The Russian sturgeon (Acipenser gueldenstaedtii, AG) is an endangered fish species increasingly raised on fish farms for black caviar. Understanding the process of sex determination in AG is, therefore, of scientific and commercial importance. AG lacks sexual dimorphism until sexual maturation and has a predominantly octoploid genome without a definite sex chromosome. A conserved short female-specific genomic sequence was recently described, leading to the development of a genetic sex marker. However, no biological function has been reported for this sequence. Thus, the mechanism of sex determination and the overall inter-sex genomic variation in AG are still unknown. To comprehensively analyze the inter-sex genomic variation and assess the overall inter-species variation between AG and A. ruthenus (AR, sterlet), a related tetraploid sturgeon species, we performed whole-genome sequencing on DNA from 10 fish-farm-raised adult AG (5 males and 5 females). We produced a partially assembled, ~2390 MBp draft genome for AG. We validated in AG the female-specific region previously described in AR. We identified ~2.8 million loci (SNP/indels) varying between the species, but only ~7400 sex-associated loci in AG. We mapped the sex-associated AG loci to the AR genome and identified 15 peaks of sex-associated variation (10 kb segments with 30 or more sex-associated variants), 1 of which matched the previously reported sex-variable region. Finally, we identified 14 known and predicted genes in proximity to these peaks. Our analysis suggests that one or more of these genes may have functional roles in sex determination and/or sexual differentiation in sturgeons. Further functional studies are required to elucidate these roles.

The blue gourami (Trichogaster trichopterus) is a model for hormonal control of reproduction in Anabantidae fish, but also relevant to other vertebrates. We analyzed the female blue gourami brain transcriptome in two developmental stages: pre-vitellogenesis (PVTL) before yolk accumulation in the oocytes, and high vitellogenesis (HVTL) at the end of yolk accumulation in the oocytes. RNA sequencing of whole-brain transcriptome identified 34,368 unique transcripts, 23,710 of which could be annotated by homology with other species. We focused on the transcripts showing significant differences between the stages. Seventeen and fourteen annotated genes were found to be upregulated in PVTL and HVTL, respectively. Five nuclear transcripts, three of which contain the homeobox domain (ARX, DLX5, CERS6), were upregulated in PVTL. Additionally, several receptors previously known to be involved in reproduction were identified, and three of these, G-protein coupled receptor 54, Membrane progesterone receptor epsilon, and Gonadotropin-releasing hormone II receptor (GPCR, mPR, and GnRHR) were measured by quantitative RT-PCR in brain, pituitary, and ovary samples from PVTL and HVTL stage females. Of these, GPCR was highly expressed in the brain and pituitary as compared to the ovary in both PVTL and HVTL. GnRHR was highly expressed in the ovary compared to the brain and pituitary, and its levels in the brain were significantly higher in PVTL than HVTL. Brain mPR mRNA levels were likewise higher in PVTL than HVTL. In conclusion, this study details changes in the female blue gourami brain transcriptome through yolk accumulation in the oocytes and identifies key genes that may mediate this process.