Cytological mechanisms of Cardinium-induced cytoplasmic incompatibility in Encarsia pergandiella (Hymenoptera: Aphelynidae): Preliminary results.

Cytoplasmic incompatibility (CI) between closely related populations occurs frequently in arthropods as a consequence of infections by Wolbachia and Cardinium endosymbionts. CI causes sterility in matings between infected males and uninfected or differently-infected females. CI-bacteria infect a large number of arthropods and strongly influence the ecology and evolution of their hosts. CI may also affect the invasiveness of alien genotypes and the effectiveness of biocontrol agents. Cytological analyses of Wolbachia CI crosses in Drosophila and Nasonia have provided insights into the mechanism of CI. These studies have shown that incompatible embryos die in the first mitotic division of the zygote, and implicate paternal chromatin remodeling and asynchrony of male and female pronuclei entering the first mitotic division. The delay leads to abnormal division or the exclusion of male chromatids. So far, nothing is known about the sequence of events in the embryogenesis of incompatible CI Cardinium crosses. We conducted a cytological analysis of reproductive incompatibility caused by CI Cardinium in Encarsia pergandiella. We set up incompatible and control matings, dissected eggs from whitefly hosts, and fixed and stained eggs with DAPI during early embryogenesis. DAPI-stained eggs were imaged with a confocal microscope. We also attempted live-staining with the nucleic acid stain Syto-11 to allow for visualization of embryogenesis in real time. This technique involves dissection in buffer followed by a short incubation before imaging with a deconvolution microscope. We observed the formation of chromatin bridges after the first mitotic division of the zygote, which is a common feature of Wolbachia-induced CI. Our preliminary results suggest a common mechanism of CI induced by the two bacterial lineages. Identifying parallels between CI caused by Wolbachia and Cardinium will shed light on fundamental mechanisms that underlie this reproductive incompatibility.