scp3 gene expression during ovarian differentiation in Hypoatherina tsurugae (Pisces; Atheriniformes)

The scp3 gene encodes the SYP3 protein, which forms the synaptonemal complex required for pairing homologous chromosomes throughout the prophase of the first meiosis. In addition, it plays a significant role in the formation of germ cells. The 978 bp scp3 mRNA transcript from Hypoatherina tsurugae was cloned and sequenced. The gene comprises an open reading frame (ORF) of 720 bp encoding 240 amino acids (AA), which are identical to those of various other fish species. A phylogenetic tree was created by comparing the mRNA sequences of 50 fish species from the taxa accessible in the NCBI database, utilizing Acipenser ruthenus as an out-group. The tree revealed a higher homology of scp3 from H. tsurugae with Maelanotaenia boesemani, the 2 forming a single clade. Using qRT-PCR, scp3 mRNA transcript expression was investigated and found to be highly expressed in amhy- (females) during the early gonadal differentiation phase, from day 0 to week 10 after hatching. In contrast, amhy+ (males) showed comparatively low expression. The differentiation of male and female gonads was determined six weeks after hatching, based on histological analysis of gonads recovered from biweekly collected larvae throughout the sex determination/differentiation stage. In this phase, primary oocytes are predictable. The data obtained in this study provide crucial information for further understanding the molecular mechanisms involved in sex differentiation and determination in fish.