LAUSR

Abstract Olive flounder Paralichthys olivaceus is an important cultured marine fish. We found that the meiosis marker scp3 and its intrinsic regulator dazl were mainly expressed in the gonads. During the ovarian differentiation, scp3 signal was detected first in pre-meiotic oogonia at 60-mm total length (TL) and then in primary oocytes at 80- and 100-mm TL, with a sharp increase in scp3 expression level observed at 80- and 100-mm TL. Dazl signal was detected in primordial germ cells at 30-mm TL and oogonia at 60-mm TL, but no significant change of expression was observed. During the testicular differentiation period, scp3 and dazl expression remained at low levels, and scp3 signal was weakly detected in spermatogonia at 80-mm TL, whereas dazl signal was not found. During the ovarian developmental stages, the highest expression levels of scp3 and dazl were detected at stages I and II, respectively, and strong signals of scp3 and dazl were detected in primary oocytes and oocytes at phases I and II. In the testis, the high expression of scp3 and dazl was detected at stages II–IV and II–III, respectively. Scp3 signal was weakly observed in pre-meiotic spermatogonia at stages I and II and strongly detected in primary spermatocytes at stages III–V. Dazl was detected in the nuclei of spermatogonia and spermatids at stages II–IV. Furthermore, scp3 expression in the ovary could be promoted by 17α-ethynylestradiol and tamoxifen, whereas dazl expression could be downregulated by tamoxifen. Summary Sentence The research shows the meiosis begins in the flounder gonads later than the gonadal differentiation. Graphical Abstract