Finding the mechanism for female bias caused by a Rickettsia bacterial endosymbiont (nr R. bellii) in the sweetpotato whitefly (Bemisia tabaci)

Mutualistic endosymbionts can have profound, positive effects on their hosts, ranging from obligate nutritional provisioning to mediating ecological interactions. However, some bacterial endosymbionts spread through populations not by providing fitness benefits to the host, but rather through reproductive manipulation. The most well-studied bacterial endosymbiont of arthropod hosts, Wolbachia, can cause feminization, male-killing, parthenogenesis, and cytoplasmic incompatibility in a variety of species. These manipulations can have costly effects to the host, and predominantly serve to propagate the endosymbiont. Rickettsia is a facultative, maternally transmitted, bacterial endosymbiont of the sweetpotato whitefly (Bemisia tabaci), that has recently been found to increase whitefly fitness but also to cause a significant female bias, simultaneously having beneficial effects and manipulating host reproduction. Understanding the mechanism underlying the reproductive manipulation in this notorious, agricultural pest is important for both applied interest and a deeper knowledge of endosymbionts. In this study, we used cytogenetic techniques to gauge if the female bias in Rickettsia-infected whiteflies is due to higher rates of fertilization. Whiteflies have haplodiploid sex determination, where fertilized eggs develop into females and unfertilized eggs develop into males.  We found no significant difference in fertilization rate between Rickettsia-infected and uninfected individuals, indicating Rickettsia does not influence the primary sex ratio. For our next steps, we hypothesized that the female bias is due to a higher female survivorship or lower male survivorship to adulthood. In order to test this, we are currently conducting survivorship assays between the offspring of virgin or mated, and Rickettsia-infected or uninfected females.