Variable viral loads and immune response in an invasive ant's native and introduced ranges

Aim: Pathogens can play an important role in biological invasions. Introduced populations may be particularly vulnerable to pathogens due to factors such as low genetic diversity and high population density. However, introduced populations that escape their natural pathogens may reallocate resources away from immunity and towards growth and reproduction. Interestingly, introduced ants have been suggested to have increased tolerance to new pathogens, contributing to their success as introduced species. In this study, we aimed to investigate whether introduced Argentine ant populations harbour different viral loads compared to native populations and if these differences were related to immunity-related gene expression. Location: The study was conducted across the native range of Argentine ants in Argentina and four introduced regions in California, France, Australia and New Zealand. Methods: We used RT-qPCR to quantify viral loads and gene expression in the ants. We analysed 15 different potentially pathogenic viruses across the Argentine ant's native and introduced ranges. Results: We found that five viruses, LhuPcV1, LhuPiLV1, LhuCV1, Kashmir Bee virus and LHUV-1, presented high loads in Argentine ants compared to the other viruses we screened. We found a significant effect of range on viral infections: high viral loads were commonly found in ants from introduced populations, which also exhibited increased immune gene expression. We found highly significant correlations between viral loads and expression of immune and metabolic genes. However, these associations were not fully consistent across the studied regions, indicating the complexity of eco-immunological phenomena. Main Conclusions: Our results suggest that introduced Argentine ant populations host different viral communities compared to native populations and that these differences are correlated with changes in immunity-related gene expression. The study highlights the complex role of pathogens in biological invasions and the importance of considering eco-immunological factors when assessing the impact of introduced species.