No Evidence for Single-Copy Immune-Gene Specific Signals of Selection in Termites

Karen Meusemann,Judith Korb,Maximilian Schughart,Fabian Staubach

doi:10.3389/fevo.2020.00026

Abstract

Selection pressures from pathogens appear to play an important role in shaping social evolution. Social behavior, in particular brood care, is associated with pathogen pressure in wood-dwelling “lower” termites. Yet, generally pathogen pressure is predicted to be low in wood-dwelling termite species that never leave the nest except for the mating flight. In comparison, pathogen pressure is predicted to be higher in species that leave the nest to forage, and thus constantly encounter a diversity of microbes from their environment. We hypothesized that such differences in predicted pathogen pressure are also reflected by differences in the intensity of natural selection on immune genes. We tested this hypothesis in a phylogenetic framework, analyzing rates of non-synonymous and synonymous substitutions on single-copy immune genes. Therefore, we leveraged recent genomic and transcriptomic data from eight termite species, representing wood-dwelling and foraging species as well as 14 additional species spanning the winged insects (Pterygota). Our results provide no evidence for a role of pathogen pressure in selection intensity on single-copy immune genes. Instead, we found evidence for a genome-wide pattern of relaxed selection in termites.

Highlights

Like in other organisms, pathogens seem to be important drivers of evolution in social insects
In this study we combined recent genomic and transcriptomic resources to test whether termite ecology, in particular exposure to pathogens, might affect the evolution of immune genes
We expected that the intensity of selection would differ between termites of the wood-dwelling and foraging life types, as foraging termites are assumed to experience higher selection pressure from pathogens due to higher exposure

Summary

Introduction

Pathogens seem to be important drivers of evolution in social insects. Social insects are well protected as they evolved “social immunity” (Traniello et al, 2002; Cremer et al, 2007), a repertoire of defensive mechanisms that work at the colony level. Molecular mechanisms of social immunity exist, for example indirect immunization of colony members (e.g., Traniello et al, 2002; Cremer et al, 2007; Masri and Cremer, 2014) or impregnation of the nest walls with fungicidal compounds (Bulmer et al, 2009; Rosengaus et al, 2011). Social immunity can be considered a selected emergent property of insect colonies where the whole is more than the sum of the individual parts (Rosengaus, personal communication). The evolution of social immunity aligns with the complexity of social organization, suggesting that selection pressure by pathogens could be a driver of complex social organization

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Conclusion