In C. elegans males, regional identities (including neurotransmitter expression, cell division, and survival) of ventral cord CP neurons are defined by overlapping zones of Hox transcription factor activity: Zone 1 is characterized by
lin-39-dependent expression of
tph-1::mCherry and
flp-22::gfp in CPs 1-4; Zone 2 is characterized by
lin-39-dependent expression of
tph-1::mCherry in CPs 5-6; Zone 3 is characterized by
mab-5-dependent expression of
flp-21::gfp in CPs 7-9. In Zone 2,
flp-22::gfp and
flp-21::gfp are each repressed in a manner that depends on
mab-5 and
lin-39 respectively, indicating that MAB-5 and LIN-39 act reciprocally to define aspects of CP fate. As LIN-39 and MAB-5 coordinately specify Zone 2 fates, we are exploring possible mechanisms for this interaction. The TALE homeodomain proteins CEH-20 and UNC-62 are good candidates to modulate MAB-5 and LIN-39 function cell-specifically. Consistent with this, we find that RNAi targeting
ceh-20 or
unc-62 phenocopies
lin-39, but not
mab-5, resulting in loss of
tph-1::mCherry and
flp-22::gfp, and expansion of
flp-21::gfp into Zone 2. Strikingly, this expression of
flp-21::gfp suggests that MAB-5 does not require
ceh-20 or
unc-62 to promote Zone 3 fates. Thus, CEH-20 and UNC-62 may be required for LIN-39 to exhibit prevalence over MAB-5 in Zone 2. The homeotic mutation
lin-39(
ccc16) specifically disrupts
lin-39's ability to prevail over MAB-5 in Zone 2, transforming Zone 2 CP fates into Zone 3 fates without altering fates of Zone 1 CPs. Because
lin-39(
ccc16) preferentially affects CPs in which both MAB-5 and LIN-39 are active, we hypothesize that the protein encoded by
lin-39(
ccc16) can specify CP fates only in cells lacking MAB-5 function (such as those in Zone 1). Consistent with this, the
mab-5(
e1239) mutation restores normal Zone 2 fates in
lin-39(
ccc16) males. We hypothesize that prevalence of LIN-39 over MAB-5 in Zone 2 depends on a cofactor interaction that is disrupted by
lin-39(
ccc16). A model in which
lin-39(
ccc16) is able to bind DNA but unable to interact with a cofactor is supported by the nature of the
ccc16 lesion, which truncates the mRNA after the homeodomain-encoding exons. We are pursuing biochemical, genetic, and cis-regulatory analysis to further explore how reciprocal activity of neighboring Hox genes is coordinated in neurons.