The C. elegans ventral nerve cord (VNC) is comprised of iterative lineages of motor neurons. A subset of neurons in the VNC are derived from the precursor cell Pn.aap and are sexually dimorphic, consisting of six VCs in hermaphrodites and nine pairs of CAs and CPs in males. In males, previous research has found that HOM-C/Hox genes
lin-39 and
mab-5 are required for survival and division of Pn.aap and
lin-39 is further required for the serotonergic fate of CP1-6. The functions of LIN-39 and MAB-5 are such that the anterior VNC is defined by
lin-39 expression in CA/CP1-4, the middle by overlapping
lin-39 and
mab-5 expression in CA/CP5-6, and the posterior by
mab-5 expression in CA/CP7-9. Thus, regionalization of the VNC along the A-P axis is apparent in the descendents of the Pn.aap lineage.
We performed a genetic screen for defective Pn.aap development using the male-specific serotonergic marker
tph-1::gfp. We isolated a mutant strain that loses expression of
tph-1::gfp in CP5-6 but retains expression in CP1-4. Whole genome sequencing revealed a G to A transition in the splice acceptor for the fifth coding exon of
lin-39. rtPCR analysis indicates that
lin-39(
ccc16) mutants make two abnormal splice isoforms, both likely truncating the protein after the homeodomain. Sequencing also revealed an amino acid substitution in the closely linked
lin-13 locus. Our mutant line is ts sterile, possibly due to the
lin-13 mutation. We are currently investigating whether this mutation also contributes to VNC phenotypes.
tph-1::gfp expression is absent in
lin-39(null) mutant CPs and hypomorphic mutations result in random loss of
tph-1::gfp expression in CPs, suggesting
lin-39(
ccc16) is not simply a weak loss-of-function mutation. Despite the normal expression of
tph-1::gfp in CP1-4,
lin-39(
ccc16) lacks expression of the CA reporter
ida-1::gfp in CA1-4, similar to null mutants. Thus, this mutant may reveal the mechanism by which
lin-39 contributes to survival, division, and fate differently in CA/CP1-4 than CA/CP5-6 and may shed light on sex-specific neuronal development in the Pn.aap lineage.