9419738 EDWARDS Over the past three decades biologists have discovered a wealth of genetic variability in natural populations of organisms. Despite this abundant variability, much of it can be accounted for by two forces: mutation, which creates new variation, and genetic drift, which eliminates variability. At a few extraordinary genes at which a superabundance of variability has been found it been necessary to invoke Darwinian natural selection as an important evolutionary force. One such group of genes, the major histocompatibility complex (MHC), is known only in vertebrates and is involved in defense against invading pathogens and parasites. Although it is clear that some sort of diversity-enhancing selection is operating--perhaps driven by the constant evasion of the immune system by parasites--the precise nature and mechanisms of selection acting in natural populations are largely unknown. This study proposes to obtain a detailed view of mechanism maintaining variability at MHC genes in a common, well-studied North American songbird, the Red-winged Blackbird (Agelaius phoeniceus). Red-winged Blackbirds are particularly useful for this type of study because, unlike many other birds, several populations have been monitored behaviorally and genetically for a number of years. Determining the pattern of allelic variability in birds with known reproductive and parasite profiles will show the mechanisms underlying the exceptionally large amount of variability occurring at this gene. This study will be the first detailed survey of variability at an MHC gene in birds. Although much is known about the extent of MHC variability in rodents, primates and other mammals, even much of this work was conducted without the advantage of recent molecular techniques that will permit variability to be quantified at the nucleotide level. Moreover, there have been only a few studies that have attempted to link MHC variability with life-history or reproductive parameters in natural populations, and these on ly in mice and humans. This study will therefore provide a critical test of the wealth of hypotheses for MHC evolution in mammals, and will extend studies of detailed mechanisms of MHC evolution to a new vertebrate class.
