Sex‐specific pheromone responses in Caenorhabditis elegans

Abstract

The characterization of receptors for sex pheromones provides important clues for understanding the mechanisms controlling animal mating and reproduction. In this issue, Wan et al 1 identify a putative sex pheromone receptor, the G protein‐coupled receptor SRD‐1, acting in a single Caenorhabditis elegans olfactory neuron class, called AWA, to mediate male attraction to volatile sex pheromones. Like in other systems, C. elegans sex pheromone elicits sex‐specific responses even though sex pheromone activates sensory neurons of both sexes.

Mate choice often relies on highly specialized chemoattraction behaviors, initiated by chemosignals termed sex pheromones. Although the existence of sex pheromones in humans is controversial to this day, their contribution to specialized sexual behaviors in most other animals is evident 2. Specific sensory neurons respond to these pheromones to produce sexually dimorphic behaviors 2. How sex pheromones can trigger distinct behavior in opposite sexes has been a matter of intense investigation. Do differences in behavioral responses arise from sexually dimorphic expression of pheromone receptors? Or are different responses the result of the engagement of distinct neuronal circuits downstream of a non‐dimorphic receptor system?

Two examples in mice and fruit flies are in support of the notion that sexually dimorphic behavioral responses to some sex pheromones derive from sexually dimorphic neural circuits and not from sexually dimorphic pheromone perception. In mice, the main olfactory epithelium and the vomeronasal organ (VNO) detect sex pheromones. One of the VNO‐detected pheromones, the male‐specific exocrine gland‐secreting peptide, is sensed by the G protein‐coupled receptor (GPCR) V2Rp5 in both females and males. However, it triggers different behaviors in the two sexes—sexual receptivity in females and enhanced aggressiveness in males (reviewed in 2). The behavioral difference is unlikely due to dimorphic receptor expression, but is rather the result of sexually dimorphic circuitry in the amygdala–hypothalamus axis 3. In fruit flies, the main olfactory organ, antenna, and the labellum and foreleg detect various types of pheromones. The male sex pheromone 11‐cis‐vaccenyl acetate, detected by the receptor Or67d, activates the same olfactory receptor neurons in the antenna of both sexes, yet it elicits sexually dimorphic behaviors (reviewed in 2). Similarly to what happens in mice, such dimorphisms arise from sexually dimorphic circuitry and different neurons in a structure analogous to the mouse amygdala 4.

Non‐dimorphic pheromone perception in both mice and flies is consistent with the notion that the vast majority of mouse and fly GPCRs do not show any sexually dimorphic expression 2, 3. Superficially, a similar situation appears to exist in Caenorhabditis elegans. Even though C. elegans also responds in a sexually dimorphic manner to sex‐specific pheromones (reviewed in 5), there are remarkably few GPCRs that are dimorphically expressed in the olfactory system of the worm 6, 7. One notable exception is the srd‐1 gene, originally noted by Troemel et al to be expressed in a sexually dimorphic manner 6. In this issue, Wan et al elucidate a function for srd‐1 in the context of male attraction to a volatile sex pheromone of presently unknown composition that is derived from sperm‐depleted C. elegans hermaphrodites and female Caenorhabditis remanei 1, 8. Using a behavioral assay that measured chemoattraction of C. elegans males, the authors defined a pair of olfactory neurons, the AWA neurons, as critical components of long‐range attraction of males to hermaphrodite/female pheromone. The AWA neurons are among the first olfactory neurons to be functionally defined in C. elegans (reviewed in 9). Male and hermaphrodite AWA neurons display differential sensitivity to food odors due to sex‐specific modulation of the expression levels of the diacetyl receptor ODR‐10 (reviewed in 10). To identify the GPCR involved in volatile sex pheromone perception, Wan et al immunoprecipitated mRNA from AWA to uncover a handful of AWA‐expressed GPCRs. In concordance with previous GPCR reporter gene studies 6, 7, most appeared to be non‐dimorphically expressed but one, SRD‐1, the GPCR previously noted to be expressed dimorphically in the ADF neurons 6, showed expression only in male, but not hermaphrodite AWA (Fig 1), thereby providing a mirror image of hermaphrodite‐specific ODR‐10 expression. Chemoattraction assays showed that loss of SRD‐1 from AWA impaired male attraction to the hermaphrodite sex pheromone (Fig 1). Moreover, ectopic expression of SRD‐1 in the AWB olfactory neurons, which normally drive aversive olfactory responses (reviewed in 9), reprogrammed pheromone attraction to pheromone repulsion (Fig 1). Furthermore, GCaMP calcium imaging experiments demonstrated that SRD‐1 was required for pheromone‐induced AWA calcium current in males (Fig 1).

Figure 1. Sexually dimorphic perception of the volatile hermaphrodite/female sex pheromone in male Caenorhabditis elegans .

Only wild‐type C. elegans males but not hermaphrodites get attracted to volatile sex pheromones (VSP) derived from self sperm‐depleted hermaphrodite C. elegans or female Caenorhabditis remanei. This dimorphic behavior is reportedly regulated through SRD‐1's functions in male AWAs.

Even though an srd‐1 reporter transgene was expressed in a sexually dimorphic manner in male but not hermaphrodite AWA neurons, and even though sex pheromones induced male‐specific behavioral responses, Wan et al found that their sex pheromone preparation could elicit SRD‐1‐dependent calcium responses in the AWA neurons of both sexes (Fig 1). It is conceivable that low levels of functionally relevant SRD‐1 expression in hermaphrodites could not be detected by reporter gene analysis, but even if so, the question remains how excitation of AWA in both sexes can lead to sex‐specific behavioral responses. A more detailed comparison of the calcium responses in males and hermaphrodites will need to be undertaken in the future to assess whether differences in the activation profiles could perhaps be responsible for triggering distinct downstream responses. Since the full spectrum of cells that respond to the sex pheromone preparation of Wan et al has not yet been reported, it is also conceivable that other neurons respond to sex pheromone to subsequently modulate the output of SRD‐1‐dependent, non‐sex‐specific AWA activation. Indeed, the ASI neurons, which express srd‐1 in both sexes, have previously been shown to modulate the AWA response to a distinct sex pheromone preparation (reviewed in 5), but differences in the assays used by these groups make these studies difficult to compare. In any case, Wan et al's findings display some conceptual similarities to the situation in mice and flies. Pheromone perception appears to be non‐dimorphic, but the manner of how pheromone perception is processed appears to be sex‐specific.

Previous studies in C. elegans point to the existence of two different types of sex pheromones. One is the volatile sex pheromone of unknown composition (reviewed in 5) shown in the current study by Wan et al to be sensed by the olfactory AWA neurons via SRD‐1. The other type consists of a group of nematode‐specific, non‐volatile small molecules termed ascarosides, which trigger sex‐specific attraction or repulsion behaviors and regulate developmental timing in a context‐ and dose‐dependent manner (reviewed in 5, 10). Consistent with their non‐volatile nature, ascarosides are not sensed by the olfactory neurons AWA, but by a distinct set of chemosensory neurons with exposed sensory endings, including the male‐specific CEM neurons and the sex‐shared ADL, ASK, and ADF neurons 5, 10. Responses to ascarosides often depend on sex‐specific perception (e.g., by the male‐specific CEM neurons or by the sex‐specific activation of a sex‐shared neuron), but there are also downstream, circuit‐based mechanisms that control the sex‐specificity of behavioral responses 5, 10. Taken together, worms generate sex‐specific pheromone responses through a number of distinct mechanisms, ranging from sex‐specific sensory neuron activation to sex‐specific modulation of downstream circuitry.

The Wan et al report also ventures into the interesting question of evolution of sex pheromone responses. The authors had previously noted that males from the two closely related species, C. elegans and C. remanei, respond to the volatile sex pheromone differentially—the latter to a greater extent than the former 8. By rescue experiments expressing srd‐1 cDNAs of either species driven by the C. elegans srd‐1 promoter, Wan et al now found that C. remanei SRD‐1 was more responsive to the pheromone than the C. elegans ortholog, indicating that disparities in perception strength were a result of differences in protein coding sequences. Through sequence alignment and in vivo validation, the authors demonstrated that it was the C‐terminal region of SRD‐1 that conferred the pheromone perception differences between the two species. These results further demonstrated that SRD‐1 is a key GPCR responsible for sex pheromone perception.

EMBO Reports (2019) 20: e47599