In metazoans, Hox transcription factors provide positional cues that define neuronal identities on the anterior-posterior (AP) body axis; disruptions to Hox genes can cause transformation of neuronal identities. In C. elegans males, regional identities of ventral cord CP neurons are defined by overlapping zones of Hox 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. The homeotic mutation
lin-39(
ccc16) transforms fates of CPs in Zone 2 into CPs with Zone 3 identities. In
lin-39(
ccc16) mutants,
tph-1::mCherry is expressed in Zone 1 CPs, but is absent in Zone 2 CPs.
lin-39(
ccc16) males express
flp-22::gfp normally in Zone 1, but fail to repress
flp-21::gfp in Zone 2. Because
lin-39(
ccc16) preferentially affects only CPs in which both MAB-5 and LIN-39 are active, we hypothesize that the protein encoded by
lin-39(
ccc16) is able to specify CP fates in the absence of MAB-5 (Zone 1) but cannot achieve the MAB-5 interactions necessary to specify CP fates in Zone 2. If this is the case,
mab-5 mutations should restore normal
tph-1::mCherry expression to Zone 2 in a
lin-39(
ccc16)
mab-5(0) double mutant. Our preliminary genetic analysis supports this idea:
mab-5(
e1239) restores
tph-1::mCherry expression in CPs 5-6 in
lin-39(
ccc16) males. To further explore regulation of neurotransmitter gene expression in the male ventral cord, we have initiated a course-based undergraduate research collaboration between students at St. Catherine University (SCU) and at Carleton College (CC). During Fall 2016, SCU genetics students constructed
tph-1::
pes-10::gfp reporters containing fragments of the
tph-1 regulatory region. These were analyzed by CC genetics students during Winter 2017, revealing preliminarily that a 485 bp
tph-1 regulatory region drives robust GFP expression in CPs 5-6. In addition to forwarding research goals, this collaboration requires students to connect concepts of cis-regulatory elements with gene expression. Our
tph-1 promoter analysis, along with the Zone-2-specificity of the
lin-39(
ccc16) phenotype, lends support to the idea of a Zone-2-distinct program of neuronal fate specification that has the potential to shed light on how activity of neighboring Hox genes is coordinated.