Sex allocation in Kelly's citrus thrips, Pezothrips kellyanus : mechanisms, constraints and the role of endosymbionts

Abstract

Kelly's citrus thrips, Pezothrips kellyanus (Bagnall) (Thysanoptera: Thripidae), native to Australia and invasive in New Zealand, other Pacific islands and Mediterranean countries, is an important pest of citrus. Despite its wide distribution, not much is known about its biology and reproductive strategies. This PhD study investigated the mechanism of sex allocation in Kelly's citrus thrips, Pezothrips kellyanus, an economically important Australian native thrips. For the first time for any haplodiploid insect, this study identified a finely tuned egg size-mediated fertilization mechanism which modulates sex ratio and is influenced by mating (see Chapter 2). The study further disentangled the underlying factor (maternal condition) that is responsible for the constrained sex allocation (extremely male-biased brood production) in a fraction of mated P. kellyanus females resulting in the expression of split sex ratios (see Chapter 3) as previously recorded in this species. Furthermore, the study assessed the role of the two common bacterial endosymbionts Wolbachia and Cardinium in the reproductive biology of this species, and the mechanism by which they manipulate sex allocation. Both endosymbionts have previously been shown to independently induce cytoplasmic incompatibility (CI) in incompatible crosses which lead to male female mortality (caused by Wolbachia) or partial male development (caused by Cardinium) of fertilised embryos. This study demonstrated that Wolbachia and Cardinium in P. kellyanus also affect resource and sex allocation as well as trade-offs between reproductive and life history/fitness traits (see Chapter 4). Overall, this thesis expands the knowledge about the evolution and mechanisms of sex allocation in thrips and haplodiploid insects; the mechanisms of constrained sex ratio in thrips and haploidploid insects; and the role of maternally inherited endosymbionts in modulating egg size and thereby sex allocation favouring their invasion of host populations.

Date of Award	2021
Original language	English