Genomic differentiation of brushtail possum (Trichosurus vulpecula) populations : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Zoology at Massey University, Manawatū, New Zealand

Abstract

Toxin resistance, an ecologically functional trait, has emerged as a result of the coevolutionary arms race between plants and herbivores. This adaptive response allows organisms to counteract the detrimental effects of toxins. Such adaptation occurs within three primary ecological contexts: predator resistance, prey resistance, and auto-resistance. In the context of plant-herbivore interactions, the production of toxic secondary metabolites by plants plays a pivotal role, triggering a dynamic arms race with herbivores. This ongoing battle leads to the development of diverse defensive strategies in plants and corresponding counter-adaptations in herbivores. Toxin resistance studies have not only shed light on coevolutionary dynamics but also provided insights into regional adaptations and population fragmentation. The case of sodium fluoroacetate, a potent toxin found in certain plant species, offers a specific example of toxin resistance in brushtail possums (Trichosurus vulpecula). Deepening our understanding of the underlying mechanisms driving toxin resistance becomes particularly interesting in this case because of the notable variation in sodium fluoroacetate resistance observed among subspecies. The brushtail possum (Trichosurus vulpecula) is protected in its natural range of Australia but as an invasive pest in New Zealand populations are controlled by the application of human made sodium fluoroacetate, providing a strong selective force for the potential parallel evolution of toxin resistance.

I first focus on the population fragmentation and past connectivity of brushtail possum subspecies in their native range using ecological niche modelling and genetic analysis. My results suggest that Pliocene and Pleistocene climate oscillations played a significant role in shaping the distribution and structure of these widespread marsupials. The findings highlight the limited gene flow between subspecies that would have facilitated their adaptations to regional plant assemblages.

I studied gene expression using RNA sequencing to reveal significant differences in transcription levels between adult and juvenile brushtail possums. These findings provide insights into the mechanisms associated with possum development, tissue development, cell cycle, and extracellular matrix. Furthermore, the downregulation of specific genes, such as Cytochrome P450, in juveniles may indicate their role in possums' dietary adaptations. Differential gene expression among subspecies of brushtail possum shed light on genomic differentiation and identified candidate genes involved in regional adaptations involving toxin tolerance. Further analyses comparing subspecies has identified genes under positive selection and enriched biological pathways that may be associated with sodium fluoroacetate resistance. These findings support the idea of genomic divergence in spatial populations and provide hypotheses on the metabolic pathways involved in toxin resistance.

Collectively, these studies offer valuable insights into the coevolutionary dynamics, toxin resistance mechanisms, gene expression and selection patterns in brushtail possums (Trichosurus vulpecula) and formulated strong hypotheses related to sodium fluoroacetate resistance. This study presents a reliable method to use transcriptomics to investigate the expression and genomic differences behind observed phenotypic variation within a single species.