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While many species of animals form only social or foraging intraspecific groups, a variety of taxa of marine invertebrates may further establish tissue unions (natural chimeras) between conspecifics. A model system is the tunicate Botryllus schlosseri, in which histocompatibility is governed by shared alleles on a single, highly polymorphic fusibility locus. Proposed benefits that have been attributed to natural chimerism have not been confirmed in this model system. Furthermore, laboratory studies on bichimeras, each consisting of two partners, have shown that fusion is usually followed by morphological elimination (resorption) of one partner. However, freely circulating stem cells from the "subordinate" partner in resorption may parasitize the soma and germline of the "winner". Here we show that multi-partner chimeras form more stable and vigorous entities than bichimeras. Multi-chimeras grow faster, reach larger sizes, do not fragment, reduce incidence of morphological resorption and show other characteristics of more equilibriated entities. It is proposed that in these multi-partner entitites, the different intraspecific conflicts alleviate each other, generating an improved entity where natural selection may act on "group" level instead of on each individual genome. The state of multi-chimerism incurs, however, some costs such as a one quarter reduction in the potential maximum chimerical size as compared to bichimeras.
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