The C. elegans Hand gene controls embryogenesis and early gonadogenesis.

The C. elegans genome encodes a single Hand bHLH transcription factor. Either hnd-1(RNAi) or a hnd-1 deletion causes partially penetrant defects in viability and gonadogenesis. Dead embryos and young larvae are often misshapen at the posterior end. Our primary focus has been the role of hnd-1 in gonadogenesis. Wild-type C. elegans has two somatic gonadal precursors and two primordial germ cells in stereotyped positions within its four-celled gonadal primordium. The hnd-1 gene affects the presence and position of both the somatic gonadal precursors and primordial germ cells within the primordium, but does not appear to have any role in later gonadogenesis. hnd-1 probably acts within the somatic gonadal precursors or their mesodermal predecessors; defects in primordial germ cells and germ line appear to be secondary. In hnd-1 mutants, somatic gonadal precursors are generated normally, but are not maintained properly and sometimes die. A similar role in controlling the maintenance of precursor fates has been described for other genes governing early organogenesis, including the zebrafish Hand gene hands off. We also report the discovery of two genes, ehn-1 and ehn-3, that have overlapping functions with hnd-1 in embryogenesis and gonadogenesis.