Multi-partner urochordate chimeras outperform two-partner chimerical entities

While many species of animals form only social or foraging intraspecific gr oups, a variety of taxa of marine invertebrates may further establish tissu e unions (natural chimeras) between conspecifics. A model system is the tun icate Botryllus schlosseri, in which histocompatibility is governed by shar ed alleles on a single, highly polymorphic fusibility locus. Proposed benef its that have been attributed to natural chimerism have not been confirmed in this model system. Furthermore, laboratory studies on bichimeras. each c onsisting of two partners, have shown that fusion is usually followed by mo rphological elimination (resorption) of one partner. However; freely circul ating stem cells from the "subordinate" partner in resorption may parasitiz e the soma and germline of the "winner". Here we show that multi-partner ch imeras form more stable and vigorous entities than bichimeras. Multi-chimer as grow Faster, reach larger sizes, do not Fragment, reduce incidence of mo rphological resorption and show other characteristics of more equilibrated entities. It is proposed that in these multi-partner entities, the differen t intraspecific conflicts alleviate each other, generating an improved enti ty where natural selection may act on "group" level instead of on each indi vidual genome. The state of multi-chimerism incurs, however, some costs suc h as a one quarter reduction in the potential maximum chimerical size as co mpared to bichimeras.