Relaxed selection underlies genome erosion in socially parasitic ant species

Abstract

Inquiline ants are highly specialized and obligate social parasites that infiltrate and exploit colonies of closely related species. They have evolved many times convergently, are often evolutionarily young lineages, and are almost invariably rare. Focusing on the leaf-cutting ant genus Acromyrmex, we compared genomes of three inquiline social parasites with their free-living, closely-related hosts. The social parasite genomes show distinct signatures of erosion compared to the host lineages, as a consequence of relaxed selective constraints on traits associated with cooperative ant colony life and of inquilines having very small effective population sizes. We find parallel gene losses, particularly in olfactory receptors, consistent with inquiline species having highly reduced social behavioral repertoires. Many of the genomic changes that we uncover resemble those observed in the genomes of obligate non-social parasites and intracellular endosymbionts that branched off into highly specialized, host-dependent niches.

The genomic basis of army ant chemosensory adaptations

“…While these were published too recently to be included in our in-depth comparative analyses, it is worth noting some striking similarities and differences in these ant groups relative to E. burchellii. Specifically, both studies found large contractions in the OR family in social parasites compared with their free-living relatives (Schrader et al, 2021, Jongepier et al, 2021. This suggests that olfactory system simplification is a common theme of ecological specialization in ants.…”

Human Follicular Mites: Ectoparasites Becoming Symbionts

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“…1). For example, socially parasitic ant species which rely on brood care and social infrastructure provided by closely related host species have lost genes associated with olfactory receptors (Schrader et al ., 2021), which underpin chemical communication and are an essential component of sensory input that stimulates expression of social behaviour (Trible et al ., 2017). Thus, selection can reduce expression of a behaviour by favouring loss of sensory inputs required to activate the behaviour, rather than loss of components of the behaviour itself.…”