A project undertaken at the Hawkesbury Institute for the Environment (HIE), Western Sydney University, and supervised by Markus Riegler (HIE), in collaboration with Duong Nguyen, James Cook (HIE), Lisa Jamieson and Max Suckling (New Zealand Institute for Plant and Food Research)
Recent breakthrough discoveries in insect science provide new avenues for highly targeted and environmental-friendly pest control. Many insects are associated with maternally inherited bacterial endosymbionts that manipulate host reproduction. Infecting >50% of insect species, Wolbachia is the most common; another example is Cardinium. The most frequently observed reproductive manipulation is cytoplasmic incompatibility (CI) which results in the embryonic mortality of eggs of uninfected females fertilised by infected males; in contrast, infected eggs can rescue CI. CI provides a highly effective way to drive reproductive parasites through host populations. Wolbachia CI can therefore be applied in mosquito control, e.g. for the spread of Wolbachia-conferred resistance to pathogens (e.g. dengue and Plasmodium).
CI can also directly suppress naturally uninfected pest populations that cannot rescue the CI induced by the release of Wolbachia-infected males – a strategy known as Incompatible Insect Technique (IIT) and tested for the control of a number of diplodiploid insect species such as fruit flies, mosquitoes and moths (e.g. Riegler and Stauffer 2002, Zabalou et al. 2004). However, endosymbiont dynamics in haplodiploid insects (such as thrips) are less studied, and application of IIT for haplodiploid pests has not yet been tested.
Figure 1.A pair of mating P. kellyanus (photo credit: D. Nguyen).
Figure 2.A developing embryo (with red eyes) and a first instar larva of P. kellyanus (photo credit: D. Nguyen).
Figure 3. Fertilised P. kellyanus embryos that remained unhatched after seven days as a consequence of cytoplasmic incompatibility (photo credit: D. Nguyen).
Our project focussed on Pezothrips kellyanus (Thysanoptera: Thripidae), an Australian-native citrus pest of economic importance which is invasive in New Zealand and Mediterranean countries (Nguyen et al. 2016). Australian populations have Cardinium and Wolbachia, and both endosymbionts independently cause CI (Nguyen et al. 2017). In contrast to Australian populations, Wolbachia is absent from invasive populations for unknown reasons (Nguyen et al. 2016).
In order to resolve this we assessed thrips genetic diversity and endosymbiont prevalence of a large number of P. kellyanus populations across its native and invasive range. We found a high mitochondrial haplotype diversity in Australia, and only one haplotype each in New Zealand and the Mediterranean region, indicative of two independent colonisation events across these two regions (Nguyen et al. 2016). Furthermore, we detected the same haploptypes of the invasive populations also in southern Australia. Interestingly all populations with these haplotypes are also without Wolbachia in Australia. Therefore, the absence of Wolbachia in invasive populations is due to the absence of Wolbachia in the Australian source populations. Furthermore, we revealed a linkage pattern between Wolbachia and co-inherited mitochondrial haplotypes. This indicates that the mitochondrial diversity in this species has experienced a selective sweep possibly as a consequence of a recent or ongoing Wolbachia invasion.
Our results indicate that the invasive populations of P. kellyanus may be amenable to suppression by a Wolbachia-based IIT approach, i.e. mass-release of male-only Wolbachia infected individuals that can be produced by virgin Wolbachia infected females. However, prior to the testing of P. kellyanus IIT, it is important to better understand the effect of endosymbionts on host reproduction and sex allocation, in particular as previous mating experiments observed unusual sex ratio patterns (Nguyen et al. 2017). Therefore, we investigated resource and sex allocation patterns in this species and found that P. kellyanus exhibits an egg size-mediated fertilisation mechanism: larger eggs are fertilised and develop as females; in contrast, smaller eggs are not fertilised and develop as males (Katlav et al. 2021a). Some females display constrained sex allocation in that they are only able to produce small eggs that remain unfertilised and thereby develop as males (Katlav et al. 2021b). Interestingly, the endosymbionts influence these sex allocation patterns: Cardinium infected females are larger and produce larger eggs resulting in more female production. In contrast, Wolbachia is more costly and does not affect egg size (Katlav et al. 2022a). Furthermore, the constrained sex allocation patterns are moderated by the endosymbionts in a temperature-dependent way (Katlav et al. 2022b).
Our findings show that endosymbionts have evolved additional strategies to colonise haplodiploid host populations and that they can affect sex ratios of haplodiploid hosts beyond the commonly recognised reproductive manipulations such as CI. Furthermore, results of this researchcontribute to a better understanding of endosymbiont population dynamics and the development of IIT for haplodiploid insect pests.