Copulation in C. elegans males requires a nuclear hormone receptor

In Caenorhabditis elegans, uncoordinated (unc)-55 encodes a nuclear hormone receptor that is necessary for coordinated movement and male mating. An unc-55 reporter gene revealed a sexually dimorphic pattern: early in post-embryonic motor neurons in both sexes; and later in a subset of male-specific cells that included an interneuron and eight muscle cells. A behavioral analysis coupled with RNA interference (RNAi) revealed that males require UNC-55 to execute copulatory motor programs. Two mRNA isoforms (unc-55a and unc-55b) were detected throughout post-embryonic development in males, whereas only one, unc-55a, was detected in hermaphrodites. In unc-55 mutant males isoform a rescued the locomotion and mating defect, whereas isoform b rescued the mating defect only. Isoform b represents the first report of male-specific splicing in C. elegans. In addition, isoform b extended the number of days that transgenic unc-55 mutant males mated when compared to males rescued with isoform a, suggesting an anabolic role for the nuclear hormone receptor. The male-specific expression and splicing is part of a regulatory hierarchy that includes two key genes, male abnormal (mab)-5 and mab-9, required for the generation and differentiation of male-specific cells. We suggest that UNC-55 acts as an interface between genes involved in male tail pattern formation and those responsible for function.