Partially redundant proneural function reveals the importance of timing during zebrafish olfactory neurogenesis

Little is known about proneural gene function during olfactory neurogenesis in zebrafish. Here, we show that the zebrafish Atonal genes neurogenin1 (neurog1) and neurod4 are redundantly required for development of both early-born olfactory neurons (EONs) and later-born olfactory sensory neurons (OSNs). We show that neurod4 expression is initially absent in neurog1 mutant embryos but recovers and is sufficient for the delayed development of OSN. By contrast, EON numbers are significantly reduced in neurog1 mutant embryos despite the recovery of neurod4 expression. Our results suggest that a shortened time window for EON development causes this reduction; the last S-phase of EON is delayed in neurog1 mutant embryos but mutant EONs are all post-mitotic at the same stage as EONs in wild-type embryos. Finally, we show that expression of certain genes, such as robo2, is never detected in neurog1 mutant EONs. Failure of robo2 expression to recover correlates with defects in the fasciculation of neurog1 mutant olfactory axonal projections and in the organisation of proto-glomeruli because projections arrive at the olfactory bulb that are reminiscent of those in robo2 mutant embryos. We conclude that the duration of proneural expression in EON progenitors is crucial for correct development of the zebrafish olfactory system.