The Evolution of Allorecognition Specificity

Intraspecific aggression between clonal cnidarians is not random with respect to genotype; instead, agonistic behavior is modified according to the relatedness of contestants such that clonemates and close relatives behave passively, whereas more distant relatives behave aggressively. Aggression is often costly, with contestants suffering injury and losing space for attachment and growth. Similarly, intraspecific somatic fusion in many sponges, cnidarians, bryozoans, and ascidians is limited to interactions between close relatives and is thought to carry substantial fitness costs. Such behavioral restriction requires the existence of both an allorecognition system and a set of cues that provides specific labels oFrelatedness. Several studies suggest that labels of identity (allotypes) are provided hy one, or a few loci. Individual specificity therefore requires that the loci co~fcrring allotypic specificity carry high levels of allelic variation. In the case of ag,:essicn, allotypic specificity directs aggression away from clonemates and close kin, and toward more distant relatives. In the context of fusion, allotypic specificity limits the potential for intraspecific fusion and its attendant costs. Taken together, these considerations :i,ipiy that rare ailorecognition alleles, and the labels of individuality that they confer, should he favored by natural selection. However, the mechanistic models of aggression analyzed in this paper show that the individual costs and benefits of aggression alone will not favor the accumulation and maintenance of allotypic specificity. In contrast, allotypic polymorphism can be maintained directly by the individual costs and benefits of fusion provided fusion carries a net fitness cost. This raises the question of how fusion conditioned on relatedness can be evolutionarily stable. Our results suggest that selection acting at the level of clonal or kin-aggregations, rather than at the level of the individual, may provide an explanation for the evolution of allotypic specificity through aggression or fusion. R K. Gmsberg and J. F. Quinn