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Despite its wide presence in cold-water bodies of central Asian mountains,  Schizothorax plagiostomus  is highly neglected Himalayan ichthyofauna. Here we have performed de novo RNA-sequencing in adult  S. plagiostomus  gonads captured from natural habitat of Garhwal Himalaya [during three distinct maturational stages (pre-spawning, spawning and post-spawning) for both sexes] and compared the season and sex specific differential transcriptomic profiles. Functional analyses of KEGG pathway and GO enrichment demonstrated augmentation in cellular signaling for multiple growth factors, chemokines, cytokines, exosomes and vascular patterning in pre-spawning gonads, rise in t-RNA biosynthesis, chaperone cofactor dependent protein refolding, steroidogenic and calcium signaling network at spawning time, whereas elevated proteolysis, autophagy and apoptotic pathways were prevalent during regressing stages. Among differentially expressed genes (DEGs) distinct sexual dimorphic pattern of male specific  Dmrt1, Sox18, Amh, Wt1, GDNF receptor α  and female specific  Foxl 2 , Gdf9, Zar1,Cyp19a1a, Wntless  were observed.Collectively, this is the first comprehensive gonadal atlas of season dependent developmental transcriptome with respective gametogenic output in any Schizothoracine species. This data will potentially serve as a vital resource for future programs in conservation management and artificial breeding of schizothoracine fishes.

Germ cells (Gc) propagate the genetic information to subsequent generations. Diploid (2n) Gc get transformed to specialized haploid (n) gametes by mitotic and meiotic divisions in adult gonads. Retinoic acid (RA), an active derivative of vitamin A (retinol), plays a critical role in organ morphogenesis and regulates the meiotic onset in developing Gc. Unlike ovaries, fetal testes express an RA-degrading enzyme CYP26B1, and thereby, male Gc fail to enter into meiosis and instead get arrested at G 0 /G 1 stage, termed as gonocytes/pro-spermatogonia by embryonic (E) 13.5 days. These gonocytes are transformed into spermatogonial stem/progenitor cells after birth (1–3 days of neonatal age). During post-natal testicular maturation, the differentiating spermatogonia enter into the meiotic prophase under the influence RA, independent of gonadotropic (both FSH and LH) support. The first pulse of RA ensures the transition of undifferentiated type A spermatogonia to differentiated A 1 spermatogonia and upregulates STRA8 expression in Gc. Whereas, the second pulse of RA induces the meiotic prophase by augmenting MEIOSIN expression in differentiated spermatogonia B. This opinion article briefly reviews our current understanding on the RA-driven spermatogonial differentiation in murine testes.

Background The classification of the sub-family Schizothoracinae has been debatable due to the overlap in morphological characters. There are discrepancies between classical taxonomy and molecular taxonomy, as well. In the present study, mitochondrial genes CO-I and Cyt b were sequenced to elucidate the phylogenetic status of three species of the genus Schizothorax . Methods and results In total, 29 samples of three species viz., S. plagiostomus , S. progastus , and S. richardsonii , were collected from rivers of Uttarakhand, India. For phylogenetic analyses, 40 sequences of CO-I and 41 sequences of Cyt b of Schizothoracinae species were downloaded from NCBI. The highest genetic divergence based on CO-I (16.08%) is between S. plagiostomus and Ptychobarbus dipogon , while the lowest divergence (0.00%) is between 10 pairs of species. The highest divergence based on Cyt b (19.43%), is between S. niger and Gymnocypris eckloni , while the lowest divergence (0.00%) is between four pairs of species. The divergence (0.00% for CO-I and 2.38% for Cyt b ) between S. chongi and S. kozlovi , seems a case of convergent molecular evolution of the CO-I gene and in this case, CO-I alone cannot be used to differentiate these two species. Conclusion The simultaneous use of two molecular markers along with morphomeristic data is a better strategy for the classification of the sub-family Schizothoracinae. These results will be a resource dataset for determining the taxonomical status of Schizothoracine species and will help in the conservation and commercial production of these commercially important fish species.