Fig. 5

Schematic diagram of the different possible pathways in inducing the pre-ovulatory LH surge along the HPG axis in female zebrafish. In general, the accepted dogma is that the hypophysiotropic GnRH and its neurons are critical for LH surge, and ultimately oocyte maturation, ovulation, and spawning (A). Our current results suggest that GnRH3 neuron-ablated zebrafish females display normal fertility, and previous studies suggest four alternative pathways (B–E): I. Hypophysiotropic GnRH3 has no role, but other brain factors (e.g., vasoactive intestinal peptide, kisspeptin, neurokinin B, and secretoneurin) trigger LH surge (B); II. Functional redundancy—GnRH3 has a role, but is not the sole and only player, thus sharing this function with other factors (C); III. Ovarian estradiol-17b (E₂) directly induces the LH surge and controls ovulation (D); IV. In GnRH3 neuron-ablated females, a few remaining hypophysiotropic GnRH3 neurons are possibly sufficient to elicit LH surge (E). The question marks indicate that these pathways remain to be established in zebrafish.