The genes governing allorecognition in both vertebrates and invertebrates can be highly polymorphic. This project will examine some of the selective forces that may have initiated and maintained this polymorphism in a model system, the colonial ascidian, Botryllus schlosseri. In this ascidian, a single and highly polymorphic allorecognition locus appears to govern the processes of colony fusion and gamete incompatibility. It is argued that a precise allorecognition system may have evolved in B. Schlosseri to reduce the costs of somatic cell parasitism associated with colony fusion and/or to decrease the level of inbreeding over that expected if male fertilities were a simple function of sperm concentration. This project will test these ideas by asking the following specific questions: 1) What are the genetic costs of colony fusion? 2) What is the frequency of genetic chimeras in natural populations? and 3) To what extent does gametic incompatibility lower male fertilities in matings among kin? To answer these questions the research will: first, determine the best combination of probes and restriction enzymes for the genetic typing of individuals by DNA fingerprinting; second, genetically characterize what happens to the tissues and germ lines of fused colonies over time; third, genetically type the tissues and progeny of a random sample of colonies to determine what percentage of a local population consists of genetic chimeras; and fourth, the project will use sperm competition experiments and paternity fertilization success in matings among kin. This project will provide two products. The first product will be a description of the genetic consequences of histocompatibility. Such information will be critical to our understanding of the adaptive significance of colony fusion and gamete incompatibility in communities of clonal marine invertebrates as well as for identifying the selective forces that initiated and maintained the high levels of allotypic polymorphism that are necessary for a precise allorecognition system. The second product will be the development of molecular techniques for the estimation of paternity. In the future, this capability will allow for more detailed genetic studies that attempt to reconstruct the genealogy of populations and assess the evolutionary fitness of different genotypes.
