Speciation in the Encarsia pergandiella complex (Hymenoptera: Aphelinidae): effect of Cardinium-induced cytoplasmic incompatibility and nuclear incompatibilities

Encarsia pergandiella (Hymenoptera: Aphelinidae) is a species complex of parasitoids of whiteflies that vary in reproductive mode and endosymbiotic bacteria infection. Whereas E. pergandiella from California is uninfected, the intracellular bacterium Cardinium causes parthenogenesis in a population from Brazil, and cytoplasmic incompatibility (CI) in a sexual population from Texas. Encarsia pergandiella from California and Texas are morphologically and genetically differentiated, yet they may mate and produce hybrid offspring. A role of CI on speciation is theoretically plausible and has been experimentally demonstrated in model insects such as Drosophila and Nasonia. However, its general importance remains a controversial issue, and is hard to assess due to scarcity of experimental data. Here we first evaluated the role of the CI-inducing endosymbiont Cardinium and genetic incompatibilities in the reproductive isolation and competitive interactions of these parasitoid populations. The level of reproductive compatibility between Cardinium-infected and uninfected E. pergandiella was assessed by both pre-zygotic (mating preferences) and post-zygotic (crossing) experiments, and the population dynamics of a secondary contact scenario of these allopatric species was studied in population cage experiments. Results suggest that, while CI played a role in the isolation of this two species at the beginning of the speciation process, at present the strong nuclear incompatibilities accumulated over time make the CI effect negligible.