Variability of immune gene expression among different groups within ant colonies show multifaceted response to infection by a non-lethal ectoparasitic fungus

Social insect colonies are known to be targeted by a wide variety of different parasites and pathogens because of their high host abundance. However, within a colony, the level of risk to exposure could vary among individuals depending on their role. Unlike many known parasites, which mostly target specific groups of individuals, e.g. foragers, the myrmecoparasitic fungus Rickia wasmannii infects entire ant colonies, being linked to subtle changes in physiology, morphology and behaviour. We investigated how different groups within the colonies respond to being exposed to the fungus by measuring the expression of the genes defensin 1 and prophenoloxidase , both vital components of ant immunity. We found that workers, queens and broods varied in their immune response. Workers displayed diverse profiles, with variable responses to infection: in same-age workers, both prophenoloxidase and defensin 1 levels exhibited increases in correlation with pathogen loads. Queens exhibited a more pronounced immune response. Highly infected queens had a heightened immune response. Larvae did not show a discernible response. Morphological and physiological characteristics had limited effects on gene expression, except in the case of queens, where larger individuals displayed higher defensin 1 expression. Our study shows that these divergent responses likely stem from the differing physiological needs and priorities of various groups within the colony. Highlights In same-age workers, prophenoloxidase and defensin 1 levels increased with pathogen loads. Body size affected defensin 1 expression in a caste-specific manner: larger queens displayed higher expression. Infection did not elicit any specific response in larvae. The diverse response to infection likely arise from distinct physiological needs and priorities within colony groups.