Characterization of scp3 and Its Expression Dynamics Identifies the Critical Period for Meiotic Initiation in Female Turbot (Scophthalmus maximus).

Synaptonemal complex protein 3 (Scp3), a structural component of the synaptonemal complex, serves as a key molecular marker for studying meiosis and germ cell development in teleosts. To elucidate the molecular mechanisms of germ cell meiosis and identify critical windows for sex differentiation in turbot (Scophthalmus maximus), a flatfish of considerable economic value in global aquaculture, this study cloned the full-length cDNA sequence of the scp3 gene. The obtained sequence is 1094 bp in length, containing an open reading frame of 717 bp that encodes a polypeptide of 238 amino acids, with bioinformatic analyses revealing characteristic coiled-coil domains. Tissue distribution analysis demonstrated that scp3 expression was predominantly restricted to the gonads. Quantitative PCR in an all-female lineage revealed that transcripts were initially detectable at 25 days post-hatching (dph), remained low until 55 dph, and increased significantly thereafter, peaking after 80 dph (p < 0.05). In situ hybridization localized scp3 mRNA to the cytoplasm of early primary oocytes. The expression window, initiating at 25 dph and intensifying from 60 dph, aligns with the initiation of meiotic prophase I and precedes morphological sex differentiation. These results indicate that scp3 functions as a reliable noninvasive molecular marker for the onset of meiosis in female turbot and may play a specific role in oocyte proliferation. This study provides a crucial theoretical foundation and key temporal information for further elucidating the molecular mechanism of sex differentiation and advancing all-female population cultivation in turbot.