Molecular basis of the nematode volatile sex pheromone perception

Both sexually matured C. remanei virgin females and self-sperm exhausted C. elegans hermaphrodites release a long-range volatile sex pheromone to attract adult males from afar. Previously, we reported that this chemotaxis behavior requires CEM, AWA and AIZ neurons. In AWA, a GPCR SRD-1 has been demonstrated as the receptor of this pheromone. Ectopic expression of srd-1 in AWB in srd-1 male elicited distinct repulsive behaviour towards pheromone; this result indicated the sufficiency of the SRD-1 receptor acting in AWA. From calcium imaging, we further confirmed the receptor was necessary for the excitation of AWA upon pheromone induction. Male C. elegans is the least attracted to pheromone in comparison to C. briggase, C. remanei, and C. brenneri. Endogenous srd-1 expression level is lower than that of its orthologue, cre-srd-1, in male C. remanei. Transformation rescue of the male srd1 mutant in C. elegans by cre-srd-1 cDNA was shown to be more responsive to pheromone than those rescued by srd-1 cDNA. Based on protein structure analysis and protein sequence alignment, SRD-1 receptors across four nematode species were highly conserved in terms of their sequences and structure, except their C-terminal region (CT). Six amino acid sequences are polymorphic in C. elegans SRD-1 extracellular loop regions (ECL). The ECL polymorphisms substituted versions of SRD-1 cDNA cannot elicit a higher responsiveness to this pheromone. However, CT truncated cre-srd-1 and srd-1 cDNA are unable to rescue srd-1 chemotaxis mutant phenotype, suggesting the CT region to be critical for SRD-1 function. Specifically, after the CT domain was swapped between Cre-SRD-1 and SRD-1, the transgenic line carrying cre-srd-1-CT-swapped-in-srd-1 cDNA is more attracted to this pheromone than the one carrying srd-1-CT-swapped-in-cre-srd-1 cDNA. Since the CT region in GPCR is usually related to the intracellular signal transduction, we envision that both the expression level of gene product and intracellular signal transduction relay component may account for the differential response of different species in sex pheromone perception. (The study is supported by Research Grants Council, Hong Kong.)