The C. elegans male tail is an ideal system to understand how developmental cues converge to regulate morphogenesis. During L4, the four hypodermal cells of the male tail tip fuse and retract to form the rounded tail tip syncytium of the adult. At the core of the regulatory network controlling this process lie three transcription factors, NHR-25, DMD-3 and MAB-3, that regulate downstream effectors of morphogenesis (see poster by Herrera et al.). The onset of morphogenesis is controlled by the heterochronic pathway, which regulates the timing of
dmd-3 expression in the tail tip. Precocious heterochronic mutants (e.g.,
lin-41(lf)) display early
dmd-3 expression and premature retraction, while retarded mutants (e.g.,
let-7 and
lin-41(gf)) exhibit delayed or absent
dmd-3 expression and unretracted and unfused ("Lep") tail tips even in adults. In mutagenesis screens for male tail mutants, we identified two alleles of a novel heterochronic gene,
lep-5. These mutants exhibit severe Lep tails and show delayed activation of
dmd-3 in the posteriormost tail tip cell,
hyp10.
lep-5 also has other heterochronic characteristics:
lep-5 mutants of both sexes undergo supernumerary molts as adults, and
lep-5 phenotypes are suppressed by transition through the dauer stage. Epistasis experiments indicate that
lep-5 acts upstream of or in parallel with
let-7 and
lin-41.
lep-5(
ny10) is a large deletion removing several genes, whereas
lep-5(
fs8) is a point mutation in the 5' regulatory region of a predicted protein of unknown function. ESTs and RT-PCR analysis have shown that
lep-5 produces a transcript that is SL1 trans-spliced and polyadenylated. However, several lines of evidence suggest that
lep-5 is not a protein, but rather a non-coding RNA. First, the single point mutation (G>A) found in
lep-5(
fs8) disrupts the predicted secondary structures of two stem loops at the 5' end of the
lep-5 transcript. Restoration of the secondary structure in this area with a compensatory mutation rescues the mutant phenotype. Second, there is no conservation of the
lep-5 sequence in Caenorhabditis at the protein level, although the nucleotide sequence is well conserved. Third, the
lep-5(
ny10) mutant can be fully rescued by the homologous sequence from C. briggsae. Interestingly, the sequence of the 5' region of the transcript (including SL1) has the potential to fold into a stem loop bearing a sequence similar to a known miRNA with which
lep-5 has some functional redundancy. We are currently testing the possibility that
lep-5 functions as a miRNA.