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A salient question in invasion biology is to elucidate what traits make some species invasive and not others. Recently, the introduction process has been hypothesised to function as a filter, selecting for traits that promote invasiveness in introduced populations. Behaviour has been increasingly recognised as a significant driver of animal invasions, but few studies focus on how the introduction process may affect behavioural variation across an invader’s range. This thesis aims at investigating the consequences of the introduction process on behavioural variation along an introduction pathway in the globally widespread Argentine ant.  In Chapter I, I investigated transcriptome-wide variations associated with morphological caste as well as worker foraging propensity. I found a number of transcripts showed caste and behaviour-specific expression profiles, and that foraging workers exhibit higher expression of genes related to chemosensory receptors and neurological pathways known to be associated with foraging behaviour and aggression. In chapter II, I investigated octopamine-mediated modulation of foraging behaviour and aggression using pharmacological manipulations using synthetic octopamine (OA) and one of its antagonists. I found that OA was associated with colony-level foraging activity, but did not appear to similarly drive changes in individual behaviour. However, my results suggest that OA is associated with individual aggressiveness, and I also found an association between OA and foraging propensity. In chapter III, I tested the hypothesis that the introduction process selected for increased risk-taking behaviour along the Argentine ant introduction pathway. I used behavioural assays combined with OA diet supplementation in four regions along the introduction pathway, and showed that both behavioural variation and plasticity were consistent across the range, not in support of our hypothesis. Finally, in chapter IV I investigated transcriptome-wide expression profiles in workers along the introduction pathway and discovered distinct transcriptomic signatures associated with range. I found that specific immune and neural pathways were differentially regulated in the introduced range compared to the native range.  Overall, my results contribute to a better understanding of caste differentiation and neuromodulation of key behaviours in invasive ants. While I did not find behavioural differentiation between native and invasive Argentine ant populations, I show that they are characterised by specific transcriptomic signatures perhaps underlying the ant’s invasion success. This is one of the first global and integrative studies to focus on changes associated with the introduction of invasive species, and may have important implications for the understanding of biological invasions.

Variation in behavioural traits and their molecular basis in the globally invasive Argentine ant