Rectal ultrastructure in salt- and freshwater mosquito larvae in relation to physiological state

Ultrastructure of the larval rectum of the scorpionfly Panorpa liui (Mecoptera: Panorpidae)

The rectum is an important part of the alimentary canal responsible for ion and water reabsorption of insects. However, it has rarely been studied in the larvae of Panorpidae, the largest family in Mecoptera. Here, we investigated the ultrastructure of larval rectum of the scorpionfly Panorpa liui Hua, 1997 using light and transmission electron microscopy. The rectum comprises tracheal muscular layers, connective tissue, non-cellular basal lamina, junctional cells, rectal epithelium, cuticle with irregular outlines, and a central lumen. The rectal epithelium is infolded to form six longitudinal rectal folds, which are distinct from rectal pads or papillae. In each rectal fold, the apical and basal plasma membranes of epithelial cells are infolded and the lateral plasma membranes form septate and scalariform junctions. The well-developed rectal folds are postulated to be closely associated with reabsorption of ions and water in the larvae. The associations of rectal folds with larval behaviors are briefly discussed in Mecoptera.

Ammonia transport in the excretory system of mosquito larvae (Aedes aegypti): Rh protein expression and the transcriptome of the rectum

The role of the mosquito excretory organs (Malpighian tubules, MT and hindgut, HG) in ammonia transport as well as expression and function of the Rhesus (Rh protein) ammonia transporters within these organs was examined in Aedes aegypti larvae and adult females. Immunohistological examination revealed that the Rh proteins are co-localized with V-type H+-ATPase (VA) to the apical membranes of MT and HG epithelia of both larvae and adult females. Of the two Rh transporter genes present in A. aegypti, AeRh50-1 and AeRh50-2, we show using quantitative real-time PCR (qPCR) and an RNA in-situ hybridization (ISH) assay that AeRh50-1 is the predominant Rh protein expressed in the excretory organs of larvae and adult females. Further assessment of AeRh50-1 function in larvae and adults using RNAi (i.e. dsRNA-mediated knockdown) revealed significantly decreased [NH4+] (mmol l-1) levels in the secreted fluid of larval MT which does not affect overall NH4+ transport rates, as well as significantly decreased NH4+ flux rates across the HG (haemolymph to lumen) of adult females. We also used RNA sequencing to identify the expression of ion transporters and enzymes within the rectum of larvae, of which limited information currently exists for this important osmoregulatory organ. Of the ammonia transporters in A. aegypti, AeRh50-1 transcript is most abundant in the rectum thus validating our immunohistochemical and RNA ISH findings. In addition to enriched VA transcript (subunits A and d1) in the rectum, we also identified high Na+-K+-ATPase transcript (α subunit) expression which becomes significantly elevated in response to HEA, and we also found enriched carbonic anhydrase 9, inwardly rectifying K+ channel Kir2a, and Na+-coupled cation-chloride (Cl-) co-transporter CCC2 transcripts. Finally, the modulation in excretory organ function and/or Rh protein expression was examined in relation to high ammonia challenge, specifically high environmental ammonia (HEA) rearing of larvae. NH4+ flux measurements using the scanning-ion selective electrode (SIET) technique revealed no significant differences in NH4+ transport across organs comprising the alimentary canal of larvae reared in HEA vs freshwater. Further, significantly increased VA activity, but not NKA, was observed in the MT of HEA-reared larvae. Relatively high Rh protein immunostaining persists within the hindgut epithelium, as well as the ovary, of females at 24-48 h post blood meal corresponding with previously demonstrated peak levels of ammonia formation. These data provide new insight into the role of the excretory organs in ammonia transport physiology and the contribution of Rh proteins in mediating ammonia movement across the epithelia of the MT and HG, and the first comprehensive examination of ion transporter and channel expression in the mosquito rectum.

Proton-driven sodium secretion in a saline water animal

Aquatic animals residing in saline habitats either allow extracellular sodium concentration to conform to environmental values or regulate sodium to lower levels. The latter strategy requires an energy-driven process to move sodium against a large concentration gradient to eliminate excess sodium that diffuses into the animal. Previous studies of invertebrate and vertebrate species indicate a sodium pump, Na+/K+ ATPase, powers sodium secretion. We provide the first functional evidence of a saline-water animal, Aedes taeniorhynchus mosquito larva, utilizing a proton pump to power this process. Vacuolar-type H+ ATPase (VHA) protein is highly expressed on the apical membrane of the posterior rectal cells, and in situ sodium flux across this epithelium increases significantly in larvae held in higher salinity and is sensitive to Bafilomycin A1, an inhibitor of VHA. We also report the first evidence of splice variants of the sodium/proton exchanger, NHE3, with both high and low molecular weight variants highly expressed on the apical membrane of the posterior rectal cells. Evidence of NHE3 function was indicated with in situ sodium transport significantly inhibited by a NHE3 antagonist, S3226. We propose that the outward proton pumping by VHA establishes a favourable electromotive gradient to drive sodium secretion via NHE3 thus producing a hyperosmotic, sodium-rich urine. This H+- driven Na+ secretion process is the primary mechanism of ion regulation in salt-tolerant culicine mosquito species and was first investigated over 80 years ago.

Physiological responses of freshwater insects to salinity: molecular-, cellular- and organ-level studies

Salinization of freshwater is occurring throughout the world, affecting freshwater biota that inhabit rivers, streams, ponds, marshes and lakes. There are many freshwater insects, and these animals are important for ecosystem health. These insects have evolved physiological mechanisms to maintain their internal salt and water balance based on a freshwater environment that has comparatively little salt. In these habitats, insects must counter the loss of salts and dilution of their internal body fluids by sequestering salts and excreting water. Most of these insects can tolerate salinization of their habitats to a certain level; however, when exposed to salinization they often exhibit markers of stress and impaired development. An understanding of the physiological mechanisms for controlling salt and water balance in freshwater insects, and how these are affected by salinization, is needed to predict the consequences of salinization for freshwater ecosystems. Recent research in this area has addressed the whole-organism response, but the purpose of this Review is to summarize the effects of salinization on the osmoregulatory physiology of freshwater insects at the molecular to organ level. Research of this type is limited, and pursuing such lines of inquiry will improve our understanding of the effects of salinization on freshwater insects and the ecosystems they inhabit.

Ultrastructure of the rectum of the soil-spraying larva in Bittacus cirratus (Mecoptera: Bittacidae)

The larvae of Bittacidae have an interesting behavior of spraying soil particles on their body surface through the anus. However, the hindgut specialization associated with this behavior has rarely been studied hitherto. Here, we investigated the fine structure of the larval rectum in the hangingfly Bittacus cirratus Tjeder using light and transmission electron microscopy. The results show that the larvae of B. cirratus have a tubular rectum without rectal pads or papillae. The rectum consists of well-developed visceral muscle layers, a non-cellular basal lamina, a single-layer epithelium with a cuticular intima, and a central lumen. The folded rectal epithelium consists of two types of flattened epithelial cells: electron-dense type I cells and electron-lucent type II cells. The apical and basal plasma membranes are infolded and are associated with mitochondria in the epithelial cells. The epithelial cells are held by septate and scalariform junctions. The lateral cell membranes are combined with mitochondria among type I cells and generate mitochondria-scalariform junction complexes. These features suggest that the epithelial cells are active in water and ion reabsorption. We conclude that the absence of rectal pads or papillae and the presence of developed circular muscles are likely morphological adaptations of these larvae to the soil-spraying behavior.

The plasticity of extracellular fluid homeostasis in insects

In chemistry, the ratio of all dissolved solutes to the solution's volume yields the osmotic concentration. The present Review uses this chemical perspective to examine how insects deal with challenges to extracellular fluid (ECF) volume, solute content and osmotic concentration (pressure). Solute/volume plots of the ECF (hemolymph) reveal that insects tolerate large changes in all three of these ECF variables. Challenges beyond those tolerances may be 'corrected' or 'compensated'. While a correction simply reverses the challenge, compensation accommodates the challenge with changes in the other two variables. Most insects osmoregulate by keeping ECF volume and osmotic concentration within a wide range of tolerance. Other insects osmoconform, allowing the ECF osmotic concentration to match the ambient osmotic concentration. Aphids are unique in handling solute and volume loads largely outside the ECF, in the lumen of the gut. This strategy may be related to the apparent absence of Malpighian tubules in aphids. Other insects can suspend ECF homeostasis altogether in order to survive extreme temperatures. Thus, ECF homeostasis in insects is highly dynamic and plastic, which may partly explain why insects remain the most successful class of animals in terms of both species number and biomass.

Histological and ultrastructural features of the rectum in Poecilimon cervus Karabağ, 1950 (Orthoptera: Tettigoniidae)

The morphology and ultrastructure of the rectum in Poecilimon cervus Karabağ, 1950 (Orthoptera, Tettigoniidae) were analyzed by light microscope, scanning (SEM) and transmission electron microscopes (TEM). The rectum is the final part of the digestive tract that plays an important role in water reabsorption in insects and so provides osmoregulation. In the transverse sections, six rectal pads and columnar epithelium can be distinguished. The cuticular intima lines the lumen at the apical side of the epithelium. In the cytoplasm, there are numerous mitochondria, some endocytic vesicles, secreting vesicles whose sizes differ according to the area in the cell, and a nucleus with globular in shape. With this study, we aimed to demonstrate the ultrastructure of the rectum of P. cervus and differences or similarities of with other species.

Tissue-specific ionomotive enzyme activity and K+ reabsorption reveal the rectum as an important ionoregulatory organ in larval Chironomus riparius exposed to varying salinity

A role for the rectum in the ionoregulatory homeostasis of larval Chironomus riparius was revealed by rearing animals in different saline environments and examining: (1) the spatial distribution and activity of keystone ionomotive enzymes Na(+)-K(+)-ATPase (NKA) and V-type H(+)-ATPase (VA) in the alimentary canal, and (2) rectal K(+) transport with the scanning ion-selective electrode technique (SIET). NKA and VA activity were measured in four distinct regions of the alimentary canal as follows: the combined foregut and anterior midgut, the posterior midgut, the Malpighian tubules and the hindgut. Both enzymes exhibited 10-20 times greater activity in the hindgut relative to all other areas. When larvae were reared in either ion-poor water (IPW) or freshwater (FW), no significant difference in hindgut enzyme activity was observed. However, in larvae reared in brackish water (BW), NKA and VA activity in the hindgut significantly decreased. Immunolocalization of NKA and VA in the hindgut revealed that the bulk of protein was located in the rectum. Therefore, K(+) transport across the rectum was examined using SIET. Measurement of K(+) flux along the rectum revealed a net K(+) reabsorption that was reduced fourfold in BW-reared larvae versus larvae reared in FW or IPW. Inhibition of NKA with ouabain, VA with bafilomycin and K(+) channels with charybdotoxin diminished rectal K(+) reabsorption in FW- and IPW-reared larvae, but not BW-reared larvae. Data suggest that the rectum of C. riparius plays an important role in allowing these larvae to cope with dilute as well as salinated environmental conditions.

On the evolution of saline tolerance in the larvae of mosquitoes in the genus Ochlerotatus

We examined physiological and morphological parameters associated with saline tolerance in the larvae of 11 species of mosquito in the genus Ochlerotatus that were collected from the wild in a variety of sites around North America. Saline tolerance was assayed, and all of the species were osmoregulators. Six of the species examined were found to be physiologically restricted to freshwater habitats, while the other five could successfully osmoregulate in both freshwater and saline water, including seawater. All larvae that were obligate freshwater forms had only one rectal segment, while all of the euryhaline osmoregulators had two. We were interested in the evolutionary pathway by which saline tolerance arose in this lineage. DNA sequence data were obtained by polymerase chain reaction amplification and sequencing of the D2 region of the 28s rDNA gene in all of the freshwater and saline-tolerant Ochlerotatus species we studied. When the morphological and physiological characters were mapped on the resultant cladogram, they revealed a complex pattern, with freshwater and saline-water forms being adjacent and interspersed through the tree. The data also demonstrate that saline tolerance has been gained and then lost at least once in this lineage. Two possible evolutionary scenarios are presented, but the one we favor is that saline tolerance arose one time in this lineage and repeated reversions to the freshwater condition have occurred.

Physiological and pharmacological characterizations of the larval Anopheles albimanus rectum support a change in protein distribution and/or function in varying salinities

Ion regulation is a biological process crucial to the survival of mosquito larvae and a major organ responsible for this regulation is the rectum. The recta of anopheline larvae are distinct from other subfamilies of mosquitoes in several ways, yet have not yet been characterized extensively. Here we characterize the two major cell types of the anopheline rectum, DAR and non-DAR cells, using histological, physiological, and pharmacological analyses. Proton flux was measured at the basal membrane of 2%- and 50%-artificial sea water-reared An. albimanus larvae using self-referencing ion-selective microelectrodes, and the two cell types were found to differ in basal membrane proton flux. Additionally, differences in the response of that flux to pharmacological inhibitors in larvae reared in 2% versus 50% ASW indicate changes in protein function between the two rearing conditions. Finally, histological analyses suggest that the non-DAR cells are structurally suited for mediating ion transport. These data support a model of rectal ion regulation in which the non-DAR cells have a resorptive function in freshwater-reared larvae and a secretive function in saline water-reared larvae. In this way, anopheline larvae may adapt to varying salinities.

Larval anopheline mosquito recta exhibit a dramatic change in localization patterns of ion transport proteins in response to shifting salinity: a comparison between anopheline and culicine larvae

Mosquito larvae live in dynamic aqueous environments, which can fluctuate drastically in salinity due to environmental events such as rainfall and evaporation. Larval survival depends upon the ability to regulate hemolymph osmolarity by absorbing and excreting ions. A major organ involved in ion regulation is the rectum, the last region for modification of the primary urine before excretion. The ultrastructure and function of culicine larval recta have been studied extensively; however, very little published data exist on the recta of anopheline larvae. To gain insight into the structure and functions of this organ in anopheline species, we used immunohistochemistry to compare the localization of three proteins [carbonic anhydrase (CA9), Na+/K+ P-ATPase and H+ V-ATPase] in the recta of anopheline larvae reared in freshwater and saline water with the localization of the same proteins in culicine larvae reared under similar conditions. Based on the following key points, we concluded that anophelines differ from culicines in larval rectal structure and in regulation of protein expression: (1) despite the fact that obligate freshwater and saline-tolerant culicines have structurally distinct recta, all anophelines examined (regardless of saline-tolerance) have a structurally similar rectum consisting of distinct DAR (dorsal anterior rectal) cells and non-DAR cells; (2) anopheline larvae undergo a dramatic shift in rectal Na+/K+-ATPase localization when reared in freshwater vs saline water. This shift is not seen in any culicine larvae examined. Additionally, we use these immunohistochemical analyses to suggest possible functions for the DAR and non-DAR cells of anopheline larvae in freshwater and saline conditions.

Cloning and characterization of AgCA9, a novel α-carbonic anhydrase from Anopheles gambiae Giles sensu stricto (Diptera:Culicidae) larvae

Mosquito larvae generate a luminal pH as high as 10.5 in the anterior region of their midgut. The mechanisms responsible for the generation and maintenance of this alkaline pH are largely unknown, but there is evidence suggesting a role for the enzyme carbonic anhydrase (CA). CA has been cloned from the alimentary canal epithelium of Anopheles gambiae larvae and can generate bicarbonate, which is implicated as a buffer for the larval lumen. The question remains as to how the bicarbonate is transported from the cells into the lumen. We hypothesize the presence of a CA within the lumen itself to generate bicarbonate from CO2 produced by the metabolically active alimentary canal cells. Here, we report the cloning and characterization of a novel cytoplasmic-type α-CA from the larval An. gambiae alimentary canal. Antibody immunolocalization reveals a unique protein distribution pattern that includes the ectoperitrophic fluid,`transitional region' of the alimentary canal, Malpighian tubules and a subset of cells in the dorsal anterior region of the rectum. Localization of this CA within the lumen of the alimentary canal may be a key to larval pH regulation,while detection within the rectum reveals a novel subset of cells in An. gambiae not described to date. Phylogenetic analysis of members of theα-CA family from the Homo sapiens, Drosophila melanogaster, Aedes aegypti and An. gambiae genomes shows a clustering of the novel CA with Homo sapiens CAs but not with other insect CAs. Finally, a universal system for naming newly cloned An. gambiae CAs is suggested.

Light and electron microscopic study of the hindgut of the ant (Formica nigricans, hymenoptera): II. Structure of the rectum

The rectum of the ant Formica nigricans is composed of six ovoid rectal papillae inserted into a rectal pouch. The wall of the rectal pouch is made up of a flat epithelium of simple rectal cells lined by cuticle, and surrounded by a circular muscle layer. Each rectal papilla is comprised by a simple columnar epithelium of principal cells facing the lumen, and a simple cuboid epithelium of secondary cells towards the hemolymph; a group of 20-25 slender junctional cells lies laterally between both epithelia enclosing an intrapapillar sinus. The muscle layer of the rectal wall also surrounds the base of the papillae. Principal cells do not exhibit extensive infoldings at the apical and basal plasma membranes. Lateral membranes, in contrast, develop highly folded mitochondria-scalariform junction complexes enclosing very narrow intercellular canaliculi between adjacent cells. These canaliculi open to wider intercellular sinuses that ultimately drain into the intrapapillar sinus at the sites of entry of tracheal cells. The lateral plasma membranes do not link to the apical or basal plasma membrane, thus originating a syncytium throughout the principal cells. The apical plasma membrane of secondary cells shows invaginations in relation with an apical tubulovacuolar system, bearing portasomes to the cytoplasmic side of the membrane. Secondary cells unite by convoluted septate junctions, and basolateral infoldings are also developed. These ultrastructural traits, some of them different from those found in other insects, are discussed and examined in relation to their role in water and solute absorption. A route for rectal transport in F. nigricans is proposed.

Cellular mechanism of HCO-3 and Cl- transport in insect salt gland

Active HCO-3 secretion in the anterior rectal salt gland of the mosquito larva, Aedes dorsalis, is mediated by a 1:1 Cl-/HCO-3 exchanger. The cellular mechanisms of HCO-3 and Cl- transport are examined using ion- and voltage-sensitive microelectrodes in conjunction with a microperfused preparation which allowed rapid saline changes. Addition of DIDS or acetazolamide to, or removal of CO2 and HCO-3 from, the serosal bath caused large (20 to 50 mV) hyperpolarizations of apical membrane potential (Va) and had little effect on basolateral potential (Vbl). Changes in luminal Cl- concentration altered Va in a rapid, linear manner with a slope of 42.2 mV/decalog a1Cl-. Intracellular Cl- activity was 23.5 mM and was approximately 10 mM lower than that predicted for a passive distribution across the apical membrane. Changes in serosal Cl- concentration had no effect on Vbl, indicating an electrically silent basolateral Cl- exit step. Intracellular pH in anterior rectal cells was 7.67 and the calculated acHCO-3 was 14.4 mM. These results show that under control conditions HCO-3 enters the anterior rectal cell by an active mechanism against an electrochemical gradient of 77.1 mV and exits the cell at the apical membrane down a favorable electrochemical gradient of 27.6 mV. A tentative cellular model is proposed in which Cl- enters the apical membrane of the anterior rectal cells by passive, electrodiffusive movement through a Cl- -selective channel, and HCO-3 exits the cell by an active or passive electrogenic transport mechanism. The electrically silent nature of basolateral Cl- exit and HCO-3 entry, and the effects of serosal addition of the Cl-/HCO-3 exchange inhibitor, DIDS, on JCO2net and transepithelial potential (Vte) suggest strongly that the basolateral membrane is the site of a direct coupling between Cl- and HCO-3 movements.

Ultrastructure of osmoregulatory organs in larvae of the brackish-water mosquito, Culiseta inornata (Williston)

The ultrastructure of the Malpighian tubules, ileum, rectum, anal canal, and anal papillae of larvae of the mosquito Culiseta inornata was examined. The Malpighian tubules, rectum, and anal papillae have many of the ultrastructural features characteristic of ion transport tissues, i.e., elaboration of the basal and apical membranes and a close association of these membranes with mitochondria. The Malpighian tubules possess two cell types, primary and stellate. The larval rectum of C. inornata is composed of a single segment containing a homogenous population of cells. In this respect, the larval rectum of C. inornata is distinct from that of saline-water species of Aedes. The cells in the larval rectum of C. inornata, however, closely resemble those of one cell type, the anterior rectal cells, of the saline-water mosquito Aedes campestris with regard to cell and nuclear size, the percentage of the cell occupied by apical folds, and mitochondrial density and distribution. No similarities can be found between the rectum of C. inornata and the posterior segment of the saline-water Aedes, which functions as a salt gland. On this basis, we have postulated that the rectum of C. inornata does not function as a site of hyperosmotic fluid secretion. The ultrastructure of the anal papillae of C. inornata is consistent with a role in ion transport. The significance of these findings to comparative aspects of osmoregulatory strategies in mosquito larvae is discussed.

An osmoregulatory syncytium and associated cells in a freshwater mosquito

In embryological terms the anal papillae are the product of eversion of the hindgut tissues. The rectum and the anal papillae have the same origin and have a marked structural similarity. The insect hindgut is very labile being able to produce salt transporting or 'chloride cells' from any of the tissues of which it is composed. The hindgut consists of four distinct regions: the ileum and part of the anal canal have a mechanical function, the rectum and the posterior anal canal contain transporting cells. Two new types, 'interstitial' and 'tertiary' are reported in the rectum. The structure of the anal papillae changes with increased salinity. Changes in the plasma membranes alter the surface area for transport. Changes in the number of mitochondria are not accompanied by changes in oxygen consumption. If mitochondria are the site of oxidative metabolism then their number does not control the level of oxygen consumption. In Aedes aegypti the papillary epithelium appears to be a syncytium. Across the lumen of the papillae there are cellular sheets supporting the tracheoles. At the base of the papillae there is a cellular transition zone; circular muscles in this region may be used to occlude the papillae. The control of salt transport may be hormonal.

The ultrastructure of the larval malpighian tubules of a saline-water mosquito

The larval Malpighian tubules of the saline-water mosquito Aedes taeniorhynchus were examined using light and electron microscopy. The tubules contain two cell types; primary cells and stellate cells. Primary cells are characterized by their size (70 microns x 70 microns x 10 microns) and an abundance of intracellular membrane-bound crystals. Two types of microvilli are found on the luminal surface of the primary cells: (1) small microvilli containing core microfilaments and extensions of endoplasmic reticulum, and (2) larger microvilli (approximately equal to 3 microns in length) which in addition to the above components contain a mitochondrion along their entire length. Both microvillar types have abundant knobs lining the cytoplasmic surface of the microvillar membrane. These knobs, which are often found in insect ion transporting tissues, have been termed 'portasomes' by Harvey (1980). The possible role of these structures in ion transport and mitochondrial positioning is discussed. The stellate cells are much smaller than the primary cells, and lack intracellular crystals. Their microvilli are smaller as well (approximately equal to 0 x 6 microns in length) and contain no endoplasmic reticulum, mitochondria or knobs. The cells types found in the saline-water mosquito larva, Aedes taeniorhynchus, are identical to those found in Aedes aegypti, indicating that the unique capacity of saline-water mosquito larvae to transport Mg2+ and SO42 is not associated with the presence of an additional cell type.