Chemical communication reflects incomplete prezygotic reproductive isolation in the parasitoid wasp Nasonia - from phenotypic to genetic architecture
In the recently diverged parasitoid jewel wasp genus Nasonia, an emerging genetic model system and my main study organism, intensively studied postzygotic reproductive isolation contrasts far less well investigated prezygotic isolation. Ubiquitous infections with species-specific Wolbachia bacteria cause bidirectional incompatibility in interspecific crosses and prevent hybridizations. However, after antibiotic treatment, the different Nasonia species regain the ability to hybridize. This hints at an incomplete level of prezygotic reproductive isolation, but its complete absence would lead to huge fitness losses in natural populations due to the ubiquitous postzygotic hybridization barrier. Hence, I hypothesized that prezygotic reproductive isolation can be observed in developing stages in the Nasonia species complex. Focusing primarily on different aspects of chemical communication in Nasonia, I attempted to investigate species-specific chemical cues, both on the phenotypic and the genetic level, that potentially contribute to prezygotic reproductive isolation.
The main part of my PhD project consisted of the determination, identification and analysis of species-specific cuticular hydrocarbon (CHC) differences in Nasonia. CHC play crucial roles in species recognition and sexual communication in a wide variety of insect species. The investigated Nasonia CHC profiles were not only distinctive enough to clearly separate the species, but also highly sex-specific. Both findings constitute important pre-requisites for a potential role of CHC in prezygotic reproductive isolation. Our chemical analyses of CHC differences in Nasonia contributed to the discovery of a hitherto unknown Nasonia species, raising the number of described species from three to four. Interestingly, CHC divergence was found to differ greatly between males and females, where only males paralleled the molecular phylogeny of Nasonia. In specifically designed bio-assays, we could demonstrate that males of certain species are able to discriminate females solely based on their species-specific CHC profile. These findings confirmed CHC as species-specific female cues involved in sexual signalling and prezygotic reproductive isolation in most instances. However, female CHC of one particular Nasonia species (N. giraulti) apparently completely lost their signalling function, hinting at an evolutionary shift from a pre-existing chemical communication channel that the receivers (males) failed to “follow”.
Furthermore, to access the genetic background of CHC variation, I analyzed artificially generated male hybrids in all possible combinations between three of the four Nasonia species and determined the gene loci (QTL) governing species-specific CHC differences. The results showed that several of the identified loci accounting for the variation of structurally similar CHC cluster in the same chromosomal regions. Those findings hint at shared biochemical pathways for the synthesis of these CHC and constitute interesting targets for pleiotropic genes contributing to prezygotic reproductive isolation via regulation of CHC variation.
The second part of my PhD project concerned the genetic manipulation of a male sex pheromone that displays two distinct species-specific phenotypes in Nasonia. Males of all species produce erythro-5-hydroxy-4-decanolide to attract females, but only males of the cosmopolitan species N. vitripennis produce the structurally identical but diastereoisomeric threo-5-hydroxy-4-decanolide as an extra compound. Females of certain (but not all) N. vitripennis strains display a clear preference for pheromone blends containing the extra compound. Preliminary analysis located the genetic region responsible for the pheromone difference on chromosome 1. We fine-mapped this particular chromosomal region of interest, reducing the number of potential candidate genes from > 50 to seven. Subsequent RNA interference experiments we conducted further reduced the number of potential genes to three putative alcohol dehydrogenases, revealing both the genetics and the potential biochemical pathway governing this species-specific pheromone difference.
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