Abstract
Chemical communication via pheromones is considered the oldest and most widespread form of communication in nature. However, the way that the enormous diversity of species-specific pheromones evolved is still of debate. One possible process driving pheromone evolution is the mate-finding and dispersal behavior, as long-distance mate-finding requires highly volatile compounds. In contrast, less volatile compounds might be sufficient attractants in species that search for mates within proximity. In the parasitoid wasp genus Leptopilina, the composition of species-specific sex pheromones ranges from highly volatile iridoid compounds through combinations of iridoids with low volatile cuticular hydrocarbons (CHCs) to only CHCs. To study the selective forces shaping the composition of sex pheromones in Leptopilina, we examined the dispersal behavior, i.e. the proportion of male and female wasps dispersing after emergence, in four species with known sex pheromone compositions. If males and females disperse immediately, long-range mate attraction might become necessary, favoring volatile iridoids over CHCs. If mating occurs directly on the host patch, short-range mate attraction by low volatile CHCs might suffice. Our analyses have revealed that the dispersal behavior of Leptopilina males and females after emergence does indeed differ between species with differently volatile sex pheromones. Specifically, males of species with iridoid sex pheromones start to disperse immediately before their females’ emergence, whereas males of species with CHC sex pheromones delay dispersal until their conspecific females emerge. While the differences in female dispersal behavior turned out to be species-specific, differences in male dispersal correlated with the volatility of female-produced sex pheromones of each species. This study significantly contributes to our understanding of the evolution of sex pheromones by differences in dispersal behavior.
Highlights
Semiochemicals that mediate a variety of intra- and interspecific communication such as the attraction and finding of mating partners, hosts, or prey, or the deterrence of enemies, are ubiquitous in nature and show a remarkable diversity (Wyatt, 2014)
Proportions of dispersed female individuals did not differ between L. pacifica and L. ryukyuensis within the first 48 h after emergence, but afterward those of L. pacifica were lower than those of L. ryukyuensis. Albeit these species-specific differences in female dispersal, we found that when we combined species according to the volatility of their sex pheromones, the mean proportion of dispersed female wasps of species with highly volatile iridoid sex pheromones, namely of L. heterotoma and L. japonica, was significantly higher than that of females of L. pacifica and L. ryukyuensis, which have sex pheromones consisting of less volatile cuticular hydrocarbons (CHCs) (Figure 5)
Within the genus Leptopilina, all species studied to date have different combinations of low volatile CHCs and/or volatile iridoids as female-produced sex pheromones (Weiss et al, 2013, 2015; Pfeiffer et al, 2018; Böttinger et al, 2019; Böttinger et al submitted)
Summary
Semiochemicals that mediate a variety of intra- and interspecific communication such as the attraction and finding of mating partners, hosts, or prey, or the deterrence of enemies, are ubiquitous in nature and show a remarkable diversity (Wyatt, 2014). Dispersal Shapes Sex Pheromone Evolution to various compound classes (see, e.g., El-Sayed, 2018 at http://www.pherobase.com, which lists >3500 semiochemical compounds), are used alone or in combination, and range from small, light, volatile molecules to large, non-volatile compounds with higher molecular weights. These chemical messengers have a variety of different physicochemical properties (e.g., solubility in water for sex pheromones in fish, volatility in airborne signals, Mollo et al, 2017), the prerequisites of semiochemicals should be similar for each form and function of chemical communication. Because of the necessity for species-specificity in mate-finding, the compounds used for sexual communication are extremely diverse and show large variations in composition, even among closely related species (Symonds and Elgar, 2008)
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