The C. elegans AWC olfactory neuron pair asymmetrically differentiates into two subtypes, default AWCOFF and induced AWCON, with distinct functions after general identity is specified. The Otx homeodomain transcription factor CEH-36 acts as a terminal selector of general AWC identity by directly regulating expression of AWC identity features. In addition, the HMX/NLX homeodomain protein MLS-2 is a transiently expressed inducer of
ceh-36 expression. However, AWC identity is only partially abolished in
mls-2 loss-of-function mutants, suggesting additional factors may have a partial redundant role to induce
ceh-36 expression in the specification of AWC identity. We identified
unc-62, encoding a homothorax homeodomain protein, from a small candidate gene approach screen for mutants of transcription factors with defective AWC identity. Like
mls-2 loss-of-function mutants,
unc-62 reduction-of-function mutants show a partial loss of general AWC identity. In addition,
unc-62 and
mls-2 double mutants show a nearly complete loss of AWC identity, suggesting that
unc-62 and
mls-2 have a partial redundant role in the specification of general AWC identity. In addition to a defect in general AWC identity, our results show that
mls-2 and
unc-62 mutants also display defective AWC asymmetry.
mls-2 and
unc-62 single mutants have a high penetrance of 2AWCOFF phenotype, in which both AWC neurons become the default AWCOFF subtype, indicating an essential role of
mls-2 and
unc-62 in the specification of the induced AWCON subtype. In addition, our genetic studies suggest that
mls-2 and
unc-62, with
mls-2 acting upstream of
unc-62, acts cell autonomously downstream of
nsy-4 (claudin-like protein) to suppress the
unc-43(CaMKII)-mediated calcium signaling in promoting the AWCON subtype. Consistent with the genetic data, we show that the number of cells expressing
mls-2 is significantly reduced in
nsy-4 mutants. Furthermore, our results suggest that
mls-2 and
unc-62 have separable functions in the specification of general AWC identity and AWC asymmetry. Thus, our study provides novel findings that
mls-2 and
unc-62 act redundantly to specify general AWC identity in early embryogenesis and are subsequently repurposed in the specification of asymmetric AWC subtypes in late embryogenesis.