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The Evolution of Social Parasitism in Insects

Session leader: Matt Johnston.

Social parasitism is an intriguing aspect in social insects. Social parasite species evolve from seemingly typical, social species but have evolved mechanisms which shortcut the usual social formalities like builiding your own nest and raising your own brood. In this sense, they are receiving all of the benefits of social organisation while giving little in the form of social costs (insert welfare joke here). Various forms of social parastism exist. Some species raid the nests of other species or their own species, taking food resources from their hosts. Other species have queens which usurp the nest along with its workers usually by kiling the host queen. Some ant species are slave-makers which steal the brood from other ant colonies. These larvae grow up in the parasite nest and do not realize that they are in the wrong place. They happily toil about doing all of the work for the parasite colony. Finally, inquilinism is an advanced form of social parasitism where the parastic species spends its entire life cycle within the nest of the host species.

Advanced social parasitism has been reported in the following taxa, keeping in mind that, at least in ants, new species are being discovered all the time:

  • Formicidae: Myrmecinae (evolved many times), Formicinae (many times), Myrmeciinae (twice), Dolichoderinae (once)
  • Vespidae: Vespinae (many times), Polistinae (debatable)
  • bees (?)

    Some of the inquiline species are completely workerless. When workers are present, they are few and have atrophied worker behaviors. The job of raising the parasite queen's brood is instead taken on by the host workers. Incredibly, the queens of some inquiline ant species coexist with the living host queen, sometimes spending their entire lives riding on the backs of the host queen.

    Emery's Rule

    The observance that inquiline species seem to always be closely related to their hosts led to the formation of Emery's Rule which, in its strict form, states that the inquiline species and the host species are each others' closest living relatives. Sometimes, Emery's Rule is used in a more relaxed context ant the two species need only be closely related. One explaination for the apparently close relatinship between social parasites and their hosts is that in order to get past the hosts' defenses, the parasite needs to have evolved communication systems similar to the host. This may be more likely if the two share a close evolutionary history. While Emery's rule was widely accepted, very few tests of its general applicability have been wanting. Recent studies have shown numerous examples where the sister species relationship does not hold. The strict form of Emery's Rule raises an interesting problem. Somehow, the host species has generated its own parasite. There have been various scenarios proposed to explain this oddity of evolution involving both allopatric and sympatric speciation.

    Remember that allopatric speciation involves the separating of one population into two different populations which are separated geographically and do not interbreed. Different mutations occur in both populations which go on different evolutionary trajectories. When the geographic barrier is overcome or removed, the two populations have diverged to point that they do not or can not interbreed and the two are now considered distinct species. Sympatric speciation involves some form of positive assortative mating where like individuals in a population only mate with like individuals. This segregation forms distinct groups, which live in the same area but do not interbreed. Different mutations build up in both groups and eventually the two groups are incapable of interbreeding.

    We will dive right into the debate as to whether inquilinism evolves via sympatry or allopatry and explore how these possibilities apply to Emery's Rule.

    Required papers

    Full references list

    Copyright 1998, Matt Johnston