The specification of cell fates on along the anteroposterior axis by Hox transcription factors is a conserved feature among metazoans. In the C. elegans ventral cord, fates of male-specific CA and CP neurons are regionally specified by Hox proteins, with overlapping domains of LIN-39 and MAB-5 activity defining three zones of neuronal fate. 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, the absence of
flp-22::gfp and
flp-21::gfp depends on
mab-5 and
lin-39 respectively, indicating that both MAB-5 and LIN-39 act reciprocally to define aspects of CP fate. Thus Zone 2 provides an opportunity to explore how activity of neighboring Hox genes is coordinated.A screen for altered CP specification identified
lin-39(
ccc16), an unusual allele that lacks in which
tph-1::mCherry is absent in Zone 2 CPs, but retained in Zone 1 CPs.
lin-39(
ccc16) mutants express
flp-22::gfp in Zone 1, but fail to repress
flp-21 in Zone 2.
lin-39(
ccc16) phenotypes differ from
lin-39(
n1760) null mutants, which lack ventral cord expression of
tph-1::mCherry and
flp-22::gfp entirely. They are also not typical of
lin-39 hypomorphs, which tend to lose expression of these reporters sporadically across Zones 1 and 2. Instead,
lin-39(
ccc16) seems to affect CPs in Zone 2 specifically. The
lin-39(
ccc16) lesion affects the splice acceptor for the final coding exon of
lin-39. rtPCR analysis indicated that
lin-39(
ccc16) mutants make two abnormal splice isoforms, both likely truncating the protein after the homeodomain. Our findings suggest a model in which the protein encoded by
lin-39(
ccc16) can specify CP fates in the absence of MAB-5 (Zone 1) but does not negotiate the MAB-5 interactions necessary to specify CP fates in Zone 2. In contrast,
lin-39(
ccc16) CAs display a phenotype similar to
lin-39(
n1760), suggesting a different mechanism at work in CAs.
lin-39(
ccc16) mutants also display regionally biased Pn.p fusion defects, with Pn.p cells in Zone 1 resembling WT and in Zone 2 demonstrating abnormal P cell fusion in both sexes. We are currently analyzing
lin-39(
ccc16)
mab-5(-) double mutants to further explore Hox interactions in male ventral cord neurons. .