Fig. 3

Gonad development in F. borealis. A Live specimen observed early during the lifecycle (2–3 dpf), with detail of the cuticle around the gonad (A’) and at the tip of the tail (A’’). Scale bar, 100 microns. B Live specimen and fluorescence-stained gonad (B’) at an advanced stage of development (5 dpf). Yellow arrowheads indicate nurse nuclei in the ovary and testis. Scale bar, 30 microns. C Ovary observed late during the lifecycle (6–7 dpf), showing oocytes growing at the periphery of the ovary (green asterisk). D Live specimen before gamete release (7 dpf, left image), and observation of ruptured gonads. At that stage, the ovary cavity consists essentially of mature oocytes, surrounded by reduced nurse nuclei (yellow arrowheads). Gametes are forced through an apical opening of the testis (right image). E Oocyte observed immediately after release and 3D reconstruction of the condensed chromosomes present in the germ vesicle (E’). Yellow arrowheads indicate reduced nurse nuclei remaining on the oocyte surface. Scale bar, 7 microns. Cyan, DAPI; magenta, phalloidin-AF488; cu, cuticle; fi, tail fin; gv, germ vesicle; ho, house; no, notochord; ol, outer layer of the ovary; op, ooplasm; ov, ovary; te, testis