Gametogenesis related genes and apoptosis marker genes during the reproductive cycle of the thicklip grey mullet (Chelon labrosus Riso, 1927) of Pasaia Harbour (Bay of Biscay, Southwestern Europe); Possibilities for development of molecular markers of xenoestrogenic exposure
Endocrine disrupting chemicals (EDCs) and in particular xenoestrogens have been a cause of major concern in the aquatic environment given that they are readily bioavailable to aquatic organisms where they exert several deleterious effects. Considering that the sexual cycle and gonadal development of teleost fishes is dependent on endocrine communication between the brain, pituitary and gonads, EDCs affecting this communicational network pose severe risks for the development, reproduction and survival of ichthyofauna in polluted areas. The present study focused on the analysis of specific transcripts along gametogenesis of female, male and intersex individuals captured in the polluted harbour of Pasaia characterizing the relevance of their transcriptional regulation as molecular markers of xenoestrogenic exposure in the thicklip grey mullet Chelon labrosus. Key genes regulating gametogenesis and follicular atresia in mullets were amplified and sequenced using degenerate primers and their gonadal transcription profile quantified by qRT-PCR in the gonads at different gametogenic stages of mature adult female, male and intersex mullets, collected during a whole reproductive cycle. Partial cDNA sequences coding for the β subunits of both pituitary gonadotropins (fshβ and lhβ), gonadal luteinizing hormone receptor (lhr), and apoptosis related E3 ubiquitin-protein ligase mdm2 were successfully sequenced and published in the Genbank. Their sequences were found to be conserved in C. labrorus when compared to orthologs in other vertebrate species, specially in other teleost fishes. Transcription profiles of lhr and the vitellogenin receptor (vtgr) were then studied in gonads showing a decline in transcription levels during late vitellogenesis with no observable differences during spermatogenesis. The apoptosis markers p53 and mdm2 showed opposite transcription profiles in late vitellogenesis, p53 increasing while mdm2 decreased in comparison to early vitellogenic ovaries, suggesting that there could an up-regulation of the apoptosis regulating cascade during this stage preparing oocytes for resorptive atresia. The transcription levels of all analysed genes were lower in intersex males in comparison to normal males, suggesting that testis feminisation and production of oocytes results in modulation of their transcription levels. This would suggest in favour of studying these genes in relation to gonadotropin signalling and apoptotic responses as molecular markers of intersex condition in monitoring programmes using C. labrosus as sentinel of exposure to xenoestrogenic compounds.
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