Loss of Wolbachia but not Cardinium in the invasive range of the Australian thrips species, Pezothrips kellyanus

Many insects are associated with maternally inherited bacterial endosymbionts that manipulate host reproduction or impact other aspects of host fitness. Despite their ecological significance, the role of endosymbionts in biological invasions by their host insects is still poorly understood. In this study, we characterised the genetic diversity of the invasive Kelly's citrus thrips, Pezothrips kellyanus (Thysanoptera: Thripidae), an important pest of citrus in Australasia and the Mediterranean region. We then surveyed native and invasive populations for the presence of the bacterial endosymbionts Cardinium and Wolbachia. Our analyses of mitochondrial and nuclear genes demonstrated that P. kellyanus originated from Australia. Furthermore, haplotype analysis revealed independent colonisation events for New Zealand and the Mediterranean region. Individuals from Australian populations had both endosymbionts at a high prevalence. In populations from New Zealand and the Mediterranean region, however, Cardinium was fixed and Wolbachia was absent. This may be due to a stochastic loss of Wolbachia prior to the establishment of invasive populations. Alternatively, Wolbachia may have been selected against by environmental factors or due to its potential role as a reproductive parasite that may constrain invasiveness of infected individuals. In contrast, Cardinium remained unaffected by the invasion process. Our study highlights that endosymbionts may be potential factors in the framework of the enemy release hypothesis that predicts success of invasive hosts in the absence of natural enemies. Therefore, the analysis of endosymbiont diversity of invasive insects may improve the understanding of host invasion biology and also deliver new diagnostic markers for biosecurity protocols.