Abstract
BackgroundInvestigating the evolution of species-specific insect genitalia is central to understanding how morphological diversification contributes to reproductive isolation and lineage divergence. While many studies evoke some form of sexual selection to explain genitalia diversity, the basis of selection and the mechanism of heterospecific mate exclusion remains vague. I conducted reciprocal mate pair trials in the Drosophila mojavensis species cluster to quantify the frequency of failed insemination attempts, historically referred to as pseudocopulation, between lineages with discrete size and shape differences of the male aedeagus.ResultsIn cross-taxon matings aedeagus size had a significant effect on pseudocopulation frequencies, while aedeagus shape and genetic distance did not. The direction of the size difference was an important factor for successful mating. When females were mated to a cross-taxon male with a larger aedeagus than males from her own species, the pair could not establish a successful mating interaction. Females mated to cross-taxon males with a smaller aedeagus than conspecific males were able to establish the mating interaction but had issues disengaging at the end of the interaction.ConclusionsThe results of this study support a role for aedeagus size in the male-female mating interaction, with a secondary role for aedeagus shape. In natural populations, mating failure based on aedeagus size could serve as an important reproductive isolating mechanism resulting in failed insemination attempts after both the male and female show a willingness to mate.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-014-0255-3) contains supplementary material, which is available to authorized users.
Highlights
Investigating the evolution of species-specific insect genitalia is central to understanding how morphological diversification contributes to reproductive isolation and lineage divergence
Pseudocopulation The sequence of events in a mate trial scored as pseudocopulation was as follows: a female accepted a courting male by spreading her wings and lowering her abdomen, the male mounted the female and tucked the tip of his abdomen downward and anteriorly to insert his aedeagus into the female vagina
When the couple was unable to establish a connection between their reproductive organs the male fell off and this was scored as pseudocopulation
Summary
Investigating the evolution of species-specific insect genitalia is central to understanding how morphological diversification contributes to reproductive isolation and lineage divergence. While many studies evoke some form of sexual selection to explain genitalia diversity, the basis of selection and the mechanism of heterospecific mate exclusion remains vague. Because insect genitalia are often species-specific, investigating genitalia diversity can enhance our understanding of the putative role these structures play in reproductive isolation and lineage divergence. While sexual selection is largely agreed upon as a general explanation, results from numerous studies covering a wide diversity of insect groups vary in their support for more specific explanations such as sperm competition, female choice, lock-and-key, or sexual conflict. Numerous studies have demonstrated the effect of size and shape variation on mating success, sperm transfer, sperm storage, and paternity (reviewed in [1])
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