Positive selection has shaped the evolution of Argentine ant immune genes both in native and introduced supercolonies.

The highly invasive Argentine ant (Linepithema humile) started its colonisation from the species' native range in South America approximately 150 years ago and has since become one of the major pests in the world. We investigated how the shifts into new ranges have affected the evolution of Argentine ants' immune genes. To the best of our knowledge, this is the first broadscale population genetic study focusing on ants' immune genes. We analysed comprehensive targeted-seq data of immune and non-immune genes containing 174 genes from 18 Argentine ant supercolonies covering the species' native and introduced ranges. We predicted that the immune gene evolution of introduced supercolonies differs from that of the native supercolonies and proposed two different, non-mutually exclusive hypotheses for this: 1) the enemy release hypothesis and 2) the higher pathogen pressure hypothesis - both of which seem to explain the observed evolutionary patterns on their behalf. Our results show that the introduced supercolonies were targeted by weaker selection than natives, but positive selection was evident among supercolonies of both ranges. Moreover, in some cases, such as the antiviral RNAi genes, introduced range supercolonies harboured a higher proportion of positively selected genes than natives. This observation was striking, knowing the recent demographic history and the detected generally lower selection efficacy of introduced supercolonies. In conclusion, it is evident that pathogen pressure is ubiquitous and strongly affects the immune gene evolution in Argentine ants.

Social environment affects the transcriptomic response to bacteria in ant queens.

Social insects have evolved enormous capacities to collectively build nests and defend their colonies against both predators and pathogens. The latter is achieved by a combination of individual immune responses and sophisticated collective behavioral and organizational disease defenses, that is, social immunity. We investigated how the presence or absence of these social defense lines affects individual-level immunity in ant queens after bacterial infection. To this end, we injected queens of the ant Linepithema humile with a mix of gram+ and gram- bacteria or a control solution, reared them either with workers or alone and analyzed their gene expression patterns at 2, 4, 8, and 12 hr post-injection, using RNA-seq. This allowed us to test for the effect of bacterial infection, social context, as well as the interaction between the two over the course of infection and raising of an immune response. We found that social isolation per se affected queen gene expression for metabolism genes, but not for immune genes. When infected, queens reared with and without workers up-regulated similar numbers of innate immune genes revealing activation of Toll and Imd signaling pathways and melanization. Interestingly, however, they mostly regulated different genes along the pathways and showed a different pattern of overall gene up-regulation or down-regulation. Hence, we can conclude that the absence of workers does not compromise the onset of an individual immune response by the queens, but that the social environment impacts the route of the individual innate immune responses.

Viruses of invasive Argentine ants from the European Main supercolony: characterization, interactions and evolution.

The Argentine ant (Linepithema humile) is a highly invasive pest, yet very little is known about its viruses. We analysed individual RNA-sequencing data from 48 Argentine ant queens to identify and characterisze their viruses. We discovered eight complete RNA virus genomes - all from different virus families - and one putative partial entomopoxvirus genome. Seven of the nine virus sequences were found from ant samples spanning 7 years, suggesting that these viruses may cause long-term infections within the super-colony. Although all nine viruses successfully infect Argentine ants, they have very different characteristics, such as genome organization, prevalence, loads, activation frequencies and rates of evolution. The eight RNA viruses constituted in total 23 different virus combinations which, based on statistical analysis, were non-random, suggesting that virus compatibility is a factor in infections. We also searched for virus sequences from New Zealand and Californian Argentine ant RNA-sequencing data and discovered that many of the viruses are found on different continents, yet some viruses are prevalent only in certain colonies. The viral loads described here most probably present a normal asymptomatic level of infection; nevertheless, detailed knowledge of Argentine ant viruses may enable the design of viral biocontrol methods against this pest.