Development of C. elegans male tail sensory rays involves multiple processes such as ray lineage, ray assembly, patterning and morphogenesis. Previously we identified
tbx-2 as a gene responsible for ray assembly during male tail development. Ray assembly defect can be found in all rays in
tbx-2 mutants, but the expression pattern of
tbx-2 in adult male rays was restricted to the structural cells of rays 1, 5, 7 and occasionally in R4B neurons, as revealed by GFP transcriptional reporter.
tbx-2 is under its own negative regulation in all rays via direct binding of the protein to a negative regulatory element on its locus region; disruption of which results in up-regulation of
tbx-2 expression in all ray structural cells in male tail. We are interested in how
tbx-2 expression pattern is only subjected to tight negative feedback in rays other than ray 1, 5, 7 and what its significance is. Several candidates of modifiers of
tbx-2 expression, including ray patterning genes such as the SMADS, and
ceh-43, a homeobox gene, were evaluated.
tbx-2 reporter activity was altered in SMADS mutants.
tbx-2 expression is reduced in ray 5 in general. Site specific mutation of a
ceh-43 putative binding site - CAATTA - on promoter of
tbx-2 transcriptional GFP reporter results in loss of GFP signal in rays, hinting a direct regulation of
tbx-2 by
ceh-43. Protein-DNA binding assays will be carried out to confirm this direct association. Meanwhile, additional upstream components, e.g.,
lin-32, and its interaction with ray assembly genes has been examined. Our final goal is to delineate the developmental program leading to ray assembly and patterning. The expression pattern of the
tbx-2 also hinted ray-specific function of
tbx-2 on ray identity control. We have investigated this feature using neurotransmitter (NT) production as a ray identity marker in
tbx-2 mutants, where changes in ray specific NT marker expression was noted. For example,
flp-5, originally expressed in neuron B of rays 1, 5, 7 is also expressed in ray 4 of
tbx-2 mutant males. Subsequent temperature shift experiment showed that
tbx-2 is required for the determination step, but not for the maintenance step. Whether such changes are a result of ray identity alteration executed by
tbx-2 expression remains to be resolved. (This study is supported by Research Grants Council, Hong Kong).