Establishment of three estrogen receptors (esr1, esr2a, esr2b) knockout lines for functional study in Nile tilapia

Estrogens play fundamental roles in regulating reproductive activities and they act through estrogen receptors (ESRs) in all vertebrates. To date, distinct roles of estrogen receptors have been characterized only in human and model organisms, including mouse, rat, zebrafish and medaka. Physiological role of estrogen/receptor signaling in reproduction remains poorly defined in non-model organisms. In the present study, we successfully generated esr1, esr2a and esr2b mutant lines in tilapia by CRISPR/Cas9 and examined their phenotypes. Surprisingly, the esr1 mutants showed no phenotypes of reproductive development and function in both females and males. The esr2a mutant females showed significantly delayed ovarian development and follicle growth at 90 and 180 dah, and the development caught up later at 360 dah. The esr2a mutant males showed no phenotypes at 90 dah, and displayed smaller gonads and efferent ducts, less spermatogonia and more abnormal sperms at 180 dah. In contrast, the esr2b mutants displayed abnormal development of ovarian ducts and efferent ducts which failed to connect to the genital orifice, and which in turn, resulted in infertility in female and male, respectively, although they produced gametes in their gonads. Taken together, our study provides evidence for differential functions of esr1, esr2a and esr2b in fish reproduction.