The basic helix-loop-helix transcription factors LIN-32 and HLH-2 functiontogether in multiple steps of a C-elegans neuronal sublineage

bHLH transcription factors function in neuronal development in organisms as diverse as worms and vertebrates. In the C. elegans male tail, a neuronal sublineage clonally gives rise to the three cell types (two neurons and a s tructural cell) of each sensory ray. We show here that the bHLH genes lin-3 2 and hlh-2 are necessary for the specification of multiple cell fates with in this sublineage, and for the proper elaboration of differentiated cell c haracteristics. Mutations in lin-32, a member of the atonal family, can cau se failures at each of these steps, resulting in the formation of rays that lack fully-differentiated neurons, neurons that lack cognate rays, and ray cells defective in the number and morphology of their processes, Mutations in hlh-2, the gene encoding the C, elegans E/daughterless ortholog, enhanc e the ray defects caused by lin-32 mutations, In vitro, LIN-32 can heterodi merize with HLH-2 and bind to an E-box-containing probe, Mutations in these genes interfere with this activity in a manner consistent with the degree of ray defects observed in vivo, We propose that LIN-32 and HLH-2 function as a heterodimer to activate different sets of targets, at multiple steps i n the ray sublineage. During ray development, lin-32 performs roles of pron eural, neuronal precursor, and differentiation genes of other systems.