The evolution of sociality and antimicrobial defences in wasps

Cuticular antimicrobial compounds are a front-line defence against disease that restrict the growth and transmission of pathogens within groups of social insects. I explore the hypothesis that these compounds were integral to both the evolution of early social aggregations and the development of more complex social systems. The strength of cuticular antimicrobials was assayed from wasp species representing a range of group sizes and life history traits. Wasps were chosen because they are a basal group within social Hymenoptera and present species with a range of social structures. Across species, nesting substrate type and levels of social complexity were strongly correlated with the strength of antimicrobial compounds. Phylogenetic analysis suggested that these compounds arose first in solitary, ground-dwelling species. Across colonies of a social species, traits varied with respect to how they influenced the strength of antimicrobial defences; group size and genetic diversity had strong but opposing correlations with antimicrobial strength. Conversely, microbial load was low across colonies and did not appear to vary with antimicrobial strength. These data suggest antimicrobial defences in wasps first evolved as a defence against soil-borne pathogens. As group sizes increased the pressure of disease risk selected for increasingly strong antimicrobial compounds. Low microbial associations indicate that these compounds are highly effective in social species however correlations with group size and genetic diversity suggest that disease risk remains a major selective pressure on maximum attainable group sizes. I propose a model of the relationship between group size, genetic diversity and the risk of disease that may be used to predict the largest stable group sizes. These findings illuminate both the historical and contemporary selective pressure that disease risk has placed on cuticular antimicrobial compounds. Further, these data provide unique insights and research avenues for both bio-prospecting and social insects in general.