Foxl2 up-regulates aromatase gene transcription in a female-specific manner by binding to the promoter as well as interacting with ad4 binding protein/steroidogenic factor 1

Increasing evidence suggests the crucial role of estrogen in ovarian differentiation of nonmammalian vertebrates including fish. The present study has investigated the plausible role of Foxl2 in ovarian differentiation through transcriptional regulation of aromatase gene, using monosex fry of tilapia. Foxl2 expression is sexually dimorphic, like Cyp19a1, colocalizing with Cyp19a1 and Ad4BP/SF-1 in the stromal cells and interstitial cells in gonads of normal XX and sex-reversed XY fish, before the occurrence of morphological sex differentiation. Under in vitro conditions, Foxl2 binds to the sequence ACAAATA in the promoter region of the Cyp19a1 gene directly through its forkhead domain and activates the transcription of Cyp19a1 with its C terminus. Foxl2 can also interact through the forkhead domain with the ligand-binding domain of Ad4BP/SF-1 to form a heterodimer and enhance the Ad4BP/SF-1 mediated Cyp19a1 transcription. Disruption of endogenous Foxl2 in XX tilapia by overexpression of its dominant negative mutant (M3) induces varying degrees of testicular development with occasional sex reversal from ovary to testis. Such fish display reduced expression of Cyp19a1 as well as a drop in the serum levels of 17beta-estradiol and 11-ketotestosterone. Although the XY fish with wild-type tilapia Foxl2 (tFoxl2) overexpression never exhibited a complete sex reversal, there were significant structural changes, such as tissue degeneration, somatic cell proliferation, and induction of aromatase, with increased serum levels of 17beta-estradiol and 11-ketotestosterone. Altogether, these results suggest that Foxl2 plays a decisive role in the ovarian differentiation of the Nile tilapia by regulating aromatase expression and possibly the entire steroidogenic pathway.

Sympatric speciation suggested by monophyly of crater lake cichlids

The existence of sympatric speciation--that populations diverge into species in the absence of physical or ecological barriers--is controversial. The East African Great Lakes harbour hundreds of cichlid species representing only a few monophyletic lineages, although palaeolimnological evidence and local restrictions on species distribution suggest that speciation in these lakes could have been allopatric. The case for sympatry in restricted areas of Lakes Malawi and Tanganyika is stronger but not unassailable. A better case might be made for cichlid species flocks in small, ecologically monotonous crater lakes. Here we present a mitochondrial DNA analysis of cichlid species flocks endemic to two such lakes in Cameroon. The results suggest that the flocks in each lake are monophyletic: the implication being that each lake was colonized once only, the size and shape of each lake being such that subsequent diversification would have been sympatric.

A Tandem Duplicate of Anti-Müllerian Hormone with a Missense SNP on the Y Chromosome Is Essential for Male Sex Determination in Nile Tilapia, Oreochromis niloticus

Variation in the TGF-β signaling pathway is emerging as an important mechanism by which gonadal sex determination is controlled in teleosts. Here we show that amhy, a Y-specific duplicate of the anti-Müllerian hormone (amh) gene, induces male sex determination in Nile tilapia. amhy is a tandem duplicate located immediately downstream of amhΔ-y on the Y chromosome. The coding sequence of amhy was identical to the X-linked amh (amh) except a missense SNP (C/T) which changes an amino acid (Ser/Leu92) in the N-terminal region. amhy lacks 5608 bp of promoter sequence that is found in the X-linked amh homolog. The amhΔ-y contains several insertions and deletions in the promoter region, and even a 5 bp insertion in exonVI that results in a premature stop codon and thus a truncated protein product lacking the TGF-β binding domain. Both amhy and amhΔ-y expression is restricted to XY gonads from 5 days after hatching (dah) onwards. CRISPR/Cas9 knockout of amhy in XY fish resulted in male to female sex reversal, while mutation of amhΔ-y alone could not. In contrast, overexpression of Amhy in XX fish, using a fosmid transgene that carries the amhy/amhΔ-y haplotype or a vector containing amhy ORF under the control of CMV promoter, resulted in female to male sex reversal, while overexpression of AmhΔ-y alone in XX fish could not. Knockout of the anti-Müllerian hormone receptor type II (amhrII) in XY fish also resulted in 100% complete male to female sex reversal. Taken together, these results strongly suggest that the duplicated amhy with a missense SNP is the candidate sex determining gene and amhy/amhrII signal is essential for male sex determination in Nile tilapia. These findings highlight the conserved roles of TGF-β signaling pathway in fish sex determination.

Unlike mammals, the identity of the master sex-determining gene varies among fish species, and it is not yet clear if there is a common molecular pathway regulating gonadal sex determination across teleosts. Here we show that a Y-linked duplicate of the anti-Mullerian hormone (amhy) is essential for male sex determination in tilapia. Mutation of amhy resulted in male to female sex reversal, while overexpression of it resulted in female to male sex reversal. A missense single nucleotide polymorphisms (SNP) (C/T) in the open reading frame (ORF) of amhy might contribute to male sex determination in tilapia. Knockout of the anti-Müllerian hormone receptor type II (amhrII) also resulted in male to female sex reversal. Taken the amhy in Patagonian pejerrey, amhrII in Takifugu rubripes, gsdf^Y in Oryzias luzonensis into consideration, these data highlight an important role for TGF-β signaling in teleost sex determination.

A genetic linkage map of a cichlid fish, the tilapia (Oreochromis niloticus)

We have constructed a genetic map for a tilapia, Oreochromis niloticus, using DNA markers. The segregation of 62 microsatellite and 112 anonymous fragment length polymorphisms (AFLPs) was studied in 41 haploid embryos derived from a single female. We have identified linkages among 162 (93.1%) of these markers. 95% of the microsatellites and 92% of the AFLPs were linked in the final map. The map spans 704 Kosambi cM in 30 linkage groups covering the 22 chromosomes of this species. Twenty-four of these linkage groups contain at least one microsatellite polymorphism. From the number of markers 15 or fewer cM apart, we estimate a total map length of approximately 1000-1200 cM. High levels of interference are observed, consistent with measurements in other fish species. This map is a starting point for the mapping of single loci and quantitative traits in cichlid fishes.

Genetic and ecological divergence of a monophyletic cichlid species pair under fully sympatric conditions in Lake Ejagham, Cameroon

Although there is mounting evidence that speciation can occur under sympatric conditions, unambiguous examples from nature are rare and it is almost always possible to propose alternative allopatric or parapatric scenarios. To identify an unequivocal case of sympatric speciation it is, therefore, necessary to analyse natural settings where recent monophyletic species flocks have evolved within a small and confined spatial range. We have studied such a case with a cichlid species flock that comprises five Tilapia forms endemic to a tiny lake (Lake Ejagham with a surface area of approximately 0.49 km2) in Western Cameroon. Analysis of mitochondrial D-Loop sequences shows that the flock is very young (approximately 10(4) years) and has originated from an adjacent riverine founder population. We have focused our study on a particular pair of forms within the lake that currently appears to be in the process of speciation. This pair is characterized by an unique breeding colouration and specific morphological aspects, which can serve as synapomorphic characters to prove monophyly. It has differentiated into a large inshore and a small pelagic form, apparently as a response to differential utilization of food resources. Still, breeding and brood care occurs in overlapping areas, both in time and space. Analysis of nuclear gene flow on the basis of microsatellite polymorphisms shows a highly restricted gene flow between the forms, suggesting reproductive isolation between them. This reproductive isolation is apparently achieved by size assortative mating, although occasional mixed pairs can be observed. Our findings are congruent with recent theoretical models for sympatric speciation, which show that differential ecological adaptations in combination with assortative mating could easily lead to speciation in sympatry.

A second-generation genetic linkage map of tilapia (Oreochromis spp.)

We constructed a second-generation linkage map of tilapia from the F(2) progeny of an interspecific cross between Oreochromis niloticus and Oreochromis aureus. The map reported here contains 525 microsatellite and 21 gene-based markers. It spans 1311 cM in 24 linkage groups, for an average marker spacing of 2.4 cM. We detected associations of sex and red color with markers on linkage group 3. This map will enable mapping and selective breeding of quantitative traits important to the economic culture of tilapia as a food fish and will contribute to the study of closely related cichlids that have undergone explosive adaptive radiation in the lakes of East Africa.

Biologically meaningful expression profiling across species using heterologous hybridization to a cDNA microarray

Background

Unravelling the path from genotype to phenotype, as it is influenced by an organism's environment, is one of the central goals in biology. Gene expression profiling by means of microarrays has become very prominent in this endeavour, although resources exist only for relatively few model systems. As genomics has matured into a comparative research program, expression profiling now also provides a powerful tool for non-traditional model systems to elucidate the molecular basis of complex traits.

Results

Here we present a microarray constructed with ~4500 features, derived from a brain-specific cDNA library for the African cichlid fish Astatotilapia burtoni (Perciformes). Heterologous hybridization, targeting RNA to an array constructed for a different species, is used for eight different fish species. We quantified the concordance in gene expression profiles across these species (number of genes and fold-changes). Although most robust when target RNA is derived from closely related species (<10 MA divergence time), our results showed consistent profiles for other closely related taxa (~65 MA divergence time) and, to a lesser extent, even very distantly related species (>200 MA divergence time).

Conclusion

This strategy overcomes some of the restrictions imposed on model systems that are of importance for evolutionary and ecological studies, but for which only limited sequence information is available. Our work validates the use of expression profiling for functional genomics within a comparative framework and provides a foundation for the molecular and cellular analysis of complex traits in a wide range of organisms.

Sexually dimorphic expression of two types of DM (Doublesex/Mab-3)-domain genes in a teleost fish, the Tilapia (Oreochromis niloticus)

Sex determination consists of somatic and germ-line sex differentiation hierarchies whose interaction is poorly understood. A single gene known to control somatic sex determination, the DM-domain containing (Doublesex/Mab-3 DNA-binding motif) gene, is highly conserved across species. Vertebrate DMRT1 (DM-related transcription factor 1) expression occurs predominantly in testis. We, however, isolated two distinct DM-domain cDNAs from tilapia testis and ovary, named tDMRT1, and tDMO (DM-domain gene in Ovary), respectively. Despite high homology in the DM-domain, there is little similarity outside the DM-domain. A male specific motif is absent in tDMO indicating a similarity with the female type of doublesex in Drosophila. In contrast to the alternatively spliced male and female types of doublesex, tDMRT1 and tDMO cDNAs are encoded by two different genes. The mutually exclusive nature of tDMRT1 and tDMO expression in the testis or ovary suggests that they both play important roles in gonadal development and/or function.

Two types of aromatase with different encoding genes, tissue distribution and developmental expression in Nile tilapia (Oreochromis niloticus)

We isolated a novel type of aromatase cDNA from a Nile tilapia (Oreochromis niloticus) ovary cDNA library. Because this aromatase is phylogenetically related to brain aromatase (CYP19b) of goldfish, zebrafish and sea bass, we named it tilapia CYP19b (tCYP19b). tCYP19b encodes a protein that is predicted to consist of 495 residues and have 63.8% homology with the aromatase (tCYP19a) we previously isolated from the same source. In vitro transient transfection of cultured COS7 cells demonstrated that tCYP19b codes a functional protein to catalyze estrogen production from an androgen substrate. RT-PCR and Northern hybridization analysis showed that tCYP19b was expressed at a high level in the brain and at a low level in a wide variety of other tissues, whereas tCYP19a was mainly present in the ovary and its level significantly increased during the vitellogenic stage. RT-PCR also detected tCYP19b expression in brain and gonad tissues of both female and male tilapia during sex differentiation, but tCYP19a was only found in the ovary of the fry at that period. These results suggest that tCYP19a plays a key role in sex differentiation and ovarian development. We also isolated genes of two tilapia aromatases. Based on the location of the transcription initiation site, we predicted that there is one promoter for tCYP19a and three promoters for tCYP19b. Although the two aromatase isoforms have similar gene structures in the coding region, we found that the binding regions of SF-1/Ad4 BP region, WT1-KTS and SRY, which are sex-determining factors in mammals, are present in the 5' flank region of tCYP19a but not tCYP19b. A similar situation is present in promoters of zebrafish and goldfish aromatase isoforms. This data indicates that CYP19a plays a decisive role in sex differentiation of those species. The unique presence of the ERE motif in the tCYP19b promoter and the high expression of tCYP19b in the brain support that CYP19b is mainly involved in estrogen-mediated neural estrogen synthesis.

Three different hepcidins from tilapia, Oreochromis mossambicus: analysis of their expressions and biological functions

Hepcidins are antimicrobial peptides that play important roles in resisting pathogenic infection. Through hybridization of a phage library, the cDNA sequences of three hepcidin-like antimicrobial peptides (named TH1-5, TH2-2, and TH2-3) in tilapia, Oreochromis mossambicus, were determined. The complete hepcidin cDNA sequences of TH1-5, TH2-2, and TH2-3 were respectively composed of 478, 533, and 583 bases, and contained a translated region of 88, 86, and 91 amino acids. An evolutionary assay of the three deduced amino acid sequences, which share eight cysteines at identical conserved positions, showed that tilapia TH2-3 is similar to Japanese flounder (Paralichthys olivaceus) JF2, tilapia TH2-2 is similar to Japanese flounder JF1, and tilapia TH1-5 is similar to seabream (Chrysophrys major) hepcidin. The predicted molecular weights of TH1-5, TH2-2, and TH2-3 are 9.5, 9.4, and 9.8 kDa, respectively. The predicted signal peptide cleavage sites in TH1-5 is between codons 24 and 25, in TH2-2, it is between codons 22 and 23, and in TH2-3, it is between codons 24 and 25. The structural models of tilapia hepcidins, constructed using the crystal structures of bass (Morone chrysopsx M. saxatilis) hepcidin as a respective template, showed that the positional cysteine residues form disulfide bonds with tilapia hepcidin, and the cysteines likely form disulfide bonds with the bass hepcidin cysteine. The tissue-specific, lipopolysaccharide (LPS) stimulation-specific, and polyinosinic-polycytidylic acid (poly I:poly C) stimulation-specific expressions of tilapia hepcidin mRNA were determined by a comparative reverse-transcription polymerase chain reaction. Results of the tissues distribution analysis revealed high expression levels of hepcidin messenger RNA (mRNA) in the liver and head kidneys for TH1-5. TH2-3 had high mRNA expression after LPS challenge in comparison to TH2-2 and TH1-5 in fish injected with 10mug/ml LPS. TH1-5 had high mRNA expression after poly I:poly C challenge in comparison to TH2-2 and TH2-3. Immunohistochemical analysis with the polyclonal antiserum of tilapia hepcidin TH1-5 (using a rabbit polyclonal antibody) showed that the peptide was localized in the spleen and head kidneys. Synthesized TH1-5 and TH2-3 peptides showed antimicrobial activity against several bacteria in this study, while the synthesized TH 2-2 peptide did not.

Evolutionary conservation of regulatory elements in vertebrate Hox gene clusters

Comparisons of DNA sequences among evolutionarily distantly related genomes permit identification of conserved functional regions in noncoding DNA. Hox genes are highly conserved in vertebrates, occur in clusters, and are uninterrupted by other genes. We aligned (PipMaker) the nucleotide sequences of the HoxA clusters of tilapia, pufferfish, striped bass, zebrafish, horn shark, human, and mouse, which are separated by approximately 500 million years of evolution. In support of our approach, several identified putative regulatory elements known to regulate the expression of Hox genes were recovered. The majority of the newly identified putative regulatory elements contain short fragments that are almost completely conserved and are identical to known binding sites for regulatory proteins (Transfac database). The regulatory intergenic regions located between the genes that are expressed most anteriorly in the embryo are longer and apparently more evolutionarily conserved than those at the other end of Hox clusters. Different presumed regulatory sequences are retained in either the Aalpha or Abeta duplicated Hox clusters in the fish lineages. This suggests that the conserved elements are involved in different gene regulatory networks and supports the duplication-deletion-complementation model of functional divergence of duplicated genes.

Induction of XY sex reversal by estrogen involves altered gene expression in a teleost, tilapia

To clarify the importance of endogenous estrogens during sex differentiation in a teleost fish, the Nile tilapia, we examined the target events for endogenous estrogens and their role during gonadal sex differentiation. The expression of CYP19a (P450arom) precedes any morphological gonadal sex differentiation. Further to these findings, the treatment of XX fry with non-steroidal aromatase inhibitor (AI), Fadrozole, from seven to 14 days after hatching caused complete sex reversal to functional males. The XX sex reversal induced by AI was rescued completely with simultaneous estrogen treatment. We also found that XY fry treated with estrogen, before the appearance of morphological sex differences, caused complete sex reversal from males to females. Taken together, these results suggest that endogenous estrogens are required for ovarian differentiation. To identify the down-stream gene products of estrogen during ovarian differentiation, we performed subtractive hybridization using mRNA derived from normal and estrogen treated XY gonads. Two out of ten gene products were expressed in germ cells, whereas the others were expressed in somatic cells.

Prolactin receptor, growth hormone receptor, and putative somatolactin receptor in Mozambique tilapia: tissue specific expression and differential regulation by salinity and fasting

In fish, pituitary growth hormone family peptide hormones (growth hormone, GH; prolactin, PRL; somatolactin, SL) regulate essential physiological functions including osmoregulation, growth, and metabolism. Teleost GH family hormones have both differential and overlapping effects, which are mediated by plasma membrane receptors. A PRL receptor (PRLR) and two putative GH receptors (GHR1 and GHR2) have been identified in several teleost species. Recent phylogenetic analyses and binding studies suggest that GHR1 is a receptor for SL. However, no studies have compared the tissue distribution and physiological regulation of all three receptors. We sequenced GHR2 from the liver of the Mozambique tilapia (Oreochromis mossambicus), developed quantitative real-time PCR assays for the three receptors, and assessed their tissue distribution and regulation by salinity and fasting. PRLR was highly expressed in the gill, kidney, and intestine, consistent with the osmoregulatory functions of PRL. PRLR expression was very low in the liver. GHR2 was most highly expressed in the muscle, followed by heart, testis, and liver, consistent with this being a GH receptor with functions in growth and metabolism. GHR1 was most highly expressed in fat, liver, and muscle, suggesting a metabolic function. GHR1 expression was also high in skin, consistent with a function of SL in chromatophore regulation. These findings support the hypothesis that GHR1 is a receptor for SL. In a comparison of freshwater (FW)- and seawater (SW)-adapted tilapia, plasma PRL was strongly elevated in FW, whereas plasma GH was slightly elevated in SW. PRLR expression was reduced in the gill in SW, consistent with PRL's function in freshwater adaptation. GHR2 was elevated in the kidney in FW, and correlated negatively with plasma GH, whereas GHR1 was elevated in the gill in SW. Plasma IGF-I, but not GH, was reduced by 4 weeks of fasting. Transcript levels of GHR1 and GHR2 were elevated by fasting in the muscle. However, liver levels of GHR1 and GHR2 transcripts, and liver and muscle levels of IGF-I transcripts were unaffected by fasting. These results clearly indicate tissue specific expression and differential physiological regulation of GH family receptors in the tilapia.

Two unlinked loci controlling the sex of blue tilapia (Oreochromis aureus)

Sex determination in the blue tilapia (Oreochromis aureus) is thought to be a WZ-ZZ (female heterogametic) system controlled by a major gene. We searched for DNA markers linked to this major gene using the technique of bulked segregant analysis. We identified 11 microsatellite markers on linkage group 3 which were linked to phenotypic sex. The putative W chromosome haplotype correctly predicts the sex of 97% of male and 85% of female individuals. Our results suggest the W locus lies within a few centimorgans of markers GM354, UNH168, GM271 and UNH131. Markers on LG1 also showed a strong association with sex, and indicate the segregation of a male-determining allele in this region. Analysis of epistatic interactions among the loci suggests the action of a dominant male repressor (the W haplotype on LG 3) and a dominant male determiner (the Y haplotype on LG1). These markers have immediate utility for studying the strength of different sex chromosome alleles, and for identifying broodstock carrying copies of the W haplotype.

Application of the RAPD technique in tilapia fish: species and subspecies identification

Random Amplified Polymorphic DNA (RAPD) analysis was applied to three species of the tilapia genus Oreochromis and four subspecies of O. niloticus. Thirteen random 10-mer primers were used to assay polymorphisms within and between populations. Different RAPD fragment patterns were observed for different species, although not always for different subspecies. Evidence is presented that RAPD markers might be useful for systemic investigation at the level of species and subspecies.

Isolation and characterization of the cDNA encoding the tilapia (Oreochromis niloticus) cytochrome P450 aromatase (P450arom): changes in P450arom mRNA, protein and enzyme activity in ovarian follicles during oogenesis

A cDNA clone encoding the complete tilapia (a teleost fish, Oreochromis niloticus) cytochrome P450 aromatase (P450arom) was isolated from an ovarian follicle cDNA library. The deduced amino acid sequence (522 amino acid residues) had 72.2% and 59.5% homology with rainbow trout and catfish P450arom respectively, and about 50% homology with mammalian and avian P450arom. Expression of this cDNA in COS-7 cells produced a protein that converted exogenous testosterone to estrogens. Northern blots using a tilapia P450arom cDNA fragment and Western blots using an antiserum against a tilapia P450arom polypeptide fragment revealed a single P450arom mRNA (2.6 kb) and a single protein (59 kDa) in tilapia ovarian tissue respectively. These analyses also revealed that the levels of both P450arom mRNA and protein were low in early vitellogenic follicles, increased in midvitellogenic follicles, and declined to non-detectable levels in post-vitellogenic follicles. Changes in the ability of follicles to convert exogenous testosterone to estrogens (aromatase activity) were similar to those of P450arom mRNA and protein. These observations indicated that the capacity of tilapia ovarian follicles to synthesize estradiol-17 beta is closely related to the contents of P450arom mRNA and protein within them.

Rapid hyperosmotic coinduction of two tilapia (Oreochromis mossambicus) transcription factors in gill cells

Gills of euryhaline teleosts are excellent models for studying osmotic-stress adaptation because they directly contact the aquatic environment and are an important effector tissue during osmotic stress. We acclimated tilapia (Oreochromis mossambicus) from fresh water (FW) to seawater (SW); performed suppression subtractive hybridization of gill mRNAs; and identified two transcription factors, osmotic stress transcription factor 1 (OSTF1) and the tilapia homolog of transcription factor II B (TFIIB), that are rapidly and transiently induced during hyperosmotic stress. mRNA levels increase 6-fold for OSTF1 and 4-fold for TFIIB, and they reach maxima 2 h after SW transfer. Protein levels increase 7.5-fold for OSTF1 and 9-fold for TFIIB, and they reach maxima 4 h after SW transfer. Induction of OSTF1 and TFIIB increases gradually with increasing salinity. Induction of OSTF1 and TFIIB is specific for osmotic stress and absent during oxidative stress (1 mM H2O2) or heat shock (+10 degrees C). Bioinformatic analysis of OSTF1 reveals that it is a transcription factor of the TGF-beta-stimulated clone 22/GILZ family. Because some mammalian homologs are strongly induced by glucocorticoids, OSTF1 may represent the molecular link between the SW hormone cortisol and transcriptional regulation of ion transport and cell differentiation in teleost gills. Coinduction of OSTF1 and TFIIB may serve to recruit TFIIB preferentially to OSTF1 target genes during hyperosmotic stress and compensate for reduced rates of transcription resulting from salt-induced chromatin compaction. We conclude that OSTF1 and TFIIB are critical elements of osmosensory signal transduction in euryhaline teleosts that mediate osmotic adaptation by means of transcriptional regulation.

Identification of a sex-determining region in Nile tilapia (Oreochromis niloticus) using bulked segregant analysis

Sex determination in the Nile tilapia (Oreochromis niloticus) is thought to be an XX-XY (male heterogametic) system controlled by a major gene. We searched for DNA markers linked to this major locus using bulked segregant analysis. Ten microsatellite markers belonging to linkage group 8 were found to be linked to phenotypic sex. The putative Y-chromosome alleles correctly predict the sex of 95% of male and female individuals in two families. Our results suggest a major sex-determining locus within a few centimorgans of markers UNH995 and UNH104. A third family from the same population showed no evidence for linkage of this region with phenotypic sex, indicating that additional genetic and/or environmental factors regulate sex determination in some families. These markers have immediate utility for studying the strength of different Y chromosome alleles, and for identifying broodstock carrying one or more copies of the Y haplotype.

Genotoxic effect of monocrotophos to sentinel species using comet assay

Monocrotophos is the single largest selling agrochemical in India. Sensitive biomarkers to study the genotoxic effects caused by monocrotophos in aquatic organisms, especially fish, are lacking. The fish used in this study are Tilapia mossambica, which are edible, commercially valuable and distributed all over India. The objective of this study was to study DNA strand breaks induced by monocrotophos in T. mossambica in vivo using single-cell micro gel electrophoresis/comet assay. Tilapia were treated orally with 0.313, 0.625, 1.25, 1.875, 2.5, 3.125, 3.75 and 4.375 ppm of monocrotophos and the assay was performed on nucleated erythrocytes after 24, 48, 72 and 96 h. A significant increase in mean comet tail-length (5.21-7.46 microM), indicating DNA damage, was observed at all the doses with monocrotophos when compared to controls (3.36 microM). The mean tail-length showed a dose-related increase and time-dependent decrease. The maximum increase in mean comet tail-length was observed at 24 h. Relative to these effects, reductions in mean comet tail-length were seen at 48 and 72 h. By 96 h, values had returned to control levels at all doses, indicating repair of the damaged DNA and/or loss of heavily damaged cells. The study reveals that the comet assay is a sensitive and rapid method to detect genotoxicity of monocrotophos and other environmental pollutants in sentinel species.

Expression of a novel piscine growth hormone gene results in growth enhancement in transgenic tilapia (Oreochromis niloticus)

Several lines of transgenic G1 and G2 tilapia fish (Oreochromis niloticus) have been produced following egg injection with gene constructs carrying growth hormone coding sequences of fish origin. Using a construct in which an ocean pout antifreeze promoter drives a chinook salmon growth hormone gene, dramatic growth enhancement has been demonstrated, in which the mean weight of the 7 month old G2 transgenic fish is more than three fold that of their non transgenic siblings. Somewhat surprisingly G1 fish transgenic for a construct consisting of a sockeye salmon metallothionein promoter spliced to a sockeye salmon growth hormone gene exhibited no growth enhancement, although salmon transgenic for this construct do show greatly enhanced growth. The growth enhanced transgenic lines were also strongly positive in a radio-immuno assay for the specific hormone in their serum, whereas the non growth enhanced lines were negative. Attempts to induce expression from the metallothionein promoter by exposing fish to increased levels of zinc were also unsuccessful. Homozygous transgenic fish have been produced from the ocean pout antifreeze/chinook salmon GH construct and preliminary trials suggest that their growth performance is similar to that of the hemizygous transgenics. No abnormalities were apparent in the growth enhanced fish, although minor changes to skull shape and reduced fertility were noted in some fish. There is also preliminary evidence for improved food conversion ratios when growth enhanced transgenic tilapia are compared to their non-transgenic siblings. The long term objective of this study is to produce lines of tilapia which are both growth enhanced and sterile, so offering improved strains of this important food fish for aquaculture.

Host genetic selection for cold tolerance shapes microbiome composition and modulates its response to temperature

The hologenome concept proposes that microbes and their host organism are an independent unit of selection. Motivated by this concept, we hypothesized that thermal acclimation in poikilothermic organisms, owing to their inability to maintain their body temperature, is connected to their microbiome composition. To test this hypothesis, we used a unique experimental setup with a transgenerational selective breeding scheme for cold tolerance in tropical tilapias. We tested the effects of the selection on the gut microbiome and on host transcriptomic response. Interestingly, we found that host genetic selection for thermal tolerance shapes the microbiome composition and its response to cold. The microbiomes of cold-resistant fish showed higher resilience to temperature changes, indicating that the microbiome is shaped by its host's selection. These findings are consistent with the hologenome concept and highlight the connection between the host and its microbiome's response to the environment.

Amh and Dmrta2 genes map to tilapia (Oreochromis spp.) linkage group 23 within quantitative trait locus regions for sex determination

Recent studies have revealed that the major genes of the mammalian sex determination pathway are also involved in sex determination of fish. Several studies have reported QTL in various species and strains of tilapia, regions contributing to sex determination have been identified on linkage groups 1, 3, and 23. Genes contributing to sex-specific mortality have been detected on linkage groups 2, 6, and 23. To test whether the same genes might control sex determination in mammals and fishes, we mapped 11 genes that are considered putative master key regulators of sex determination: Amh, Cyp19, Dax1, Dmrt2, Dmrta2, Fhl3l, Foxl2, Ixl, Lhx9, Sf1, and Sox8. We identified polymorphisms in noncoding regions of these genes and genotyped these sites for 90 individuals of an F2 mapping family. Mapping of Dax1 joined LG16 and LG21 into a single linkage group. The Amh and Dmrta2 genes were mapped to two distinct regions of LG23. The Amh gene was mapped 5 cM from UNH879 within a QTL region for sex determination and 2 cM from UNH216 within a QTL region for sex-specific mortality. Dmrta2 was mapped 4 cM from UNH848 within another QTL region for sex determination. Cyp19 was mapped to LG1 far from a previously reported QTL region for sex determination on this chromosome. Seven other candidate genes mapped to LG4, -11, -12, -14, and -17.

Insulin-like growth factor I in the teleost Oreochromis mossambicus, the tilapia: gene sequence, tissue expression, and cellular localization

Using reverse transcription-PCR and molecular cloning, the complementary DNA sequence encoding preproinsulin-like growth factor I (IGF-I) of a teleost, the tilapia (Oreochromis mossambicus) was established from liver. At the amino acid level, tilapia IGF-I shows all residues necessary for the maintenance of tertiary structure and shares about 80% identity with IGF-I from other teleosts. The B and A domains of tilapia IGF-I show more than 90% homology to those of other teleosts and 86-93% to those of human. However, in contrast to salmonids, the C domain of tilapia is truncated. Reverse transcription-PCR analysis followed by Southern blotting with an internal probe specific for tilapia IGF-I indicated a transcript in liver, pancreas, gut, kidney, head kidney, gill, ovary, testis, eye, and brain. In correlation, parenchymal cells were identified as likely local production sites by the use of immunohistochemistry. IGF-I immunoreactivity was confined to D cells in pancreatic islets, gastroentero-endocrine cells, cells of renal proximal tubules, interrenal cells of the head kidney, gill chondrocytes, chloride cells of the gill epithelium, granulosa cells in the ovary, spermatocytes and Sertoli cells in testis, and neurons in retina and brain. The local production of IGF-I in multiple organs of the tilapia indicates paracrine/autocrine actions of IGF-I involved in organ-specific functions. The results further demonstrate that the primary structure of IGF-I, especially in the B and A domains, is highly conserved during phylogeny.

Regulation of gonadotropin subunit genes in tilapia

A steroidogenic tilapia gonadotropin (taGtH=LH) was purified from pituitaries of hybrid tilapia (Oreochromis niloticus x O. aureus) and a homologous RIA was established. This RIA enabled the study of the endocrine regulation of GtH release, the transduction pathways involved in its secretion and its profile during the spawning cycle. Discrepancies between steroid and taGtH peaks during the cycle led to the conclusion that an additional gonadotropin similar to salmonid FSH operates early in the cycle. In order to identify this hormone and to study the endocrine control of synthesis of all gonadotropin (GtH) subunits, a molecular approach was taken. The cDNA sequences and the entire gene sequences encoding the FSHbeta and LHbeta subunits, as well as an incomplete sequence of the glycoprotein hormone alpha subunit (GPalpha), were cloned. Salmon gonadotropin-releasing hormone (sGnRH) elevated mRNA steady-state levels of all three GtH subunits in cultured pituitary cells. Pituitary adenylate cyclase-activating polypeptide (PACAP) and neuropeptide Y (NPY) also stimulated the expression of these subunits and potentiated the effect of GnRH, except that NPY did not affect FSHbeta. The GnRH and NPY effects were found to be mediated mainly through protein kinase C (PKC), while protein kinase A (PKA) cascade was involved to a lesser extent. Mitogen-activated protein kinase (MAPK) cascade takes part in mediating GnRH effects, possibly via PKC. Testosterone (T) and estradiol (E2), but not 11-ketotestosterone (KT), are able to elevate GPalpha and LHbeta mRNAs in pituitary cells of early maturing or regressing males. Low levels of T exposure are associated with elevated FSHbeta mRNA in cells of mature fish, while higher levels suppress it, but elevate LHbeta mRNA. In vivo observations also showed the association of low T levels with increased FSHbeta mRNA and high T levels with elevated LHbeta mRNA. In accordance with these findings, analysis of LHbeta and FSHbeta 5' gene-flanking regions revealed on both gene promoters a GtH-specific element (GSE), half site estrogen response elements (ERE), cAMP response element (CRE) and AP1. In vitro experiments showed that recombinant human activin-A leads to higher levels of GPalpha, FSHbeta and LHbeta mRNAs in pituitary cell culture. Porcine inhibin marginally decreased the mRNA levels of GPalpha and FSHbeta, but at a low level (1 ng/ml) it stimulated that of LHbeta. These results shed some light on certain hypothalamic and gonadal hormones regulating the expression of GtH subunit genes in tilapia. In addition, they provide evidence for their differential regulation, and insight into their mode of action.

Effects of transfer from seawater to fresh water on the growth hormone/insulin-like growth factor-I axis and prolactin in the Tilapia, Oreochromis mossambicus

The effect of freshwater (FW) transfer on growth and on the growth hormone (GH)/insulin-like growth factor-I (IGF-I) axis was examined in the tilapia, Oreochromis mossambicus. Tilapia were raised in seawater (SW) for 5 months and then transferred to FW for an additional 40 days. The growth rate of the fish transferred to FW was significantly reduced compared with the growth rate of fish that remained in SW. Plasma levels of GH were significantly elevated in FW-transferred fish, as were plasma IGF-I levels. Pituitary GH and liver IGF-I mRNA levels, on the other hand, were significantly reduced in the fish transferred to FW. There was a significant correlation between body mass and mRNA levels of GH and IGF-I, but not with plasma levels of GH and IGF-I. Fish transferred to FW had significantly higher prolactin (PRL)(177) levels than the SW control fish, although there was no difference in plasma PRL(188) levels. Consistent with the hyperosmoregulatory effects of PRL, mRNA levels of both PRL(177) and PRL(188) were significantly higher in FW-transferred fish than in the fish in SW. These results suggest that transferring tilapia from SW to FW activates the GH/IGF-I axis, but growth is still inhibited, possibly due to the greater metabolic cost of osmoregulation in FW than in SW.