9628865 WEEKS Evolutionary biologists seek to understand circumstances favoring self-fertilization as a viable form of reproduction. Commonly, offspring produced through self-fertilization have reduced fitness compared to outcrossed counterparts. This "inbreeding depression" is thought to be caused by either expression of recessive deleterious genes (partial dominance model) or by reduced heterozygote advantage (overdominance model). Inbreeding depression should be reduced in a regularly inbreeding population in the former case, whereas fitness continues to decrease with continued inbreeding as long as there is genetic variation in the latter scenario. Quantification of the long-term effects of inbreeding in regularly self-fertilizing species is fundamental to distinguishing between these two models, and thus to increased understanding of mating system evolution. The current proposal is designed to explore factors contributing to the maintenance of outcrossing in a naturally selfing species of shrimp. Eulimnadia texana. By exploiting both E. texana's unique breeding system and its extraordinarily short generation time (5-10 days), the consequences of inbreeding over several generations of self-fertilization will be examined. Data on several fitness components (egg production, age at maturity, survivorship, and growth) will be collected. The fitness responses will be compared for 15 generations in four populations differing in their history of previous inbreeding. These fitness data will be used to compare the predictions of two models for inbreeding depression, and thus to begin to understand the forces contributing to the maintenance of the observed polymorphism in natural populations. This project should lead to a significant increase in our understanding of the evolution of cross fertilization in plants and animals.
