Medaka factor in the germline alpha (figα) is indispensable for oogenesis but not for gonadal sex differentiation

A basic helix-loop-helix transcription factor, figα, is one of the earliest marker genes of oocyte differentiation in vertebrates. In the present study, we made figα knockout medaka by CRISPR/Cas9, expecting aborted progress of oogenesis, to see if differentiation of somatic ovarian tissues is affected. Figα knockout male gonads differentiated normally into testes with functional sperm. The females, on the other hand, were sterile; there are oocytes only up to pachytene. No growing oocytes in diplotene were found. The phenotype was already apparent at 10 days after hatching, when diplotene oocytes start to develop in the control. Furthermore, several putative target genes of figα were not expressed in the mutant female gonads. Previous studies showed that medaka lacking germ cells have morphologically abnormal gonads and female to male sex reversal occurred. Figα knockout female gonads differentiated morphologically into ovaries, suggesting that in medaka, figα knockout prevents oogenesis progress into diplotene stage, and ovarian differentiation do not need developed oocytes beyond pachytene stage. This ovarian phenotype reminded us of "pachytene checkpoint", which blocks meiotic progress into diplotene when chromosomal abnormalities are present. Therefore, we further analyzed phenotype of oocytes in the mutant ovaries. The number of oocytes at each meiotic stage suggested that oogenesis normally proceeded up to pachytene stage. The number of apoptotic oocytes in the knockout ovaries was small. Finally, we did not detect any abnormal pairings or unrepaired double-strand breaks by immunostaining. Taken together, figα knockout likely stop oocyte growth at or around pachytene via unknown mechanisms other than "pachytene checkpoint" and arrested oocytes remained in the ovary. This is in contrast to figα knockout mice where all oocytes underwent apoptosis quickly after birth.