Genetic variation and inbreeding depression in the rare California endemic, Astragalus agnicidus (Leguminosae)

Thesis (M.A.)--Humboldt State University, Biology, 1997Predictions of low genetic variation and low levels of inbreeding depression in small plant populations were tested on Astragalus agnicidus, a species with only one known population. Loss of alleles through drift and increased inbreeding (followed by selection) may leave small populations genetically depauperate, but with negligible genetic load as well. This scenario results in low levels of inbreeding depression as selfed and outcrossed progeny become more equivalent in overall fitness (i.e., the difference in progeny fitness between pollination treatments decreases). As predicted, gel electrophoresis of isozymes indicated that A. agnicidus has low genetic variation. Five fitness variables: seed set, seed weight, germination, survival, and seedling weight were used to test for fitness differences between open-pollinated and self-pollinated progeny. Some degree of fitness reduction for selfed progeny was seen in all variables, but only in seedling survival was the difference significant. Results indicated that the effect of the maternal parent was more important than pollination treatment on determining the fitness of an individual. Therefore, there is low enzyme diversity within the population, but genetic load is still maintained in the population and varies between individuals. The role of the mating system in influencing the level of genetic variation and inbreeding was also examined. One polymorphic enzyme, IDH, was used as the genetic marker in estimating an outcrossing rate of .59, which indicates a mixed mating system. The advantages of a mixed mating system include maintenance of moderate levels of heterozygosity (and thus, genetic variation) through outcrossing and elimination of genetic load and retention of optimum genotypes through selfing. Evolutionary models predict that A. agnicidus, with a high level of self-compatability and an inbreeding depression level of