The Evolution of Allorecognition Specificity in Clonal Invertebrates

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9402797 Grosberg Allorecognition is defined as the ability to distinguish self from other members of the same species. Although allorecognition was once thought to be the hallmark of the vertebrate immune system, it is now clear that virtually all encrusting clonal organisms possess allorecognition systems whose specificity rivals that of the vertebrate immune system. A growing body of evidence supports the conclusion that allorecognition specificity in clonal organisms is genetically based, and that relatively few loci control the expression of this specificity. Consequently, the loci that confer allorecognition specificity must carry levels of genetic variation that greatly exceed levels characteristic of other polymorphic loci. The major goal of this proposal is to analyze -- from both a genetical and ecological perspective -- how such extraordinarily high levels of genetic variation are evolutionarily maintained. Three types of hyporthesis have been advanced to explain the evolution of allorecognition specificity. The first, that the genetic variation is functionally unimportant, is not a viable explanation, given what is known about mutation rates, population sizes, and patterns of nucleotide substitution at allorecognition loci. The second two hypotheses invoke natural selection; they differ with respect to whether selection acts directly to regulate the outcomes of tissue interactions, or whether selection acts indirectly via a correlated effect of allorecognition, such as the control of mating preferences. In contrast to vertebrates, in which sustained tissue interactions are rare, such encounters occur frequently among clonial invertebrates as they compete with conspecies for habitable space. Allorecognition systems mediate the nature and outcomes of such interactions by dictating whether they will lead to intergenotypic fusion, rejection, or aggression. Quantitative models confirm that allorecognition specificity can evolve, through its effects on the outcomes of competitive interactions. The key determinants of how strongly selection promotes the accumulation of specificity include the costs and benefits of fusion, rejections, and rejection,and the frequencies with which compatible (i.e. fusible) and incompatible (i.e. rejecting or aggressive) genotypes interact. This project will evaluate how allorecognition specificity evolves by estimating these costs, benefits, and interaction frequencies in two species of encrusting, clonal invertebrate. In one species the colonial sea squirt Botryllus schlosseri, allorecognition systems control whether conspecific encounters lead to fusion or rejection. In the other species, the cnidarian Hydractinia symbiolongicarpus, the alternative outcomes of conspecific encounters are fusion versus aggression. It will also evaluate the merits of one widely invoked pleiotropic mechanism by which allorecognition specificity could also be maintained, namely the control of gametic interations. By comparing how allotypic diversity evolves in both species, this research will build a foundation for understanding how and why individuals of nearly every species can distinguish self from nonself.