Abstract
Some parasitoid wasps possess soldier castes during their parasitic larval stage, but are often neglected from our evolutionary theories explaining caste systems in animal societies. This is primarily due to the polyembryonic origin of their societies. However, recent discoveries of polyembryonic trematodes (i.e. flatworms) possessing soldier castes require us to reconsider this reasoning. I argue we can benefit from including these polyembryonic parasites in eusocial discussions, for polyembryony and parasitism are taxonomically vast and influence the evolution of social behaviours and caste systems in various circumstances. Despite their polyembryony, their social evolution can be explained by theories of eusociality designed for parent–offspring groups, which are the subjects of most social evolution research. Including polyembryonic parasites in these theories follows the trend of major evolutionary transitions theory expanding social evolution research into all levels of biological organization. In addition, these continued discoveries of caste systems in parasites suggest social evolution may be more relevant to parasitology than currently acknowledged.
Highlights
Some parasitoid wasps possess soldier castes during their parasitic larval stage, but are often neglected from our evolutionary theories explaining caste systems in animal societies
Their social evolution can be explained by theories of eusociality designed for parent– offspring groups, which are the subjects of most social evolution research
How do we account for larval colonies of parasites that have converged upon sterile helper castes absent of the family-living context we observe in all other eusocial systems? Answering this is important, as polyembryonic parasites are useful exceptions to the eusocial norms receiving much attention from research on reproductive divisions of labour
Summary
Eusociality is one of the most substantial guiding paradigms for social evolutionary research. Larval colonies of trematodes (i.e. flatworms, blood flukes) are argued to be eusocial, following the discovery of morphologically distinct soldier castes [7], and this claim has received growing support [8,9,10,11,12,13]. This discovery is unexpected and exciting, extending our social evolutionary theories into a phylum (Platyhelminthes) that seemingly had no relevance to social evolution research. Parasites are potentially full of undiscovered systems possessing behaviours convergent to social and eusocial taxa [34,35,36,37], and if we are going to include this incredibly common lifestyle in our social evolutionary theories, we can start by understanding the weird eusociality found in polyembryonic parasites
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