title: Chemical Ecology and Pheromone Evolution in Leptopilina, a Genus of Parasitoid Wasps
Creators: name: Böttinger, Lea
Publisher: name: University of Bayreuth
Year: 2020
Subjects: subject: Pheromone
subject: Evolution
subject: Mating System
subject: Iridoids
subject: Iridomyrmecin
subject: Citral
subject: Cuticular hydrocarbons
subject: Sex pheromone
subject: Chemical defense
subject: Competition avoidance
subject: Dispersal
subject: Insects
subject: Drosophila
subject: 570
subject: 590

description: Chemical communication is ubiquitous and considered the oldest form of communication in nature. The way in which the remarkable diversity of information-transmitting pheromone compounds evolved is still a major topic in chemical ecology. To date, studies concerning differences in pheromone compositions between closely related species have so far focused on phylogenetic comparisons and genetic analyses with explanations such as large, saltational shifts in pheromone evolution during speciation or differential gene regulation leading to changes between species in enzymatic pathways generating the pheromone. Another explanation for the chemical composition of pheromones is the precursor hypothesis, which states that pheromones evolve from compounds formerly used for non-communicative purposes in the same organism. Finding case studies in which one can elucidate this mode of evolution is difficult, as one would need to find a species in which both functions of the same compound are still active, but mostly only the derived function of a pheromone is present and its former function lost. In a species of parasitoid wasps of Drosophila larvae, Leptopilina heterotoma, a rare example of a threefold semiochemical parsimony was found, as the compound (–)-iridomyrmecin serves as defensive secretion, as a cue for female competition avoidance and as part of the female sex pheromone to attract mates from a distance and to elicit the courtship behavior of males in close proximity. This way, an evolutionary route from the non-communicative defensive secretion over a cue to a sex pheromone can be assumed. Iridoids were also found in three other species of the same genus, and while the usage as defensive secretion was found in all species of Leptopilina wasps, the iridoid compounds do not in all species serve as cue for female competition avoidance. Only in L. heterotoma the females’ iridoids evolved as sex pheromones, whereas other species use a combination of cuticular hydrocarbons (CHCs) and iridoids or only CHCs as sex pheromones. To shed light on the pheromone evolution and the way in which the different usages of iridoids evolved in the genus Leptopilina, I studied the chemical ecology of the three species L. japonica, L. pacifica, and L. ryukyuensis. I found that all Leptopilina species studied here produce iridoid compounds apart from CHCs, and I proved that these iridoids are emitted for defense in L. japonica, L. pacifica, and L. ryukyuensis. Therefore, the allomone function can be assumed to be the primary function of iridoids in Leptopilina wasps. Conspecific female competitors were not avoided by female L. japonica wasps, but L. ryukyuensis females avoid host patches already exploited by other conspecific females. Living female conspecifics of L. pacifica were not avoided by female wasps, however, the odor of female conspecifics’ extracts was avoided. While L. ryukyuensis and L. pacifica use CHCs as female sex pheromones, in the species L. japonica the defensive iridoid compounds evolved a second function as sex pheromones. Interestingly, I found in all species, also in those that use CHCs as sex pheromones, that iridoids evolved as cues for males to locate female conspecifics. However, research is limited regarding the degree to which the evolution of the impressive variety of sex pheromone compositions is driven by ecological or life-history traits and behavior. Here, I propose the dispersal behavior of organisms for mate-finding as a process driving pheromone evolution, as it is restricted by and hence presumably correlated with the volatility of pheromonal compounds. Therefore, I studied the dispersal behavior of male and female wasps after emergence in the four species of the genus Leptopilina, L. heterotoma and L. japonica, which have highly volatile iridoids as sex pheromones, and L. ryukyuensis and L. pacifica, whose sex pheromones are composed of CHCs, which can only be perceived in close vicinity of the releasing individual. My analyses revealed that the dispersal behavior of Leptopilina males and females after emergence does significantly differ between species with differently volatile sex pheromones. Specifically, males of species with volatile iridoid sex pheromones start to disperse immediately before their females’ emergence from the natal patch, whereas males of species with low volatile CHC-based 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. Probably, a tradeoff between the volatility of the sex pheromone for long range attraction and the information content for inbreeding avoidance plays an important role. My studies significantly contribute to the understanding of the evolutionary forces shaping the composition of sex pheromones.