Abstract

Gonadogenesis in zebrafish goes through an initial ovarian phase then subsequently into either ovarian or testicular phases. How germ cells choose to commit to an oogenic fate and enter meiosis or alternatively not enter meiosis and commit to a spermatogenetic fate remains a key question. This study investigated events of early gonadogenesis in zebrafish with the aim of unraveling the events surrounding the mitotic/meiotic transition in juvenile ovaries. Primordial germ cells were identified at eight days post fertilization (dpf).  Mitotic divisions were apparent at 15 dpf, and meiosis initiated in some gonads after 22 dpf. After 40 dpf, female gonads contained various germ cells including oogonia, post-pachytene and early pre-vitellogenic oocytes, whereas in some gonads, degenerative post-pachytene oocytes and proliferating germ were observed. The occurrence degenerating oocytes as well as a relatively larger proportion of proliferating "gonial" germ cells in the latter gonads was considered the first indication of spermatogenic activity, marking the onset of secondary (testicular) gonadogenesis. It could not be determined here whether primordial germ cells were involved in secondary gonadogenesis. While the mechanisms of this phenomenon in zebrafish ovaries are not well addressed, here it can be seen in the context of an apoptotic regulation.

Key words: Danio rerio, gonad, mitotic/meiotic transition, development, sex inversion.