Immune response varies among members of a fungus-infected ant society

Social insect colonies are known to be targeted by a wide variety of different parasites and pathogens because of the dense cohabitation of many highly related individuals. However, within a colony, the level of risk of exposure could vary among different groups depending on their social role. Unlike many parasites, which mostly affect specific groups of individuals, e.g. foragers, the myrmecoparasitic fungus Rickia wasmannii infects entire ant colonies, causing subtle changes in physiology, morphology and behaviour. We investigated how different groups within a colony respond to fungal infection by measuring the expression of the genes defensin 1 and prophenoloxidase, both vital components of ant immunity. We found that workers, queens and larvae varied considerably in defensin levels, though not in prophenoloxidase expression. In same-age workers, both prophenoloxidase and defensin 1 levels exhibited increases in correlation with parasite loads. In queens, only the body size appeared to influence immune gene expression, with larger individuals exhibiting higher levels of defensin 1. However, in workers, morphological and physiological traits did not correlate with gene expression. Larvae did not show any discernible response in immune gene expression, reflecting that they are typically not infected by the fungus. Our study reveals considerable variation in the immune response among different groups within ant colonies. These diverging reactions likely stem from differences in exposure to parasites, as well as differences in immunocompetence.