9728855 Waller Small populations face many hazards including the loss of genetic variation and the genetic effects of inbreeding. Most populations experience moderate to severe inbreeding depression which occurs when accumulated deleterious mutations are exposed via inbreeding. Populations with a prior history of inbreeding, however, may experience less inbreeding depression if their load of mutations has already been expressed and selected out of the population. Some scientists argue that such selection can effectively eliminate inbreeding depression, allowing opportunities to purge threatened populations of their load via intentional inbreeding. Others argue from theory that purging will be weak and ineffective under most circumstances and may actually increase the number of mildly deleterious mutations fixed in small populations. This project will explore just how effectively selection can purge real populations of their inbreeding depression. To save space and allow rapid progress, the investigator will use strains of a small plant in the cabbage family (Brassica rapa) that has been bred to grow quickly in the laboratory. Multiple lines will be subjected to maximal inbreeding via self-fertilization, while other populations of various size will experience alternating selfing and outcrossing or pure outcrossing for six generations. The investigator will then estimate how much inbreeding depression has declined in each treatment by comparing the size and performance of seedlings derived from self- and cross-fertilized seeds from the first, third, and sixth generations. In addition, he will examine whether the high fitness expected in seedlings derived from crosses among the inbred lines is maintained over subsequent generations of random mating (a result suggesting that purging was effective) or deteriorates (reflecting the decay of temporary hybrid vigor). To address another controversy, the investigator will also compar e the amount of inbreeding depression observed when plants are grown under different growing conditions. These experiments should demonstrate how quickly and completely inbreeding can purge populations of their genetic load. Empirical results like these should help to resolve the controversy over how effective purging can be and how quickly it can operate. Such results will help us to understand how mating systems that involve a mixture of selfing and outcrossing evolve. They will also illuminate the genetic hazards faced by small populations and demonstrate whether intentional inbreeding can provide a useful breeding strategy for small populations in threatened or endangered species.
