The alloherpesviral counterparts of interleukin 10 in European eel and common carp

Viral interleukin 10 (IL-10) like open reading frames have been identified in several pox- and herpesviruses, including the fish herpesviruses Anguillid herpesvirus 1 (AngHV-1) and Cyprinid herpesvirus 3 (CyHV-3). European eel (Anguilla anguilla) IL-10 was sequenced, in order to compare European eel and common carp (Cyprinus carpio) IL-10 with their alloherpesviral counterparts. Homology between the virus and host IL-10 amino acid sequences is low, which is confirmed by phylogenetic analysis. However, the three dimensional structures of the fish and alloherpesviral IL-10 proteins as predicted by modeling are highly similar to human IL-10. Closely related AngHV-1 and CyHV-3 are expected to have obtained their viral IL-10 genes independently in the course of coexistence with their respective hosts. The presence and structural conservation of these alloherpesviral IL-10 genes suggest that they might play an important role in the evolution of pathogenesis.

Localization of diversified relaxin gene transcripts in the brain of eels

Relaxin 3 (RLN3) is a newly-discovered member of the insulin superfamily. We isolated three RLN3-like cDNAs from the brain of the Japanese eel (Anguilla japonica). The deduced amino acid sequences of the RLN3-like cDNAs contained the two-chain structure common to relaxin including a RXXXRXXI/V motif in the B-chain. Phylogenetic analysis assigned the two prepropeptides into teleost/mammalian RLN3 group, which are a pair of duplicates generated by the teleost-specific third-round whole genome duplication, and the other one into teleost RLN group. Therefore, they have been named eel rln3a, rln3b and rln. rln3a transcripts were abundant in the middle-posterior region of the brain and detected at lower levels in the gills, head kidney and kidney. rln3b transcripts were also detected in the middle-posterior region of the brain, but the expression levels were lower than those of rln3a. Low levels of rln transcripts were detected in all brain areas, pituitary, digestive tract and gonad. Quantitative PCR analysis did not detect differences in expression of any rln3 or rln gene between freshwater- and seawater-acclimated eels. In situ hybridization showed that rln3a was expressed in neurons of the lateral lemniscus of the midbrain and of the griseum centrale (GC) of the hindbrain, while low amounts of rln transcripts were found in neurons of the periventricular nucleus of the posterior tuberculum of the diencephalon and the GC. These results suggest that the multiple RLN3-like peptides may play regulatory roles in the brain of euryhaline fish.

Expression and localization of aquaporin 1b during oocyte development in the Japanese eel (Anguilla japonica)

To elucidate the molecular mechanisms underling hydration during oocyte maturation, we characterized the structure of Japanese eel (Anguilla japonica) novel-water selective aquaporin 1 (AQP1b) that thought to be involved in oocyte hydration. The aqp1b cDNA encodes a 263 amino acid protein that includes the six potential transmembrane domains and two Asn-Pro-Ala motifs. Reverse transcription-polymerase chain reaction showed transcription of Japanese eel aqp1b in ovary and testis but not in the other tissues. In situ hybridization studies with the eel aqp1b cRNA probe revealed intense eel aqp1b signal in the oocytes at the perinucleolus stage and the signals became faint during the process of oocyte development. Light microscopic immunocytochemical analysis of ovary revealed that the Japanese eel AQP1b was expressed in the cytoplasm around the yolk globules which were located in the peripheral region of oocytes during the primary yolk globule stage; thereafter, the immunoreactivity was observed throughout the cytoplasm of oocyte as vitellogenesis progressed. The immunoreactivity became localized around the large membrane-limited yolk masses which were formed by the fusion of yolk globules during the oocyte maturation phase. These results together indicate that AQP1b, which is synthesized in the oocyte during the process of oocyte growth, is essential for mediating water uptake into eel oocytes.

Oceanic spawning ecology of freshwater eels in the western North Pacific

The natural reproductive ecology of freshwater eels remained a mystery even after some of their offshore spawning areas were discovered approximately 100 years ago. In this study, we investigate the spawning ecology of freshwater eels for the first time using collections of eggs, larvae and spawning-condition adults of two species in their shared spawning area in the Pacific. Ovaries of female Japanese eel and giant mottled eel adults were polycyclic, suggesting that freshwater eels can spawn more than once during a spawning season. The first collection of Japanese eel eggs near the West Mariana Ridge where adults and newly hatched larvae were also caught shows that spawning occurs during new moon periods throughout the spawning season. The depths where adults and newly hatched larvae were captured indicate that spawning occurs in shallower layers of 150-200 m and not at great depths. This type of spawning may reduce predation and facilitate reproductive success.

Characterization of androgen receptor structure and nucleocytoplasmic shuttling of the rice field eel

Androgen receptor (AR) plays a critical role in prostate cancer and male sexual differentiation. We have identified AR from a primitive vertebrate with a sex reversal characteristic, the rice field eel. AR of this species (eAR) is distinct from human AR, especially in the ligand binding domain (LBD), and its expression in gonads shows an increasing tendency during gonadal transformation from ovary via ovotestis to testis. eAR has a restricted androgen-dependent transactivation function after a nuclear translocation upon dihydrotestosterone exposure. A functional nuclear localization signal was further identified in the DNA binding domain and hinge region. Although nuclear export is CRM1-independent, eAR has a novel nuclear export signal, which is negatively charged, indicating that a nuclear export pathway may be mediated by electrostatic interaction. Further, our studies have identified critical sequences for ligand binding in the C terminus. A structure of three α-helices in the LBD has been conserved from eels to humans during vertebrate evolution, despite a distinct amino acid sequence. Mutation analysis confirmed that the LBD is essential for dihydrotestosterone-induced nuclear import of eAR and following transactivation function in the nucleus. In addition, eAR interacts with both Sox9a1 and Sox9a2, and their interaction regulates transactivation of eAR. Our data suggest that the primitive species conserves and especially acquires key novel domains, the nuclear export signal and LBD, for the eAR function in spite of a rapid sequence evolution.

Molecular phylogenetics of moray eels (Muraenidae) demonstrates multiple origins of a shell-crushing jaw (Gymnomuraena, Echidna) and multiple colonizations of the Atlantic Ocean

Moray eels (Muraenidae) are apex predators on coral reefs around the world, but they are not well studied because their cryptic habitats and occasionally aggressive behaviors make them difficult to collect. We provide a molecular phylogeny of moray eels including 44 species representing two subfamilies, eight genera, and all tropical ocean basins. Phylogenetic relationships among these taxa are estimated from portions of mitochondrial loci cytochrome b (632 bp) and cytochrome oxidase subunit 1 (596 bp), and portions of the nuclear loci RAG-1 (421 bp) and RAG-2 (754 bp). We test four sets of contrasting phylogenetic hypotheses using Bayes Factors, Shimodaira-Hasegawa tests, and Templeton tests. First, our results support the subfamily-level taxonomic distinction between true morays (Muraeninae) and snakemorays (Uropterygiinae), statistically rejecting hypotheses of non-monophyly for each subfamily. Second, we reject a monophyletic grouping of the genera Gymnomuraena and Echidna, which share a durophagous (shell-crushing) cranial morphology and dentition, indicating that the durophagous characters are not homologous. Third, we demonstrate that durophagous feeding habits and associated morphological characters have evolved in parallel in an ancestor of Gymnomuraena and at least three additional times within the genus Echidna. Finally, the tree topology indicates multiple invasions of the Atlantic from the Indo-Pacific, one of these occurring immediately prior to formation of the Isthmus of Panama approximately 2.8 MYA (million years ago) and one or two others occurring in the early to mid Miocene. Cladogenesis occurring within the Atlantic during the mid Miocene and Pliocene also contributed to moray species diversity. These data include a pair of sister species separated by the Isthmus of Panama, allowing a time-calibrated tree with an estimated crown age for Muraenidae at between 41 and 60 MYA, consistent with fossil evidence. Most lineage accumulation within morays occurred from the late Oligocene (24-27 MYA) through the Miocene (5-23 MYA) to the late Pliocene (∼ 2.5 MYA).

Production of recombinant Japanese eel gonadotropins by baculovirus in silkworm larvae

Recombinant follicle-stimulating hormone (reFSH) and luteinizing hormone (reLH) of the Japanese eel Anguilla japonica were produced by baculovirus in silkworm Bombyx mori larvae. cDNAs encoding Japanese eel gonadotropin subunits (i.e., FSH beta, LH beta, and common alpha) were introduced into the baculovirus, which was infected into silkworm larvae after propagation of the recombinant virus in B. mori culture cells. A 100ml solution of pooled hemolymph from silkworm larvae containing reFSH or reLH were obtained from approximately 250 infected larvae. Ten milliliters of hemolymph were applied to Ni-affinity choromatography, and 5.6 and 3.5mg of partially purified reFSH and reLH were obtained, respectively. Using Western blot analysis concentrations of reFSH and reLH in the original hemolymph was estimated to be 2.2 and 1.1mg/ml, respectively. Biological activities of reFSH and reLH were assessed in vitro and in vivo. Purified reFSH and reLH induced eel oocyte maturation in vitro, and administration of hemolymph containing reFSH or reLH induced spermatogenesis in vivo in sexually immature Japanese eel. The present study indicates that a baculovirus-silkworm system could produce large amounts of biologically active recombinant fish gonadotropins for use in investigations in reproductive endocrinology and/or aquaculture of fish.

Swimming suppresses hepatic vitellogenesis in European female silver eels as shown by expression of the estrogen receptor 1, vitellogenin1 and vitellogenin2 in the liver

BACKGROUND

When European silver eels (Anguilla anguilla) venture into the Atlantic Ocean for their 6,000 km semelparous spawning run to the Sargasso Sea, they are still in a prepubertal stage. Further sexual development appears to be blocked by dopaminergic inhibition of hypothalamus and pituitary activity. Recently, we found that swimming for several weeks in freshwater stimulated the incorporation of fat droplets in the oocytes. So, it was hypothesized that long term swimming in seawater would release the inhibition further and would also stimulate the production of vitellogenin by the liver.

METHODS

For this study a swim-flume was constructed to allow simulated migration of migratory female silver eels for 3 months (1,420 km) in natural seawater at 20 degrees C. Primers were designed for polymerase chain reactions to measure the mRNA expression of estrogen receptor 1 (esr1), vitellogenin1 (vtg1) and vitellogenin2 (vtg2) genes in the liver of European female silver eels.

RESULTS

In comparison to resting eels, swimming eels showed a diminished expression of esr1, vtg1 and vtg2 in the liver. They also had lower plasma calcium (Ca; indicative of vitellogenin) levels in their blood. This showed that vitellogenesis is more strongly suppressed in swimming than in resting eels. However, when eels were subsequently stimulated by 3 weekly carp pituitary extract injections, the expression of the same genes and plasma levels of Ca strongly increased in both groups to similar levels, thus equalizing the initial differences between resting and swimming.

CONCLUSIONS

It is concluded that vitellogenesis remains suppressed during resting and even more during swimming. The fact that swimming stimulates fat deposition in the oocytes but suppresses vitellogenesis indicates that these events are separated in nature and occur sequentially. Swimming-suppressed vitellogenesis may imply that in nature eels undergo vitellogenesis and final maturation near or at the spawning grounds.

Trypsin is a multifunctional factor in spermatogenesis

Trypsin is well known as a pancreatic enzyme that is typically secreted into the intestine to digest proteins. We show in our current study, however, that trypsin is also a key factor in the control of spermatogenesis. A progestin in teleost fish, 17alpha, 20beta-dihydroxy-4-pregnen-3-one (DHP), is an essential component of the spermatogenesis pathway, particularly during the initiation of the first meiotic division. In the course of our investigations into the mechanisms underlying progestin-stimulated spermatogenesis, we identified that eel trypsinogen is upregulated in eel testis by DHP treatment. Trypsinogen is expressed in the Sertoli cells surrounding spermatogonia and in the membranes of spermatids and spermatozoa. Using an in vitro eel testicular culture system, we further analyzed the roles of trypsin in spermatogenesis. The inhibition of trypsin using specific antibodies or serine protease inhibitors was found to compromise DHP-induced spermatogenesis. A low dose of trypsin induces DNA synthesis and the expression of Spo11, a molecular marker of meiosis, in germ cells. By comparison, a higher dose of trypsin partially induced spermiogenesis. Furthermore, trypsin was detectable in the membranes of the spermatozoa and found to be associated with fertilization in fish. Our results thus demonstrate that trypsin and/or a trypsin-like protease is an essential and multifunctional factor in spermatogenesis.

Homologue of mammalian apolipoprotein A-II in non-mammalian vertebrates

Although apolipoprotein with molecular weight 14 kDa (apo-14 kDa) is associated with fish plasma high-density lipoproteins (HDLs), it remains to be determined whether apo-14 kDa is the homologue of mammalian apoA-II. We have obtained the full cDNA sequences that encode Japanese eel and rainbow trout apo-14 kDa. Homologues of Japanese eel apo-14 kDa sequence could be found in 14 fish species deposited in the DDBJ/EMBL/GenBank or TGI database. Fish apo-14 kDa lacks propeptide and contains more internal repeats than mammalian apoA-II. Nevertheless, phylogenetic analysis allowed fish apo-14 kDa to be the homologue of mammalian apoA-II. In addition, in silico cloning of the TGI, Ensembl, or NCBI database revealed apoA-IIs in dog, chicken, green anole lizard, and African clawed frog whose sequences had not so far been available, suggesting both apoA-I and apoA-II as fundamental constituents of vertebrate HDLs.

Two distinct dopamine D2 receptor genes in the European eel: molecular characterization, tissue-specific transcription, and regulation by sex steroids

Two full-length cDNA encoding putative dopamine D2-like receptors were cloned from the brain of female European eel. The deduced protein sequences, termed D2A- and D2B-R, exhibit closer phylogenetic relationships to vertebrate D2 receptors compared with D3 and D4 or D1 receptors. The two protein sequences share 100% identity within the transmembrane domains containing the highly conserved amino acids involved in dopamine binding. Accordingly, an apparent single population of sites on eel brain membranes bound [(3)H]spiperone, a D2-R-specific antagonist, with a K(d) of 0.2 +/- 0.04 nM. However, D2A- and D2B-R significantly differ within the amino terminus and the third intracellular loop. As analyzed by quantitative PCR and in situ hybridization, both receptor transcripts were found, with different relative abundance, in the majority of brain areas and in the pituitary, whereas in the retina, olfactory epithelium, spinal cord, and adipose tissue, only D2A-R gene was expressed. Because sex steroid hormones recently have been shown to regulate eel brain dopamine systems, we analyzed the effect of steroids on the amount of D2-R transcripts by quantitative PCR and in situ hybridization. In eels treated with testosterone, the gene expression of the D2B-R, but not D2A-R, was increased in a region-dependent manner. The effect of testosterone on D2B-R transcript levels was mimicked by dihydrotestosterone, a nonaromatizable androgen, whereas estradiol had no stimulatory action, evidencing an androgen receptor-dependent mechanism. Although functionality of the two receptors awaits determination of D2-R proteins, we hypothesize that differences in the tissue expression pattern and hormonal regulation of eel D2A- and D2B-R gene expression could represent selective forces that have contributed to the conservation of the duplicated D2-R.

LPS induces gene expression of interleukin-1beta in conger eel (Conger myriaster) macrophages: first cytokine sequence within Anguilliformes

Our aim was to comprehensively analyze the immune response in Anguilliformes macrophages and to survey cytokine genes expressed from them. We therefore used suppression subtractive hybridization (SSH) to randomly clone molecules that are specifically expressed in conger eel (Conger myriaster) macrophages when cells are stimulated by LPS. As a result, we succeeded in identifying a conger eel IL-1beta. This is the first report on cytokines in Anguilliformes, which is the most ancient order in living teleosts.

Reconstruction of a probable ancestral form of conger eel galectins revealed their rapid adaptive evolution process for specific carbohydrate recognition

Recently, many cases of rapid adaptive evolution, which is characterized by the higher substitution rates of nonsynonymous substitutions to synonymous ones, have been identified in the various genes of venomous and biodefense proteins, including the conger eel galectins, congerins I and II (ConI and ConII). To understand the evolutionary process of congerins, we prepared a probable ancestral form, Con-anc, corresponding to the putative amino acid sequence at the divergence of ConI and ConII in phylogenetic tree with 76% and 61% sequence identities to the current proteins, respectively. Con-anc and ConII had comparable thermostability and similar carbohydrate specificities in general, whereas ConI was more thermostable and showed different carbohydrate specificities. Con-anc showed decreased specificity to oligosaccharides with alpha 2,3-sialyl galactose moieties. These suggest that ConI and ConII have evolved via accelerated evolution under significant selective pressure to increase the thermostability and to acquire the activity to bind to alpha2,3-sialyl galactose present in pathogenic bacteria, respectively. Furthermore, comparative mutagenesis analyses of Con-anc and congerins revealed the structural basis for specific recognition of ConII to alpha2,3-sialyl galactose moiety, and strong binding ability of ConI to oligosaccharides including lacto-N-biosyl (Galbeta1-3GlcNAc) or lacto-N-neobiosyl (Galbeta1-4GlcNAc) residues, respectively. Thus, protein engineering using a probable ancestral form presented here is a powerful approach not only to determine the evolutionary process but also to investigate the structure-activity relationships of proteins.

Development of real-time RT-PCR assays for eel gonadotropins and their application to the comparison of in vivo and in vitro effects of sex steroids

Gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), are key factors in the brain-pituitary-gonad axis and understanding their regulation remains essential for future management of eel reproduction. In this regard, we developed quantitative real-time RT-PCR (qrtRT-PCR) assays for the expression of European eel LHbeta, FSHbeta and GPalpha subunits, using the Light Cycler system. The qrtRT-PCR was adapted to permit detection of the three gonadotropin subunit mRNAs in individual pituitaries and in dispersed pituitary cells. The validated assays were applied to investigate the effects of sex steroids (estrogens and androgens) on gonadotropin subunit expression, in vivo in steroid-injected eels, and in vitro by steroid treatments of primary cultures of eel pituitary cells. In vivo, a stimulation of LHbeta mRNA was observed after estradiol (E2) treatments, while testosterone (T) or the non-aromatizable androgen dihydrotestosterone (DHT) had no effect. Concerning FSHbeta expression, slight but non-significant decreases were observed after both E2 and androgen treatments. Different results were obtained in vitro: E2 induced an increase in FSHbeta mRNA levels but had no effect on LHbeta expression. In contrast, androgens (T and DHT) stimulated LHbeta expression while no significant variation was observed on FSHbeta mRNA levels following androgen treatment. Concerning the GPalpha mRNA, no significant effect of sexual steroids was observed in vivo or in vitro. This demonstrated specific direct actions of steroids on gonadotropin subunit expression. The differences observed between in vivo and in vitro experiments may be explained by the involvement of cerebral control, including GnRH and dopamine neurons, and their specific regulation by sex steroids. The data indicate that sex steroid feedbacks on gonadotropins are exerted via multiple pathways, indirectly at the brain level and directly on pituitary gonadotrope cells.

Evolution of the CT/CGRP family: comparative study with new data from models of teleosts, the eel, and cephalopod molluscs, the cuttlefish and the nautilus

In mammals, alternative splicing of the calcitonin gene generates two distinct peptides: calcitonin (CT), synthesised in the thyroid C cells and involved in the regulation of calcium metabolism, and calcitonin gene-related peptide (CGRP), brain neuromediator synthesised in the peripheral and central nerves. CGRP is well represented and molecularly conserved during evolution whereas CT has not been detected in any of the invertebrates analysed so far. In order to better understand the evolution of this CT/CGRP peptide family we reviewed the major data concerning its evolution from the literature and our recent data obtained in models of teleosts and cephalopod molluscs. The presence of both CGRP-like molecules and its specific bindings sites in the central nervous system of eel, cuttlefish and nautilus, suggests that the brain neurotransmitter role of CGRP could represent an ancient role in metazoa, already present in cephalopods and conserved among vertebrates, as still observed in mammals. In contrast, the presence of CGRP specific binding sites, and not the peptide itself, in the gills suggests an endocrine role for CGRP, in cephalopods and teleosts, that may have been lost during the evolution of the tetrapod lineage. These data, and the absence of CT-like molecules that we observed in cephalopods, support the hypothesis that CGRP represents the ancestral molecule of the CT/CGRP family, appeared in metazoa before the vertebrate emergence. The distinction between CT and CGRP receptors appears to be an event posterior to the emergence of ecdysozoan and lophotrochozoan protostomes, probably in relation to the CT appearance. The evolution of the CT/CGRP peptide family is probably similar to the evolution of the CT/CGRP receptor family. In fact, the genic duplication that induced the appearance of the two separate molecules, CT and CGRP, may constitute an event close to that, which induced the appearance of the two specific receptors. These events remain to be further studied in order to better understand the peptide and receptor evolution of the CT/CGRP family.

Transcriptomic approach to the study of osmoregulation in the European eel Anguilla anguilla

In euryhaline teleosts, osmoregulation is a fundamental and dynamic process that is essential for the maintenance of ion and water balance, especially when fish migrate between fresh water (FW) and sea water (SW) environments. The European eel has proved to be an excellent model species to study the molecular and physiological adaptations associated with this osmoregulatory plasticity. The life cycle of the European eel includes two migratory periods, the second being the migration of FW eels back to the Sargasso Sea for reproduction. Various anatomical and physiological changes allow the successful transition to SW. The aim of this study was to use a microarray approach to screen the osmoregulatory tissues of the eel for changes in gene expression following acclimation to SW. Tissues were sampled from fish at selected intervals over a 5-mo period following FW/SW transfer, and RNA was isolated. Suppressive subtractive hybridization was used for enrichment of differentially expressed genes. Microarrays comprising 6,144 cDNAs from brain, gill, intestine, and kidney libraries were hybridized with appropriate targets and analyzed; 229 differentially expressed clones with unique sequences were identified. These clones represented the sequences for 95 known genes, with the remaining sequences (59%) being unknown. The results of the microarray analysis were validated by quantification of 28 differentially expressed genes by Northern blotting. A number of the differentially expressed genes were already known to be involved in osmoregulation, but the functional roles of many others, not normally associated with ion or water transport, remain to be characterized.

Yeast-binding C-type lectin with opsonic activity from conger eel (Conger myriaster) skin mucus☆

A mannose-specific lectin with agglutination activity against a yeast, Saccharomyces serevisiae, was purified from conger eel (Conger myriaster) skin mucus. The lectin, named conCL-s, has a tetrameric structure consisting of two non-covalently associated dimers whose constituent monomers with a molecular mass of about 16,000 Da are linked by a disulphide bond. conCL-s is composed of 173 amino acid residues including a 24-residue signal peptide, and belongs to the C-type lectin family, which is typically characterized by binding to sugar in a Ca2+-dependent manner. Nevertheless, conCL-s showed Ca2+-independent activity in its yeast-binding. The gene expression of the lectin was widely detected in external and internal mucosal tissues, i.e., skin, gills, tongue, buccal cavity wall and esophagus. In these tissues, mRNA of conCL-s was exclusively distributed in club cells, which are one of the major components of the epidermis and mucosal epithelium in Anguilliforme species. conCL-s showed agglutination activity to a bacterium, Escherichia coli. Furthermore, attachment of the lectin to microspheres significantly enhanced their phagocytosis in conger eel macrophages. These findings suggest that conCL-s acts as an opsonin and plays an important role in the innate immunity on the body surface in conger eels.

Solution structure and functional importance of a conserved RNA hairpin of eel LINE UnaL2

The eel long interspersed element (LINE) UnaL2 and its partner short interspersed element (SINE) share a conserved 3′ tail that is critical for their retrotransposition. The predicted secondary structure of the conserved 3′ tail of UnaL2 RNA contains a stem region with a putative internal loop. Deletion of the putative internal loop region abolishes UnaL2 mobilization, indicating that this putative internal loop is required for UnaL2 retrotransposition; the exact role of the putative internal loop in retrotransposition, however, has not been elucidated. To establish a structure-based foundation on which to address the issue of the putative internal loop function in retrotransposition, we used NMR to determine the solution structure of a 36 nt RNA derived from the 3′ conserved tail of UnaL2. The region forms a compact structure containing a single bulged cytidine and a U–U mismatch. The bulge and mismatch region have conformational flexibility and molecular dynamics simulation indicate that the entire stem of the 3′ conserved tail RNA can anisotropically fluctuate at the bulge and mismatch region. Our structural and mutational analyses suggest that stem flexibility contributes to UnaL2 function and that the bulged cytidine and the U–U mismatch are required for efficient retrotransposition.

Construction of a BAC library and identification of Dmrt1 gene of the rice field eel, Monopterus albus

A bacterial artificial chromosome (BAC) library was constructed using nuclear DNA from the rice field eel (Monopterus albus). The BAC library consists of a total of 33,000 clones with an average insert size of 115 kb. Based on the rice field eel haploid genome size of 600 Mb, the BAC library is estimated to contain approximately 6.3 genome equivalents and represents 99.8% of the genome of the rice field eel. This is first BAC library constructed from this species. To estimate the possibility of isolating a specific clone, high-density colony hybridization-based library screening was performed using Dmrt1 cDNA of the rice field eel as a probe. Both library screening and PCR identification results revealed three positive BAC clones which were overlapped, and formed a contig covering the Dmrt1 gene of 195 kb. By sequence comparisons with the Dmrt1 cDNA and sequencing of first four intron-exon junctions, Dmrt1 gene of the rice field eel was predicted to contain four introns and five exons. The sizes of first and second intron are 1.5 and 2.6 kb, respectively, and the sizes of last two introns were predicted to be about 20 kb. The Dmrt1 gene structure was conserved in evolution. These results also indicate that the BAC library is a useful resource for BAC contig construction and molecular isolation of functional genes.

Trends in the evolution of the prodynorphin gene in teleosts: cloning of eel and tilapia prodynorphin cDNAs

The detection of the prodynorphin gene in anuran amphibians and lungfishes may indicate that this gene arose as a result of the duplication of the proenkephalin gene early during the divergence of the Sarcopterygii, or that this gene may predate the divergence of the ray-finned fish and the lobe-finned fish. The cloning of prodynorphin-related genes from the pufferfish and zebrafish supports the latter hypothesis. This study analyzes trends in the radiation of the prodynorphin gene in teleosts. Prodynorphin cDNAs were cloned from the brain of the eel Anguilla rostrata and the Nile tilapia, Oreochromis niloticus. These teleost prodynorphin sequences have distinct alpha-neoendorphin, dynorphin A, and dynorphin B sequences, and a novel opioid sequence, YGGFI. The relationship of these teleost prodynorphin sequences to other actinopterygian and sarcopterygian prodynorphin sequences will be discussed.

Cloning and characterization of vitellogenin cDNA from the common Japanese conger (Conger myriaster) and vitellogenin gene expression during ovarian development

The major yolk protein precursor, vitellogenin (VTG) was detected in plasma from vitellogenic females and estradiol-17beta (E2)-treated immature females, but not in males and immature females by Western blotting in common Japanese conger Conger myriaster. Its molecular mass was approximately 180 kDa under denaturing and reducing conditions. The common Japanese conger VTG cDNA was cloned from the liver of vitellogenic female. It contains 5110 nucleotides including an open reading frame that encodes 1663 amino acids. The deduced amino acid sequence of the common Japanese conger VTG shares 80% identity with that of eel Anguilla japonica VTG-1, and 45-55%, 32-34% and 27-29% identity with the deduced amino acid sequences of other fish, amphibian and avian VTG with polyserine domain, respectively. In female common Japanese conger, VTG gene was highly expressed in the liver of this species similar with other oviparous vertebrates. The expression levels of VTG gene in the liver increased from the oil droplet stage to the tertiary yolk globule stage and were maintained until the migratory nucleus stage.

Molecular cloning and characterization of growth hormone receptor and its homologue in the Japanese eel (Anguilla japonica)

Two cDNAs encoding growth hormone receptor (GHR)-like genes, eGHR1 and eGHR2, were isolated from Japanese eel (Anguilla japonica) liver tissue. The putative eel GHR proteins showed conserved structural features of vertebrate GHRs, including six cysteine residues and a YGEFS motif in the extracellular domain, a single transmembrane region, and proline-rich box 1 and box 2 domains. Northern blot analysis showed a single eGHR1 transcript in liver, while two sizes of eGHR2 transcripts, thought to be produced by alternative splicing, were present. RT-PCR revealed that eGHR1 and eGHR2 transcripts were widely distributed throughout the whole body of the Japanese eel. Moreover, the results of binding assays showed the specific binding of growth hormone to recombinant eGHR1. Since these putative eGHR proteins show all characteristics of the GHR family, we conclude that eGHR1 and eGHR2 cDNA encode two different GHRs in Japanese eel. We confirmed the ligand specificity of eGHR1 by binding assay, and further research is needed to allow characterization of the binding capability of eGHR2.

Cloning and Expression of Guanylin-like Peptides in Teleost Fish

Homologues of the guanylin peptide family are expressed in teleost fish. Using the eel as a model euryhaline species, cDNAs for three peptides, guanylin, uroguanylin, and renoguanylin, were cloned and found to be expressed within both renal and intestinal epithelia. Seawater (SW) acclimation resulted in upregulation of uroguanylin mRNA expression in the intestinal epithelia of both immature "yellow" and sexually mature "silver" eels. SW acclimation also resulted in an increase in guanylin mRNA in silver eel intestine, whereas there was no change in renoguanylin mRNA expression under any condition. No changes in expression were found in the kidney following SW acclimation.

[Characterization of a multimer type III antifreeze protein gene from the Antarctic eel pout (Lycodichthys dearborni)]

To survive the freezing marine environment, the Antarctic eel pout, Lycodichthys dearborni synthesizes high concentration of type III antifreeze proteins (AFP III). In the process of characterizing the various types of AFP III mRNA present in the L. dearboni liver, a 2.87 kb mRNA encodes for multiple domains of AFP III was identified. This cDNA encodes 12 tandemly repeated segments, each translates into a 7 kD AFP III molecule plus a 9-amino acid linker. This naturally occurred and functional multimer type III antifreeze protein gene is the first of this kind being identified. The organization strongly mimics the polyprotein structure found in the genes for another type of bio-antifreezes, the antifreeze glycoprotein, AFGP. The AFP III and AFGP are compositionally and structurally completely different, and synthesized by fishes in different suborders. The presence of the similar polyprotein structures in the different types of antifreeze genes may imply a common organizational mechanism in the fish genomes for adapting to the extremely cold polar environment.

[Characterization of the beta-actin gene of the rice field eel and its phylogeny in fish]

Beta-actin is a member of the actin family of genes,which play important roles in maintaining cytoskeletal structure, cell motility, cell division, intracellular movements and contractile processes. We report here the identification of a beta-actin cDNA of the rice field eel,a teleost fish with a characteristic of natural sex reversal. The cDNA sequence of this gene was 1860 bp in length,encoding a 375 amino acid protein. Amino acid identities of the beta-actin between the rice field eel and other vertebrates including human, chicken, and other fishes, were more than 98%. RT-PCR showed expression of the rice field eel beta-actin in testis, ovotestis, ovary, heart, liver, spleen and brain,suggesting a ubiquitous expression pattern. Phylogenetic tree including all beta-actin cds of teleost fishes was constructed,which suggested consistently that teleost beta-actin can be classified into four types,but no fish has been found contains all the four types of beta-actin gene in its genome. The results that suggest lineage-specific beta-actin loss might happen in the radical evolution of teleost fishes.