, although specialized for absorption, prevent dangerous levels of PO₂, PCO₂ and ammonia from equilibrating with the blood circulation.

Localization of the Neuropeptide Arginine Vasotocin and Its Receptor in the Osmoregulatory Organs of Black Porgy, Acanthopagrus schlegelii: Gills, Kidneys, and Intestines

The neurohypophysial hormone arginine vasotocin (avt) and its receptor (avtr) regulates ions in the osmoregulatory organs of euryhaline black porgy (Acanthopagrus schlegelii). The localization of avt and avtr transcripts in the osmoregulatory organs has yet to be demonstrated. Thus, in the present study, we performed an in situ hybridization analysis to determine the localization of avt and avtr in the gills, kidneys, and intestines of the black porgy. The avt and avtr transcripts were identified in the filament and lamellae region of the gills in the black porgy. However, the basal membrane of the filament contained more avt and avtr transcripts. Fluorescence double tagging analysis revealed that avt and avtr mRNAs were partially co-localized with α-Nka-ir cells in the gill filament. The proximal tubules, distal tubules, and collecting duct of the kidney all had positive hybridization signals for the avt and avtr transcripts. Unlike the α-Nka immunoreactive cells, the avt and avtr transcripts were found on the basolateral surface of the distal convoluted tubule and in the entire cells of the proximal convoluted tubules of the black porgy kidney. In the intestine, the avt and avtr transcripts were found in the basolateral membrane of the enterocytes. Collectively, this study provides a summary of evidence suggesting that the neuropeptides avt and avtr with α-Nka-ir cells may have functions in the gills, kidneys, and intestines via ionocytes.

Na+/HCO3- cotransporter 1 (nbce1) isoform gene expression during smoltification and seawater acclimation of Atlantic salmon

The life history of Atlantic salmon (Salmo salar) includes an initial freshwater phase (parr) that precedes a springtime migration to marine environments as smolts. The development of osmoregulatory systems that will ultimately support the survival of juveniles upon entry into marine habitats is a key aspect of smoltification. While the acquisition of seawater tolerance in all euryhaline species demands the concerted activity of specific ion pumps, transporters, and channels, the contributions of Na⁺/HCO₃^- cotransporter 1 (Nbce1) to salinity acclimation remain unresolved. Here, we investigated the branchial and intestinal expression of three Na⁺/HCO₃^- cotransporter 1 isoforms, denoted nbce1.1, -1.2a, and -1.2b. Given the proposed role of Nbce1 in supporting the absorption of environmental Na⁺ by ionocytes, we first hypothesized that expression of a branchial nbce1 transcript (nbce1.2a) would be attenuated in salmon undergoing smoltification and following seawater exposure. In two separate years, we observed spring increases in branchial Na⁺/K⁺-ATPase activity, Na⁺/K⁺/2Cl^- cotransporter 1, and cystic fibrosis transmembrane regulator 1 expression characteristic of smoltification, whereas there were no attendant changes in nbce1.2a expression. Nonetheless, branchial nbce1.2a levels were reduced in parr and smolts within 2 days of seawater exposure. In the intestine, gene transcript abundance for nbce1.1 increased from spring to summer in the anterior intestine, but not in the posterior intestine or pyloric caeca, and nbce1.1 and -1.2b expression in the intestine showed season-dependent transcriptional regulation by seawater exposure. Collectively, our data indicate that tissue-specific modulation of all three nbce1 isoforms underlies adaptive responses to seawater.

An update on the evolutionary origin of aglomerular kidney with structural and ultrastructural descriptions of the kidney in three fish species

The kidney of fish contains numerous nephrons, each of which is divided into the renal corpuscle and renal tubules. This glomerular structure is the filtration unit of the nephron and is important for the kidney function, but it has been reported that the renal corpuscle was lost in at least four independent linages of fish (i.e., aglomerular kidney). In this study, the authors newly described renal structures for three species by histological and ultrastructural observations: two aglomerular kidneys from a seahorse Hippocampus barbouri and a toadfish Allenbatrachus grunniens and a glomerular kidney from a snake eel Pisodonophis boro. The renal development of H. barbouri was also described during 1-35 days after birth. In all species tested, the anterior kidney was comprised of haematopoietic tissues and a few renal tubules, whereas the posterior kidney contained more renal tubules. Although the glomerular structure was present in P. boro, light microscopic observations identified no glomeruli in the kidney of H. barbouri and A. grunniens. Ultrastructurally, abundant deep basal infoldings with mitochondria in the renal tubules were observed in A. grunniens compared to H. barbouri and P. boro, suggesting the possible role of basal infoldings in maintaining the osmotic balance. By integrating the results from the three species and comprehensive literature search, the authors further showed that 56 species have been reported to be aglomerular, and that the aglomerular kidney has evolved at least eight times in bony fishes.

Histological effects of sublethal concentrations of insecticide Lindane on intestinal tissue of grass carp (Ctenopharyngodon idella)

This study investigates acute toxicity and histological effects of Lindane insecticide on intestinal tissues of immature grass carp (Ctenopharyngodon idella). For these purposes, 105 fish were exposed to different concentrations of Lindane for 96 h in 5 groups with 3 replications. Samples of intestinal tissues were prepared in both control and exposure groups during and after the toxicity test. Vaculation, necrosis, bleeding and epithelial degeneration were apparent in the intestine tissue of treated individuals. The LC₅₀ 96 h of Lindane was 0.788 ppm in the present study, also the results showed besides pesticide-induced lesions, symptoms of abnormal swimming, anxiety, tendency to swim near the surface, and death due to terminal toxicity with mouth widely open. The exposed fish were pale in color, showed oxygen deficit and severe respiratory problems. Overall, our results confirm the toxicity of different concentrations of Lindane insecticide affecting behavior, intestinal function and inducing tissue lesions of immature grass carp.

Molecular Characterization of Carbonic Anhydrase II (CA II) and Its Potential Involvement in Regulating Shell Formation in the Pacific Abalone, Haliotis discus hannai

Carbonic anhydrases (CAs) are a family of metalloenzymes that can catalyze the reversible interconversion of CO₂/HCO₃^–, ubiquitously present in both prokaryotes and eukaryotes. In the present study, a CA II (designated as HdhCA II) was sequenced and characterized from the mantle tissue of the Pacific abalone. The complete sequence of HdhCA II was 1,169 bp, encoding a polypeptide of 349 amino acids with a NH₂-terminal signal peptide and a CA architectural domain. The predicted protein shared 98.57% and 68.59% sequence identities with CA II of Haliotis gigantea and Haliotis tuberculata, respectively. Two putative N-linked glycosylation motifs and two cysteine residues could potentially form intramolecular disulfide bond present in HdhCA II. The phylogenetic analysis indicated that HdhCA II was placed in a gastropod clade and robustly clustered with CA II of H. gigantea and H. tuberculata. The highest level of HdhCA II mRNA expression was detected in the shell forming mantle tissue. During ontogenesis, the mRNA of HdhCA II was detected in all stages, with larval shell formation stage showing the highest expression level. The in situ hybridization results detected the HdhCA II mRNA expression in the epithelial cells of the dorsal mantle pallial, an area known to express genes involved in the formation of a nacreous layer in the shell. This is the first report of HdhCA II in the Pacific abalone, and the results of this study indicate that this gene might play a role in the shell formation of abalone.

Use of a carbonic anhydrase Ca17a knockout to investigate mechanisms of ion uptake in zebrafish (Danio rerio)

In fishes, branchial cytosolic carbonic anhydrase (CA) plays an important role in ion and acid-base regulation. The Ca17a isoform in zebrafish (Danio rerio) is expressed abundantly in Na⁺-absorbing/H⁺-secreting H⁺-ATPase-rich (HR) cells. The present study aimed to identify the role of Ca17a in ion and acid-base regulation across life stages using CRISPR/Cas9 gene editing. However, in preliminary experiments, we established that ca17a knockout is lethal with ca17a^-/- mutants exhibiting a significant decrease in survival beginning at ∼12 days postfertilization (dpf) and with no individuals surviving past 19 dpf. Based on these findings, we hypothesized that ca17a^-/- mutants would display alterations in ion and acid-base balance and that these physiological disturbances might underlie their early demise. Na⁺ uptake rates were significantly increased by up to 300% in homozygous mutants compared with wild-type individuals at 4 and 9 dpf; however, whole body Na⁺ content remained constant. While Cl^- uptake was significantly reduced in ca17a^-/- mutants, Cl^- content was unaffected. Reduction of CA activity by Ca17a morpholino knockdown or ethoxzolamide treatments similarly reduced Cl^- uptake, implicating Ca17a in the mechanism of Cl^- uptake by larval zebrafish. H⁺ secretion, O₂ consumption, CO₂ excretion, and ammonia excretion were generally unaltered in ca17a^-/- mutants. In conclusion, while the loss of Ca17a caused marked changes in ion uptake rates, providing strong evidence for a Ca17a-dependent Cl^- uptake mechanism, the underlying causes of the lethality of this mutation in zebrafish remain unclear.

Effect of dietary soybean lecithin on fish performance, hemato-immunological parameters, lipid biochemistry, antioxidant status, digestive enzymes activity and intestinal histomorphometry of pre-spawning Caspian brown trout (Salmo trutta caspius)

A feeding experiment was conducted to evaluate the effects of dietary soybean lecithin (SBL) supplementation on performance, hemato-immunological parameters, lipid biochemistry, antioxidant status, digestive enzymes activity and intestinal histomorphometry of Caspian brown trout, Salmo trutta caspius in the pre-spawning stage. The basal diet was supplemented with 0% (control), 3%, 6%, 9% and 12% of SBL to obtain five experimental diets. Fish with an average weight of 350 ± 10 g were randomly distributed among five experimental groups and fed for 90 days. Dietary SBL resulted in better performance including specific growth rate (SGR), weight gain (WG) and feed conversion ratio (FCR) (p < 0.05). Among the different hemato-immunological parameters, white blood cell counts (WBC), lysozyme, alternative complement activity (ACH50) and total immunoglobulin (IgM) content of serum were significantly increased with dietary SBL inclusion (p < 0.05). For antioxidant enzymes, glutathione S-transferase (GST) and catalase (CAT) showed significant differences among various experimental diets (p < 0.05). Furthermore, digestive enzymes activity including alkaline protease, lipase and amylase were increased in those fish received SBL supplemented diets (p < 0.05). Our results revealed that the dietary SBL improved some physiological responses of the fish and indicate 6-9% dietary SBL supplementation would improve the physiological competence of the pre-spawning Caspian brown trout breeders.

Zonation of Ca2+ transport and enzyme activity in the caeca of rainbow trout - a simple structure with complex functions

Trout caeca are vermiform structures projecting from the anterior intestine of the gastrointestinal tract. Despite their simple gross morphology, these appendages are physically distinct along the anterior-posterior axis, and ultrastructural evidence suggests zonation of function within the structures. Individual caeca from three sections (anterior, middle and posterior) were removed from the intestine of freshwater rainbow trout and investigated for ion transport and enzyme activity. Ca²⁺ absorption appeared as a combination of active and passive movement, with Michaelis-Menten kinetics observable under symmetrical conditions, and was inhibited by several pharmacological agents (ouabain, La³⁺ and a calmodulin antagonist). There was a decrease in ion transport function from adjacent to the intestine (proximal) to the distal tip of each caecum, along with decreasing transport from anterior to posterior for the proximal portion alone. Feeding increased the J _Max and K _M for Ca²⁺ absorption within all sections, whereas ion-poor water (IPW) exposure further increased the J _Max and K _M for Ca²⁺ transport in the anterior and middle sections. Increased Na⁺/K⁺-ATPase (NKA) and citrate synthase (CS) activity rates paralleled trends seen in Ca²⁺ transport. Feeding in freshwater and IPW exposure increased the glycolytic capacity of the caeca via increased pyruvate kinase (PK) and decreased lactate dehydrogenase (LDH) activity, while amino acid metabolism increased with IPW exposure through increased glutamate dehydrogenase (GDH) activity. Overall, feeding and IPW exposure each altered ionoregulation within the caeca of freshwater rainbow trout in a zone-specific pattern, with the anterior and proximal portions of the caeca being most affected. Increased carbohydrate and protein metabolism fueled the increased ATP demand of NKA through CS.

Intestinal Na+, K+, 2Cl- cotransporter 2 plays a crucial role in hyperosmotic transitions of a euryhaline teleost

Euryhaline fishes, such as the red drum (Sciaenops ocellatus), must quickly transition between hyperosmotic and hypoosmotic physiological strategies. When freshwater individuals transition to seawater they are exposed to increased diffusive water loss and ion gain. To maintain osmoregulatory balance these animals must drink and absorb seawater through the intestine, followed by ion excretion at the gills. The ability of fishes to transition between strategies can limit the magnitude of osmotic shock that can be tolerated. Here, we demonstrate that red drum can tolerate direct transfer from freshwater to full‐strength seawater with marginal impacts on osmotic balance, as indicated by plasma and muscle ion concentration, as well as muscle water. Seawater transition is concurrent with a significant increase in intestinal fluid volume. Typical patterns of osmoregulatory plasticity were observed in the gill with increased expression of nkcc1 and cftr. Expression changes in the anterior intestine were observed after 24 h for nkcc2 with smaller and later responses observed for slc26a3, slc26a6, and nbc. Immunofluorescence staining demonstrated similar patterns of NKCC localization in freshwater and seawater intestines; however, reduced basolateral staining of V‐type ATPase was observed in seawater. Electrophysiological preparations demonstrated that seawater fish had increased absorptive current in the anterior intestine, which was significantly reduced in the presence of 10 μmol/L bumetanide. Overall, these results suggest that nkcc2 plays a crucial role during hyperosmotic transitions, and may be a more important complement to the well‐known bicarbonate secretion pathway than generally considered.

A fish intestinal epithelial barrier model established from the rainbow trout (Oncorhynchus mykiss) cell line, RTgutGC

The intestine of fish is a multifunctional organ: lined by only a single layer of specialized epithelial cells, it has various physiological roles including nutrient absorption and ion regulation. It moreover comprises an important barrier for environmental toxicants, including metals. Thus far, knowledge of the fish intestine is limited largely to in vivo or ex vivo investigations. Recently, however, the first fish intestinal cell line, RTgutGC, was established, originating from a rainbow trout (Oncorhynchus mykiss). In order to exploit the opportunities arising from RTgutGC cells for exploring fish intestinal physiology and toxicology, we present here the establishment of cells on commercially available permeable membrane supports and evaluate its suitability as a model of polarized intestinal epithelia. Within 3 weeks of culture, RTgutGC cells show epithelial features by forming tight junctions and desmosomes between adjacent cells. Cells develop a transepithelial electrical resistance comparable to in vivo measured values, reflecting the leaky nature of the fish intestine. Immunocytochemistry reveals evidence of polarization, such as basolateral localization of Na⁺/K⁺-ATPase (NKA) and apical localization of the tight junction protein ZO-1. NKA mRNA abundance was induced as physiological response toward a saltwater buffer, mimicking the migration of rainbow trout from fresh to seawater. Permeation of fluorescent molecules proved the barrier function of the cells, with permeation coefficients being comparable to those reported in fish. Finally, we demonstrate that cells on permeable supports are more resistant to the toxicity elicited by silver ions than cells grown the conventional way, likely due to improved cellular silver excretion.

High rates of intestinal bicarbonate secretion in seawater tilapia (Oreochromis mossambicus)

Osmoregulation in fish is a complex process that requires the orchestrated cooperation of many tissues. In fish facing hyperosmotic environments, the intestinal absorption of some monovalent ions and the secretion of bicarbonate are key processes to favor water absorption. In the present study, we showed that bicarbonate levels in the intestinal fluid are several fold higher in seawater than in freshwater acclimated tilapia (Oreochromis mossambicus). In addition, we analyzed gene expression of the main molecular mechanisms involved in HCO₃^- movements i.e. slc26a6, slc26a3, slc4a4 and v-type H-ATPase sub C in the intestine of tilapia acclimated to both seawater and freshwater. Our results show an anterior/posterior functional regionalization of the intestine in tilapia in terms of expression patterns, which is affected by environmental salinity mostly in the anterior and mid intestine. Analysis of bicarbonate secretion using pH-Stat in tissues mounted in Ussing chambers reveals high rates of bicarbonate secretion in tilapia acclimated to seawater from anterior intestine to rectum ranging between ~900 and ~1700nmolHCO₃^-cm^-2h^-1. However, a relationship between the expression of slc26a6, slc26a3, slc4a4 and the rate of bicarbonate secretion seems to be compromised in the rectum. In this region, the low expression of the bicarbonate transporters could not explain the high bicarbonate secretion rates here described. However, we postulate that the elevated v-type H-ATPase mRNA expression in the rectum could be involved in this process.

A Carbonic Anhydrase Serves as an Important Acid-Base Regulator in Pacific Oyster Crassostrea gigas Exposed to Elevated CO2: Implication for Physiological Responses of Mollusk to Ocean Acidification

Carbonic anhydrases (CAs) have been demonstrated to play an important role in acid-base regulation in vertebrates. However, the classification and modulatory function of CAs in marine invertebrates, especially their responses to ocean acidification remain largely unknown. Here, a cytosolic α-CA (designated as CgCAII-1) was characterized from Pacific oyster Crassostrea gigas and its molecular activities against CO₂ exposure were investigated. CgCAII-1 possessed a conserved CA catalytic domain, with high similarity to invertebrate cytoplasmic or mitochondrial α-CAs. Recombinant CgCAII-1 could convert CO₂ to HCO₃^- with calculated activity as 0.54 × 10³ U/mg, which could be inhibited by acetazolamide (AZ). The mRNA transcripts of CgCAII-1 in muscle, mantle, hepatopancreas, gill, and hemocytes increased significantly after exposure to elevated CO₂. CgCAII-1 could interact with the hemocyte membrane proteins and the distribution of CgCAII-1 protein became more concentrated and dense in gill and mantle under CO₂ exposure. The intracellular pH (pHi) of hemocytes under CO₂ exposure increased significantly (p < 0.05) and CA inhibition reduced the pHi value. Besides, there was no increase in CA activity in gill and mantle after CO₂ exposure. The impact of CO₂ exposure on CA activity coupled with the mRNA expression level and protein translocation of CgCAII-1 provided evidences that CgCAII-1 could respond to ocean acidification and participate in acid-base regulation. Such cytoplasmic CA-based physiological regulation mechanism might explain other physiological responses of marine organisms to OA.

Salinity-Dependent Shift in the Localization of Three Peptide Transporters along the Intestine of the Mozambique Tilapia (Oreochromis mossambicus)

The peptide transporter (PepT) systems are well-known for their importance to protein absorption in all vertebrate species. These symporters use H⁺ gradient at the apical membrane of the intestinal epithelial cells to mediate the absorption of small peptides. In fish, the intestine is a multifunctional organ, involved in osmoregulation, acid-base regulation, and nutrient absorption. Therefore, we expected environmental stimuli to affect peptide absorption. We examined the effect of three environmental factors; salinity, pH and feeding, on the expression, activity and localization of three PepT transporters (PepT1a, PepT1b, PepT2) along the intestine of the Mozambique tilapia (Oreochromis mossambicus). Quantitative real time PCR (qPCR) analysis demonstrated that the two PepT1 variants are typical to the proximal intestinal section while PepT2 is typical to the distal intestinal sections. Immunofluorescence analysis with custom-made antibodies supported the qPCR results, localized both transporters on the apical membrane of enterocytes and provided the first evidence for the participation of PepT2 in nutrient absorption. This first description of segment-specific expression and localization points to a complementary role of the different peptide transporters, corresponding to the changes in nutrient availability along the intestine. Both gene expression and absorption activity assays showed that an increase in water salinity shifted the localization of the PepT genes transcription and activity down along the intestinal tract. Additionally, an unexpected pH effect was found on the absorption of small peptides, with increased activity at higher pH levels. This work emphasizes the relationships between different functions of the fish intestine and how they are affected by environmental conditions.

Temperature Modulates the Effects of Ocean Acidification on Intestinal Ion Transport in Atlantic Cod, Gadus morhua

CO₂-driven seawater acidification has been demonstrated to enhance intestinal bicarbonate secretion rates in teleosts, leading to an increased release of CaCO₃ under simulated ocean acidification scenarios. In this study, we investigated if increasing CO₂ levels stimulate the intestinal acid–base regulatory machinery of Atlantic cod (Gadus morhua) and whether temperatures at the upper limit of thermal tolerance stimulate or counteract ion regulatory capacities. Juvenile G. morhua were acclimated for 4 weeks to three CO₂ levels (550, 1200, and 2200 μatm) covering present and near-future natural variability, at optimum (10°C) and summer maximum temperature (18°C), respectively. Immunohistochemical analyses revealed the subcellular localization of ion transporters, including Na⁺/K⁺-ATPase (NKA), Na⁺/H⁺-exchanger 3 (NHE3), Na⁺/HCO3− cotransporter (NBC1), pendrin-like Cl⁻/HCO3− exchanger (SLC26a6), V-type H⁺-ATPase subunit a (VHA), and Cl⁻ channel 3 (CLC3) in epithelial cells of the anterior intestine. At 10°C, proteins and mRNA were generally up-regulated for most transporters in the intestinal epithelium after acclimation to higher CO₂ levels. This supports recent findings demonstrating increased intestinal HCO3− secretion rates in response to CO₂ induced seawater acidification. At 18°C, mRNA expression and protein concentrations of most ion transporters remained unchanged or were even decreased, suggesting thermal compensation. This response may be energetically favorable to retain blood HCO3− levels to stabilize pH_e, but may negatively affect intestinal salt and water resorption of marine teleosts in future oceans.

Physiological impacts of elevated carbon dioxide and ocean acidification on fish

Most fish studied to date efficiently compensate for a hypercapnic acid-base disturbance; however, many recent studies examining the effects of ocean acidification on fish have documented impacts at CO2 levels predicted to occur before the end of this century. Notable impacts on neurosensory and behavioral endpoints, otolith growth, mitochondrial function, and metabolic rate demonstrate an unexpected sensitivity to current-day and near-future CO2 levels. Most explanations for these effects seem to center on increases in Pco2 and HCO3− that occur in the body during pH compensation for acid-base balance; however, few studies have measured these parameters at environmentally relevant CO2 levels or directly related them to reported negative endpoints. This compensatory response is well documented, but noted variation in dynamic regulation of acid-base transport pathways across species, exposure levels, and exposure duration suggests that multiple strategies may be utilized to cope with hypercapnia. Understanding this regulation and changes in ion gradients in extracellular and intracellular compartments during CO2 exposure could provide a basis for predicting sensitivity and explaining interspecies variation. Based on analysis of the existing literature, the present review presents a clear message that ocean acidification may cause significant effects on fish across multiple physiological systems, suggesting that pH compensation does not necessarily confer tolerance as downstream consequences and tradeoffs occur. It remains difficult to assess if acclimation responses during abrupt CO2 exposures will translate to fitness impacts over longer timescales. Nonetheless, identifying mechanisms and processes that may be subject to selective pressure could be one of many important components of assessing adaptive capacity.

A new role for carbonic anhydrase 2 in the response of fish to copper and osmotic stress: implications for multi-stressor studies

The majority of ecotoxicological studies are performed under stable and optimal conditions, whereas in reality the complexity of the natural environment faces organisms with multiple stressors of different type and origin, which can activate pathways of response often difficult to interpret. In particular, aquatic organisms living in estuarine zones already impacted by metal contamination can be exposed to more severe salinity variations under a forecasted scenario of global change. In this context, the present study aimed to investigate the effect of copper exposure on the response of fish to osmotic stress by mimicking in laboratory conditions the salinity changes occurring in natural estuaries. We hypothesized that copper-exposed individuals are more sensitive to osmotic stresses, as copper affects their osmoregulatory system by acting on a number of osmotic effector proteins, among which the isoform two of the enzyme carbonic anhydrase (CA2) was identified as a novel factor linking the physiological responses to both copper and osmotic stress. To test this hypothesis, two in vivo studies were performed using the euryhaline fish sheepshead minnow (Cyprinodon variegatus) as test species and applying different rates of salinity transition as a controlled way of dosing osmotic stress. Measured endpoints included plasma ions concentrations and gene expression of CA2 and the α1a-subunit of the enzyme Na+/K+ ATPase. Results showed that plasma ions concentrations changed after the salinity transition, but notably the magnitude of change was greater in the copper-exposed groups, suggesting a sensitizing effect of copper on the responses to osmotic stress. Gene expression results demonstrated that CA2 is affected by copper at the transcriptional level and that this enzyme might play a role in the observed combined effects of copper and osmotic stress on ion homeostasis.

Osmoregulatory bicarbonate secretion exploits H(+)-sensitive haemoglobins to autoregulate intestinal O2 delivery in euryhaline teleosts

Marine teleost fish secrete bicarbonate (HCO3 (-)) into the intestine to aid osmoregulation and limit Ca(2+) uptake by carbonate precipitation. Intestinal HCO3 (-) secretion is associated with an equimolar transport of protons (H(+)) into the blood, both being proportional to environmental salinity. We hypothesized that the H(+)-sensitive haemoglobin (Hb) system of seawater teleosts could be exploited via the Bohr and/or Root effects (reduced Hb-O2 affinity and/or capacity with decreasing pH) to improve O2 delivery to intestinal cells during high metabolic demand associated with osmoregulation. To test this, we characterized H(+) equilibria and gas exchange properties of European flounder (Platichthys flesus) haemoglobin and constructed a model incorporating these values, intestinal blood flow rates and arterial-venous acidification at three different environmental salinities (33, 60 and 90). The model suggested red blood cell pH (pHi) during passage through intestinal capillaries could be reduced by 0.14-0.33 units (depending on external salinity) which is sufficient to activate the Bohr effect (Bohr coefficient of -0.63), and perhaps even the Root effect, and enhance tissue O2 delivery by up to 42 % without changing blood flow. In vivo measurements of intestinal venous blood pH were not possible in flounder but were in seawater-acclimated rainbow trout which confirmed a blood acidification of no less than 0.2 units (equivalent to -0.12 for pHi). When using trout-specific values for the model variables, predicted values were consistent with measured in vivo values, further supporting the model. Thus this system is an elegant example of autoregulation: as the need for costly osmoregulatory processes (including HCO3 (-) secretion) increases at higher environmental salinity, so does the enhancement of O2 delivery to the intestine via a localized acidosis and the Bohr (and possibly Root) effect.

An in vitro investigation of gastrointestinal Na(+) uptake mechanisms in freshwater rainbow trout

In vitro gut-sac preparations of all four sections (stomach, anterior, mid, and posterior intestine) of the gastrointestinal tract (GIT) of freshwater rainbow trout, together with radiotracer ((22)Na) techniques, were used to study unidirectional Na(+) uptake rates (UR, mucosal → blood space) and net absorptive fluid transport rates (FTR) under isosmotic conditions (mucosal = serosal osmolality). On an area-specific basis, unidirectional Na(+) UR was highest in the mid-intestine, but when total gut area was taken into account, the three intestinal sections contributed equally, with very low rates in the stomach. The theoretical capacity for Na(+) uptake across the whole GIT is sufficient to supply all of the animal's nutritive requirements for Na(+). Transport occurs by low affinity systems with apparent K m values 2-3 orders of magnitude higher than those in the gills, in accord with comparably higher Na(+) concentrations in chyme versus fresh water. Fluid transport appeared to be Na(+)-dependent, such that treatments which altered unidirectional Na(+) UR generally altered FTR in a comparable fashion. Pharmacological trials (amiloride, EIPA, phenamil, bafilomycin, furosemide, hydrochlorothiazide) conducted at a mucosal Na(+) concentration of 50 mmol L(-1) indicated that GIT Na(+) uptake occurs by a variety of apical mechanisms (NHE, Na(+) channel/H(+) ATPase, NCC, NKCC) with relative contributions varying among sections. However, at a mucosal Na(+) concentration of 10 mmol L(-1), EIPA, phenamil, bafilomycin, and hydrochlorothiazide were no longer effective in inhibiting unidirectional Na(+) UR or FTR, suggesting the contribution of unidentified mechanisms under low Na(+) conditions. A preliminary model is presented.

Esophageal desalination is mediated by Na⁺, H⁺ exchanger-2 in the gulf toadfish (Opsanus beta)

Esophageal desalination is a crucial step in the gastrointestinal water absorption pathway, as this pre-intestinal processing establishes the osmotic conditions necessary for water absorption. Previous work has shown that esophageal Na(+) absorption is amiloride sensitive; however, it is as yet unclear if Na(+), H(+) exchangers (NHE) or Na(+) channels (ENaC) are responsible. The purpose of the current study was therefore to investigate the roles that NHE isoforms may play in this process in a marine teleost, the gulf toadfish (Opsanus beta), as well as what role NHE isoforms may play in the downstream intestinal Na(+) transport. A combination of symmetrical current clamp and asymmetrical voltage clamp experiments showed the esophagus to contain both an ion absorptive current (I(sc)=0.83±0.68) and serosal side negative transepithelial potential (TEP=-4.9±0.6). (22)Na uptake (J(Na)(m→s)) was inhibited by 0.5 mM EIPA, with no effect of 0.1 mM amiloride, 1 mM furosemide or 1 mM thiazide. A Cl(-) free saline reduced J(Na)(m→s) by 40% while also reducing conductance and reversing TEP. These results suggest that both transcellular and paracellular components contribute to esophageal Na(+) transport, with transcellular transport mediated by NHE. The NHE1, NHE2 and NHE3 genes were amplified and tissue distribution analysis by real-time PCR showed high NHE2 expression levels in the esophagus and stomach. Little NHE3 expression was observed throughout the gastrointestinal tract, and NHE2 expression was absent from the intestine. Hypersalinity (60 ppt) had no effect on the expression profile of NHE2, slc4a2, scl26a6, CAc or V-type ATPase (β-subunit), suggesting that esophageal desalination is less flexible in response to osmotic stress than the intestine.

An in vitro analysis of intestinal ammonia handling in fasted and fed freshwater rainbow trout (Oncorhynchus mykiss)

Ammonia transport and metabolism were investigated in the intestinal tract of freshwater rainbow trout which had been either fasted for 7 days, or fasted then fed a satiating meal of commercial trout pellets. In vivo, total ammonia concentrations (T amm) in the chyme were approximately 1 mmol L(-1) across the entire intestine at 24 h after the meal. Highest chyme pH and P NH3 values occurred in the posterior intestine. In vitro gut sac experiments examined ammonia handling with mucosal (Jmamm) and serosal (Js amm) fluxes under conditions of fasting and feeding, with either background (control ≤ 0.013 mmol L(-1)) or high luminal ammonia concentrations (HLA = 1 mmol L(-1)), the latter mimicking those seen in chyme in vivo. Feeding status (fasted or fed) appeared to influence ammonia handling by each individual section. The anterior intestine exhibited the greatest Jm amm and Js amm values under fasted control conditions, but these differences tended to disappear under typical post-feeding conditions when total endogenous ammonia production (Jt amm = Js amm - Jm amm, signs considered) was greatly elevated in all intestinal sections. Under fasted conditions, glutamate dehydrogenase (GDH) and glutaminase (GLN) activities were equal across all sections, but the ammonia-trapping enzyme glutamine synthetase (GS) exhibited highest activity in the posterior intestine, in contradiction to previous literature. Feeding clearly stimulated the total rate of endogenous ammonia production (Jt amm), even in the absence of a high luminal ammonia load. This was accompanied by an increase in GDH activity of the anterior intestine, which was also the site of the largest Jt amm. In all sections, during HLA exposure, either alone or in combination with feeding, there were much larger increases in endogenous Jt amm, most of which was effluxed to the serosal solution. This is interpreted as a response to avoid potential cytotoxicity due to overburdened detoxification mechanisms in the face of elevated mucosal ammonia. Thus T amm of the intestinal tissue remained relatively constant regardless of feeding status and exposure to HLA. Ammonia production by the gut may explain up to 18 % of whole-body ammonia excretion in vivo under fasting conditions, and 47 % after feeding, of which more than half originates from endogenous production rather than from absorption from the lumen.

The divergence, actions, roles, and relatives of sodium-coupled bicarbonate transporters

The mammalian Slc4 (Solute carrier 4) family of transporters is a functionally diverse group of 10 multi-spanning membrane proteins that includes three Cl-HCO3 exchangers (AE1-3), five Na(+)-coupled HCO3(-) transporters (NCBTs), and two other unusual members (AE4, BTR1). In this review, we mainly focus on the five mammalian NCBTs-NBCe1, NBCe2, NBCn1, NDCBE, and NBCn2. Each plays a specialized role in maintaining intracellular pH and, by contributing to the movement of HCO3(-) across epithelia, in maintaining whole-body pH and otherwise contributing to epithelial transport. Disruptions involving NCBT genes are linked to blindness, deafness, proximal renal tubular acidosis, mental retardation, and epilepsy. We also review AE1-3, AE4, and BTR1, addressing their relevance to the study of NCBTs. This review draws together recent advances in our understanding of the phylogenetic origins and physiological relevance of NCBTs and their progenitors. Underlying these advances is progress in such diverse disciplines as physiology, molecular biology, genetics, immunocytochemistry, proteomics, and structural biology. This review highlights the key similarities and differences between individual NCBTs and the genes that encode them and also clarifies the sometimes confusing NCBT nomenclature.

Intestinal fluid absorption in anadromous salmonids: importance of tight junctions and aquaporins

The anadromous salmonid life cycle includes both fresh water (FW) and seawater (SW) stages. The parr-smolt transformation (smoltification) pre-adapt the fish to SW while still in FW. The osmoregulatory organs change their mode of action from a role of preventing water inflow in FW, to absorb ions to replace water lost by osmosis in SW. During smoltification, the drinking rate increases, in the intestine the ion and fluid transport increases and is further elevated after SW entry. In SW, the intestine absorbs ions to create an inwardly directed water flow which is accomplished by increased Na⁺, K⁺-ATPase (NKA) activity in the basolateral membrane, driving ion absorption via ion channels and/or co-transporters. This review will aim at discussing the expression patterns of the ion transporting proteins involved in intestinal fluid absorption in the FW stage, during smoltification and after SW entry. Of equal importance for intestinal fluid absorption as the active absorption of ions is the permeability of the epithelium to ions and water. During the smoltification the increase in NKA activity and water uptake in SW is accompanied by decreased paracellular permeability suggesting a redirection of the fluid movement from a paracellular route in FW, to a transcellular route in SW. Increased transcellular fluid absorption could be achieved by incorporation of aquaporins (AQPs) into the enterocyte membranes and/or by a change in fatty acid profile of the enterocyte lipid bilayer. An increased incorporation of unsaturated fatty acids into the membrane phospholipids will increase water permeability by enhancing the fluidity of the membrane. A second aim of the present review is therefore to discuss the presence and regulation of expression of AQPs in the enterocyte membrane as well as to discuss the profile of fatty acids present in the membrane phospholipids during different stages of the salmonid lifecycle.

Prolactin regulates luminal bicarbonate secretion in the intestine of the sea bream (Sparus aurata L.)

The pituitary hormone prolactin is a pleiotropic endocrine factor that plays a major role in the regulation of ion balance in fish, with demonstrated actions mainly in the gills and kidney. The role of prolactin in intestinal ion transport remains little studied. In marine fish, which have high drinking rates, epithelial bicarbonate secretion in the intestine produces luminal carbonate aggregates believed to play a key role in water and ion homeostasis. The present study was designed to establish the putative role of prolactin in the regulation of intestinal bicarbonate secretion in a marine fish. Basolateral addition of prolactin to the anterior intestine of sea bream mounted in Ussing chambers caused a rapid (<20 min) decrease of bicarbonate secretion measured by pH-stat. A clear inhibitory dose-response curve was obtained, with a maximal inhibition of 60-65% of basal bicarbonate secretion. The threshold concentration of prolactin for a significant effect on bicarbonate secretion was 10 ng ml(-1), which is comparable with putative plasma levels in seawater fish. The effect of prolactin on apical bicarbonate secretion was independent of the generation route for bicarbonate, as shown in a preparation devoid of basolateral HCO(3)(-)/CO(2) buffer. Specific inhibitors of JAK2 (AG-490, 50 μmol l(-1)), PI3K (LY-294002, 75 μmol l(-1)) or MEK (U-012610, 10 μmol l(-1)) caused a 50-70% reduction in the effect of prolactin on bicarbonate secretion, and demonstrated the involvement of prolactin receptors. In addition to rapid effects, prolactin has actions at the genomic level. Incubation of intestinal explants of anterior intestine of the sea bream in vitro for 3 h demonstrated a specific effect of prolactin on the expression of the Slc4a4A Na(+)-HCO(3)(-) co-transporter, but not on the Slc26a6A or Slc26a3B Cl(-)/HCO(3)(-) exchanger. We propose a new role for prolactin in the regulation of bicarbonate secretion, an essential function for ion/water homeostasis in the intestine of marine fish.

Challenges for the development of a biotic ligand model predicting copper toxicity in estuaries and seas

An effort is ongoing to develop a biotic ligand model (BLM) that predicts copper (Cu) toxicity in estuarine and marine environments. At present, the BLM accounts for the effects of water chemistry on Cu speciation, but it does not consider the influence of water chemistry on the physiology of the organisms. We discuss how chemistry affects Cu toxicity not only by controlling its speciation, but also by affecting the osmoregulatory physiology of the organism, which varies according to salinity. In an attempt to understand the mechanisms of Cu toxicity and predict its impacts, we explore the hypothesis that the common factor linking the main toxic effects of Cu is the enzyme carbonic anhydrase (CA), because it is a Cu target with multiple functions and salinity-dependent expression and activity. According to this hypothesis, the site of action of Cu in marine fish may be not only the gill, but also the intestine, because in this tissue CA plays an important role in ion transport and water adsorption. Therefore, the BLM of Cu toxicity to marine fish should also consider the intestine as a biotic ligand. Finally, we underline the need to incorporate the osmotic gradient into the BLM calculations to account for the influence of physiology on Cu toxicity.

The influence of 17β-estradiol on intestinal calcium carbonate precipitation and osmoregulation in seawater-acclimated rainbow trout (Oncorhynchus mykiss)

The intestine of marine teleosts produces carbonate precipitates from ingested calcium as part of their osmoregulatory strategy in seawater. The potential for estrogens to control the production of intestinal calcium carbonate and so influence osmoregulation was investigated in seawater-acclimated rainbow trout following intraperitoneal implantation of 17β-estradiol (E2) at two doses (0.1 and 10 μg E2 g(-1)). Levels of plasma vitellogenin provided an indicator of estrogenic effect, increasing significantly by three and four orders of magnitude at the low and high doses, respectively. Plasma osmolality and muscle water content were unaffected, whereas E2-treated fish maintained lower plasma [Na(+)] and [Cl(-)]. Plasma [Ca(2+)] and [Mg(2+)] and muscle [Ca(2+)] increased with vitellogenin induction, whereas the intestinal excretion of calcium carbonate was reduced. This suggests that elevated levels of circulating E2 may enhance Ca(2+) uptake via the gut and simultaneously reduce CaCO(3) formation, which normally limits intestinal availability of Ca(2+). Increasing E2 caused an elevation of [Na(+)] and [Cl(-)] and a reduction of [HCO(3(-))] in intestinal fluid. We speculate that E2 may influence a number of intestinal ion transport processes that ultimately may influence water absorption: (1) reduced NaCl cotransport, (2) reduced Cl(-) uptake via Cl(-)/HCO(3(-)) exchange and (3) reduced precipitation of Ca(2+) and Mg(2+) carbonates. Despite these effects on intestinal ion and water transport, overall osmoregulatory status was not compromised in E2-treated fish, suggesting the possibility of compensation by other organs.

Digestion of a single meal affects gene expression of ion and ammonia transporters and glutamine synthetase activity in the gastrointestinal tract of freshwater rainbow trout

Experiments on freshwater rainbow trout, Oncorhynchus mykiss, demonstrated how digestion affected the transcriptional expression of gastrointestinal transporters following a single satiating meal (~3% body mass ration) after a 1-week fast. Quantitative real-time polymerase chain reaction was employed to measure the relative mRNA expression of three previously cloned and sequenced transporters [H(+)-K(+)-ATPase (HKA), Na(+)/HCO(3)(-) cotransporter (NBC), and the Rhesus glycoprotein (Rhbg1; an ammonia transporter)] over a 24-h time course following feeding. Plasma total ammonia increased about threefold from pre-feeding levels to 288 μmol l(-1), whereas total ammonia levels in chyme supernatant reached a sixfold higher value (1.8 mmol l(-1)) than plasma levels. Feeding did not appear to have a statistically significant effect on the relative mRNA expression of the gastric HKA or Rhbg1. However, the relative mRNA expression of gastric NBC was increased 24 h following the ingestion of a meal. Along the intestinal tract, feeding increased the relative mRNA expression of Rhbg1, but had no effect on the expression of NBC. Expression of the gastric HKA was undetectable in the intestinal tract of freshwater rainbow trout. Digestion increased the activity of glutamine synthetase in the posterior intestine at 12 and 24 h following feeding. This study is among the first to show that there are digestion-associated changes in gene expression and enzyme activity in the gastrointestinal tract of teleost fish illustrating the dynamic plasticity of this organ. These post-prandial changes occur over the relative short-term duration of digesting a single meal.

Compensatory regulation of acid-base balance during salinity transfer in rainbow trout (Oncorhynchus mykiss)

In seawater-acclimated rainbow trout (Oncorhynchus mykiss), base secretion into the intestine is a key component of the intestinal water absorption that offsets osmotic water loss to the marine environment. Acid-base balance is maintained by the matched excretion of acid equivalents via other routes, presumably the gill and/or kidney. The goal of the present study was to examine acid-base balance in rainbow trout upon transfer to more dilute environments, conditions under which base excretion into the intestine is predicted to fall, requiring compensatory adjustments of acid excretion at the gill and/or kidney if acid-base balance is to be maintained. Net acid excretion via the gill/kidney and rectal fluid, and blood acid-base status were monitored in seawater-acclimated rainbow trout maintained in seawater or transferred to iso-osmotic conditions. As predicted, transfer to iso-osmotic conditions significantly reduced base excretion into the rectal fluid (by ~48%). Transfer to iso-osmotic conditions also significantly reduced the excretion of titratable acidity via extra-intestinal routes from 183.4 ± 71.3 to -217.5 ± 42.7 μmol kg(-1) h(-1) (N = 7). At the same time, however, ammonia excretion increased significantly during iso-osmotic transfer (by ~72%) so that the apparent overall reduction in net acid excretion (from 419.7 ± 92.9 to 189.2 ± 76.5 μmol kg(-1 )h(-1); N = 7) was not significant. Trout maintained blood acid-base status during iso-osmotic transfer, although arterial pH was significantly higher in transferred fish than in those maintained in seawater. To explore the mechanisms underlying these adjustments of acid-base regulation, the relative mRNA expression and where possible, activity of a suite of proteins involved in acid-base balance were examined in intestine, gill and kidney. At the kidney, reduced mRNA expression of carbonic anhydrase (CA; cytosolic and membrane-associated CA IV), V-type H(+)-ATPase, and Na(+)/HCO(3) (-) co-transporter were consistent with a reduced role in net acid excretion following iso-osmotic transfer. Changes in relative mRNA expression and/or activity at the intestine and gill were consistent with the roles of these organs in osmotic rather than acid-base regulation. Overall, the data emphasize the coordination of acid-base, osmoregulatory and ionoregulatory processes that occur with salinity transfer in a euryhaline fish.

Regulation of apical H⁺-ATPase activity and intestinal HCO₃⁻ secretion in marine fish osmoregulation

The absorption of Cl(-) and water from ingested seawater in the marine fish intestine is accomplished partly through Cl(-)/HCO(3)(-) exchange. Recently, a H(+) pump (vacuolar-type H(+)-ATPase) was found to secrete acid into the intestinal lumen, and it may serve to titrate luminal HCO(3)(-) and facilitate further Cl(-)/HCO(3)(-) exchange, especially in the posterior intestine, where adverse concentration gradients could limit Cl(-)/HCO(3)(-) exchange. The H(+) pump is expressed in all intestinal segments and in gill tissue of gulf toadfish (Opsanus beta) maintained in natural seawater. After acute transfer of toadfish to 60 ppt salinity, H(+) pump expression increased 20-fold in the posterior intestine. In agreement with these observations was a fourfold-increased H(+)-ATPase activity in the posterior intestine of animals acclimated to 60 ppt salinity. Interestingly, Na(+)-K(+)-ATPase activity was elevated in the anterior intestine and gill, but not in the posterior intestine. Apical acid secretion by isolated intestinal tissue mounted in Ussing chambers fitted with pH-stat titration systems increased after acclimation to hypersalinity in the anterior and posterior intestine, titrating >20% of secreted bicarbonate. In addition, net base secretion increased in hypersalinity-acclimated fish and was ∼70% dependent on serosal HCO(3)(-). Protein localization by immunohistochemistry confirmed the presence of the vacuolar-type H(+)-ATPase in the apical region of intestinal enterocytes. These results show that the H(+) pump, especially in the posterior intestine, plays an important role in hypersaline osmoregulation and that it likely has significant effects on HCO(3)(-) accumulation in the intestinal lumen and, therefore, the continued absorption of Cl(-) and water.

Acid-base responses to feeding and intestinal Cl- uptake in freshwater- and seawater-acclimated killifish, Fundulus heteroclitus, an agastric euryhaline teleost

Marine teleosts generally secrete basic equivalents (HCO(3)(-)) and take up Na(+) and Cl(-) in the intestine so as to promote absorption of H(2)O. However, neither the integration of these functions with feeding nor the potential role of the gut in ionoregulation and acid-base balance in freshwater have been well studied. The euryhaline killifish (Fundulus heteroclitus) is unusual in lacking both an acid-secreting stomach and a mechanism for Cl(-) uptake at the gills in freshwater. Responses to a satiation meal were evaluated in both freshwater- and seawater-acclimated killifish. In intact animals, there was no change in acid or base flux to the external water after the meal, in accord with the absence of any post-prandial alkaline tide in the blood. Indeed, freshwater animals exhibited a post-prandial metabolic acidosis ('acidic tide'), whereas seawater animals showed no change in blood acid-base status. In vitro gut sac experiments revealed a substantially higher rate of Cl(-) absorption by the intestine in freshwater killifish, which was greatest at 1-3 h after feeding. The Cl(-) concentration of the absorbate was higher in preparations from freshwater animals than from seawater killifish and increased with fasting. Surprisingly, net basic equivalent secretion rates were also much higher in preparations from freshwater animals, in accord with the 'acidic tide'; in seawater preparations, they were lowest after feeding and increased with fasting. Bafilomycin (1 micromol l(-1)) promoted an 80% increase in net base secretion rates, as well as in Cl(-) and fluid absorption, at 1-3 h post-feeding in seawater preparations only, explaining the difference between freshwater and seawater fish. Preparations from seawater animals at 1-3 h post-feeding also acidified the mucosal saline, and this effect was associated with a marked rise in P(CO(2)), which was attenuated by bafilomycin. Measurements of chyme pH from intact animals confirmed that intestinal fluid (chyme) pH and basic equivalent concentration were lowest after feeding in seawater killifish, whereas P(CO(2)) was greatly elevated (80-95 Torr) in chyme from both seawater and freshwater animals but declined to lower levels (13 Torr) after 1-2 weeks fasting. There were no differences in pH, P(CO(2)) or the concentrations of basic equivalents in intestinal fluid from seawater versus freshwater animals at 12-24 h or 1-2 weeks post-feeding. The results are interpreted in terms of the absence of gastric HCl secretion, the limitations of the gills for acid-base balance and Cl(-) transport, and therefore the need for intestinal Cl(-) uptake in freshwater killifish, and the potential for O(2) release from the mucosal blood flow by the high P(CO(2)) in the intestinal fluids. At least in seawater killifish, H(+)-ATPase running in parallel to HCO(3)(-):Cl(-) exchange in the apical membranes of teleost enterocytes might reduce net base secretion and explain the high P(CO(2)) in the chyme after feeding.

Parathyroid hormone-related protein-stanniocalcin antagonism in regulation of bicarbonate secretion and calcium precipitation in a marine fish intestine

Bicarbonate secretion in the intestine (duodenum) of marine fish has been suggested to play a major role in regulation of calcium availability for uptake. However, while the end process may lead to carbonate precipitation, regulation of transport of calcium and/or bicarbonate may actually result in fine-tuning of calcium availability for transport. To test this hypothesis, sea bream ( Sparus auratus ) duodenal preparations were mounted in Ussing-type chambers and the effect of parathyroid hormone-related protein (PTHrP) and stanniocalcin 1 (STC 1) on the control of intestinal bicarbonate secretion and calcium transport was analyzed. As expected, PTHrP increased net calcium uptake, as a result of an increase of calcium uptake without changes in calcium efflux. In contrast, purified sea bream STC 1 caused a minor decrease of calcium uptake and a two- to threefold increase in calcium efflux. As a result, STC 1 was able to invert the calcium flux from net calcium uptake to net calcium loss, which is in keeping with its known actions as a hypocalcemic factor. Furthermore, both PTHrP and STC 1 regulate intestinal bicarbonate secretion. PTHrP increased calcium uptake and simultaneously reduced the single factor that induces calcium precipitation, bicarbonate secretion. In contrast, STC 1, while reversing the calcium net flux to make it secretory, promoted intestinal bicarbonate secretion, both actions directed to decrease the calcium gradient across the epithelium and promote immobilization in the form of bicarbonate in the intestinal lumen. Together our results provide robust evidence to support an antagonistic action of PTHrP and STC 1 in the fine control of movements of both calcium and bicarbonate in the intestine of seawater fish.

Molecular cloning and functional expression of atlantic salmon peptide transporter 1 in Xenopus oocytes reveals efficient intestinal uptake of lysine-containing and other bioactive di- and tripeptides in teleost fish

Atlantic salmon (Salmo salar L.) is one of the most economically important cultured fish and also a key model species in fish nutrition. During digestion, dietary proteins are enzymatically cleaved and a fraction of degradation products in the form of di- and tripeptides translocates from the intestinal lumen into the enterocyte via the Peptide Transporter 1 (PepT1). With this in mind, a full-length cDNA encoding the Atlantic salmon PepT1 (asPepT1) was cloned and functionally characterized. When overexpressed in Xenopus laevis oocytes, asPepT1 operated as a low-affinity/high-capacity transport system, and its maximal transport activity slightly increased as external proton concentration decreased (varying extracellular pH from 6.5 to 8.5). A total of 19 tested di- and tripeptides, some with acknowledged bioactive properties, some containing lysine, which is conditionally growth limiting in fish, were identified as well transported substrates, with affinities ranging between approximately 0.5 and approximately 1.5 mmol/L. Analysis of body tissue distribution showed the highest levels of asPepT1 mRNA in the digestive tract. In particular, asPepT1 mRNA was present in all segments after the stomach, with higher levels in the pyloric caeca and midgut region and lower levels in the hindgut. Depriving salmon of food for 6 d resulted in a approximately 70% reduction of intestinal PepT1 mRNA levels. asPepT1 will allow systematic in vitro analysis of transport of selected di- and tripeptides that may be generated in Atlantic salmon intestine during gastrointestinal transit. Also, asPepT1 will be useful as a marker to estimate protein absorption function along the intestine under various physiological and pathological conditions.

Basolateral NBCe1 plays a rate-limiting role in transepithelial intestinal HCO3- secretion, contributing to marine fish osmoregulation

Although endogenous CO2 hydration and serosal HCO3- are both known to contribute to the high rates of intestinal HCO3- secretion important to marine fish osmoregulation, the basolateral step by which transepithelial HCO3- secretion is accomplished has received little attention. Isolated intestine HCO3- secretion rates, transepithelial potential (TEP) and conductance were found to be dependent on serosal HCO3- concentration and sensitive to serosal DIDS. Elevated mucosal Cl- concentration had the unexpected effect of reducing HCO3- secretion rates, but did not affect electrophysiology. These characteristics indicate basolateral limitation of intestinal HCO3- secretion in seawater gulf toadfish, Opsanus beta. The isolated intestine has a high affinity for serosal HCO3- in the physiological range (Km=10.2 mmol l(-1)), indicating a potential to efficiently fine-tune systemic acid-base balance. We have confirmed high levels of intestinal tract expression of a basolateral Na+/HCO3- cotransporter of the electrogenic NBCe1 isoform in toadfish (tfNBCe1), which shows elevated expression following salinity challenge, indicating its importance in marine fish osmoregulation. When expressed in Xenopus oocytes, isolated tfNBCe1 has transport characteristics similar to those in the isolated tissue, including a similar affinity for HCO3- (Km=8.5 mmol l(-1)). Reported affinity constants of NBC1 for Na+ are generally much lower than physiological Na+ concentrations, suggesting that cotransporter activity is more likely to be modulated by HCO3- rather than Na+ availability in vivo. These similar functional characteristics of isolated tfNBCe1 and the intact tissue suggest a role of this cotransporter in the high HCO3- secretion rates of the marine fish intestine.

Population genomics of parallel adaptation in threespine stickleback using sequenced RAD tags

Next-generation sequencing technology provides novel opportunities for gathering genome-scale sequence data in natural populations, laying the empirical foundation for the evolving field of population genomics. Here we conducted a genome scan of nucleotide diversity and differentiation in natural populations of threespine stickleback (Gasterosteus aculeatus). We used Illumina-sequenced RAD tags to identify and type over 45,000 single nucleotide polymorphisms (SNPs) in each of 100 individuals from two oceanic and three freshwater populations. Overall estimates of genetic diversity and differentiation among populations confirm the biogeographic hypothesis that large panmictic oceanic populations have repeatedly given rise to phenotypically divergent freshwater populations. Genomic regions exhibiting signatures of both balancing and divergent selection were remarkably consistent across multiple, independently derived populations, indicating that replicate parallel phenotypic evolution in stickleback may be occurring through extensive, parallel genetic evolution at a genome-wide scale. Some of these genomic regions co-localize with previously identified QTL for stickleback phenotypic variation identified using laboratory mapping crosses. In addition, we have identified several novel regions showing parallel differentiation across independent populations. Annotation of these regions revealed numerous genes that are candidates for stickleback phenotypic evolution and will form the basis of future genetic analyses in this and other organisms. This study represents the first high-density SNP-based genome scan of genetic diversity and differentiation for populations of threespine stickleback in the wild. These data illustrate the complementary nature of laboratory crosses and population genomic scans by confirming the adaptive significance of previously identified genomic regions, elucidating the particular evolutionary and demographic history of such regions in natural populations, and identifying new genomic regions and candidate genes of evolutionary significance.

Population genomics of parallel adaptation in threespine stickleback using sequenced RAD tags

Next-generation sequencing technology provides novel opportunities for gathering genome-scale sequence data in natural populations, laying the empirical foundation for the evolving field of population genomics. Here we conducted a genome scan of nucleotide diversity and differentiation in natural populations of threespine stickleback (Gasterosteus aculeatus). We used Illumina-sequenced RAD tags to identify and type over 45,000 single nucleotide polymorphisms (SNPs) in each of 100 individuals from two oceanic and three freshwater populations. Overall estimates of genetic diversity and differentiation among populations confirm the biogeographic hypothesis that large panmictic oceanic populations have repeatedly given rise to phenotypically divergent freshwater populations. Genomic regions exhibiting signatures of both balancing and divergent selection were remarkably consistent across multiple, independently derived populations, indicating that replicate parallel phenotypic evolution in stickleback may be occurring through extensive, parallel genetic evolution at a genome-wide scale. Some of these genomic regions co-localize with previously identified QTL for stickleback phenotypic variation identified using laboratory mapping crosses. In addition, we have identified several novel regions showing parallel differentiation across independent populations. Annotation of these regions revealed numerous genes that are candidates for stickleback phenotypic evolution and will form the basis of future genetic analyses in this and other organisms. This study represents the first high-density SNP-based genome scan of genetic diversity and differentiation for populations of threespine stickleback in the wild. These data illustrate the complementary nature of laboratory crosses and population genomic scans by confirming the adaptive significance of previously identified genomic regions, elucidating the particular evolutionary and demographic history of such regions in natural populations, and identifying new genomic regions and candidate genes of evolutionary significance.

HCO (3)(-) secretion and CaCO3 precipitation play major roles in intestinal water absorption in marine teleost fish in vivo

The intestine of marine teleosts must effectively absorb fluid from ingested seawater to avoid dehydration. This fluid transport has been almost exclusively characterized as driven by NaCl absorption. However, an additional feature of the osmoregulatory role of the intestine is substantial net HCO(3)(-) secretion. This is suggested to drive additional fluid absorption directly (via Cl(-)/HCO(3)(-) exchange) and indirectly by precipitating ingested Ca(2+) as CaCO(3), thus creating the osmotic gradient for additional fluid absorption. The present study tested this hypothesis by perfusing the intestine of the European flounder in vivo with varying [Ca(2+)]: 10 (control), 40, and 90 mM. Fractional fluid absorption increased from 47% (control) to 73% (90 mM Ca(2+)), where almost all secreted HCO(3)(-) was excreted as CaCO(3). This additional fluid absorption could not be explained by NaCl cotransport. Instead, a significant positive relationship between Na(+)-independent fluid absorption and total HCO(3)(-) secretion was consistent with the predicted roles for anion exchange and CaCO(3) precipitation. Further analysis suggested that Na(+)-independent fluid absorption could be accounted for by net Cl(-) and H(+) absorption (from Cl(-)/HCO(3)(-) exchange and CO(2) hydration, respectively). There was no evidence to suggest that CaCO(3) alone was responsible for driving fluid absorption. However, by preventing the accumulation of luminal Ca(2+) it played a vital role by dynamically maintaining a favorable osmotic gradient all along the intestine, which permits substantially higher rates of solute-linked fluid absorption. To overcome the resulting hyperosmotic and highly acidic absorbate, it is proposed that plasma HCO(3)(-) buffers the absorbed H(+) (from HCO(3)(-) production), and consequently reduces the osmolarity of the absorbed fluid entering the body.

Intestinal osmoregulatory acclimation and nitrogen metabolism in juveniles of the freshwater marble goby exposed to seawater

The objective of this study was to elucidate the role of the intestine from juveniles of the marble goby, Oxyeleotris marmorata, during seawater (SW) exposure. It has been reported elsewhere that SW-exposed juvenile O. marmorata exhibits hypoosmotic and hypoionic regulation, with the induction of branchial Na(+)/K(+)-ATPase (NKA), Na(+):K(+):2Cl(-) cotransporter (NKCC), and cystic fibrosis transmembrane receptor-like chloride channels. Here, we report that SW exposure also led to significant increases in the activity and protein abundance of NKA in, and probably an increase in Na(+) uptake through, its intestine. Additionally, there was an increase in apical NKCC immunoreactivity in the intestinal epithelium, indicating that there could be increased Cl(-) uptake through the intestine. These results suggest that absorption of ions, and hence water, from the intestinal lumen could be an essential part of the osmoregulatory process in juvenile O. marmorata during exposure to SW. Furthermore, there were significant increases in the glutamate content, and the aminating activity and protein abundance of glutamate dehydrogenase (GDH) in the intestine of fish exposed to SW. Since the intestinal glutamine synthetase activity and protein abundance decreased significantly, and the intestinal glutamine content remained unchanged, in the SW-exposed fish, excess glutamate formed via increased GDH activity in the intestine could be channeled to other organs to facilitate the increased synthesis of amino acids. Taken together, our results indicate for the first time that, besides absorbing ions and water during SW exposure, the intestine of juvenile O. marmorata also participated in altered nitrogen metabolism in response to salinity changes.

The involvement of H+-ATPase and carbonic anhydrase in intestinal HCO3- secretion in seawater-acclimated rainbow trout

Pyloric caeca and anterior intestine epithelia from seawater-acclimated rainbow trout exhibit different electrophysiological parameters with lower transepithelial potential and higher epithelial conductance in the pyloric caeca than the anterior intestine. Both pyloric caeca and the anterior intestine secrete HCO(3)(-) at high rates in the absence of serosal HCO(3)(-)/CO(2), demonstrating that endogenous CO(2) is the principal source of HCO(3)(-) under resting control conditions. Apical, bafilomycin-sensitive, H(+) extrusion occurs in the anterior intestine and probably acts to control luminal osmotic pressure while enhancing apical anion exchange; both processes with implications for water absorption. Cytosolic carbonic anhydrase (CAc) activity facilitates CO(2) hydration to fuel apical anion exchange while membrane-associated, luminal CA activity probably facilitates the conversion of HCO(3)(-) to CO(2). The significance of membrane-bound, luminal CA may be in part to reduce HCO(3)(-) gradients across the apical membrane to further enhance anion exchange and thus Cl(-) absorption and to facilitate the substantial CaCO(3) precipitation occurring in the lumen of marine teleosts. In this way, membrane-bound, luminal CA thus promotes the absorption of osmolytes and reduction on luminal osmotic pressure, both of which will serve to enhance osmotic gradients to promote intestinal water absorption.

High rates of HCO3- secretion and Cl- absorption against adverse gradients in the marine teleost intestine: the involvement of an electrogenic anion exchanger and H+-pump metabolon?

Anion exchange contributes significantly to intestinal Cl(-) absorption in marine teleost fish and is thus vital for successful osmoregulation. This anion exchange process leads to high luminal HCO(3)(-) concentrations (up to approximately 100 mmol l(-1)) and high pH and results in the formation of CaCO(3) precipitates in the intestinal lumen. Recent advances in our understanding of the transport processes involved in intestinal anion exchange in marine teleost fish include the demonstration of a role for the H(+)-pump (V-ATPase) in apical H(+) extrusion and the presence of an electrogenic (nHCO(3)(-)/Cl(-)) exchange protein (SLC26a6). The H(+)-V-ATPase defends against cellular acidification, which might otherwise occur as a consequence of the high rates of base secretion. In addition, apical H(+) extrusion probably maintains lower HCO(3)(-) concentrations in the unstirred layer at the apical surface than in the bulk luminal fluids and thus facilitates continued anion exchange. Furthermore, H(+)-V-ATPase activity hyperpolarizes the apical membrane potential that provides the driving force for apical electrogenic nHCO(3)(-)/Cl(-) exchange, which appears to occur against both Cl(-) and HCO(3)(-) electrochemical gradients. We propose that a similar coupling between apical H(+) extrusion and nHCO(3)(-)/Cl(-) exchange accounts for Cl(-) uptake in freshwater fish and amphibians against very steep Cl(-) gradients.

Mechanisms of seawater acclimation in a primitive, anadromous fish, the green sturgeon

Relatively little is known about salinity acclimation in the primitive groups of fishes. To test whether physiological preparative changes occur and to investigate the mechanisms of salinity acclimation, anadromous green sturgeon, Acipenser medirostris (Chondrostei) of three different ages (100, 170, and 533 dph) were acclimated for 7 weeks to three different salinities (<3, 10, and 33 ppt). Gill, kidney, pyloric caeca, and spiral intestine tissues were assayed for Na(+), K(+)-ATPase activity; and gills were analyzed for mitochondria-rich cell (MRC) size, abundance, localization and Na(+), K(+)-ATPase content. Kidneys were analyzed for Na(+), K(+)-ATPase localization and the gastro-intestinal tract (GIT) was assessed for changes in ion and base content. Na(+), K(+)-ATPase activities increased in the gills and decreased in the kidneys with increasing salinity. Gill MRCs increased in size and decreased in relative abundance with fish size/age. Gill MRC Na(+), K(+)-ATPase content (e.g., ion-pumping capacity) was proportional to MRC size, indicating greater abilities to regulate ions with size/age. Developmental/ontogenetic changes were seen in the rapid increases in gill MRC size and lamellar length between 100 and 170 dph. Na(+), K(+)-ATPase activities increased fourfold in the pyloric caeca in 33 ppt, presumably due to increased salt and water absorption as indicated by GIT fluids, solids, and ion concentrations. In contrast to teleosts, a greater proportion of base (HCO(3) (-) and 2CO(3) (2-)) was found in intestinal precipitates than fluids. Green sturgeon osmo- and ionoregulate with similar mechanisms to more-derived teleosts, indicating the importance of these mechanisms during the evolution of fishes, although salinity acclimation may be more dependent on body size.

Evolutionary history of Na,K-ATPases and their osmoregulatory role

The Na/K pump, or Na,K-ATPase, is a key enzyme to the homeostasis of osmotic pressure, cell volume, and the maintenance of electrochemical gradients. Its alpha subunit, which holds most of its functions, belongs to a large family of ATPases known as P-type, and to the subfamily IIC, which also includes H,K-ATPases. In this study, we attempt to describe the evolutionary history of IIC ATPases by doing phylogenetic analysis with most of the currently available protein sequences (over 200), and pay special attention to the relationship between their diversity and their osmoregulatory role. We include proteins derived from many completed or ongoing genome projects, many of whose IIC ATPases have not been phylogenetically analyzed previously. We show that the most likely origin of IIC proteins is prokaryotic, and that many of them are present in non-metazoans, such as algae, protozoans or fungi. We also suggest that the pre-metazoan ancestor, represented by the choanoflagellate Monosiga brevicollis, whose genome has been sequenced, presented at least two IIC-type proteins. One of these proteins would have given rise to most current animal IIC ATPases, whereas the other apparently evolved into a lineage that, so far, has only been found in nematodes. We also propose that early deuterostomes presented a single IIC gene, from which all the extant diversity of vertebrate IIC proteins originated by gene and genome duplications.