9629287 Johnson One of the outstanding problems in biology today is why sexual reproduction is so common in many organisms when there appear to be considerable costs to sexual reproduction relative to alternatives like parthenogenetic reproduction (individuals develop from unfertilized eggs). Of the many hypotheses developed to explain the widespread maintenance of sexual reproduction in many organisms, the parasite theory appears to be the most plausible and testable. According to the parasite theory, sexual reproduction is favored over parthenogenetic reproduction in hosts under high risk of parasitism because sexual offspring may be less susceptible to infection by parasites. Parthenogenesis should be favored when there is minimal risk of parasitism. Parthenogens may, however, suffer high levels of parasitism if parasites recolonize parthenogenetic populations with limited genetic variation. Field and laboratory experiments will be undertaken to distinguish among the causes of higher levels of parasitic flatworms in parthenogenetic population of the North American freshwater snail, Campeloma decisum. Parthenogenesis may be favored due to sperm limitation caused by a trematode worm or, alternatively, parthenogens may have had an advantage colonizing lakes and rivers after glaciers receded. High parasitism results from lack of genetic variation in asexual populations. Field and laboratory experiments will test the prediction from the parasite theory that sexual individuals should experience lower levels of infection and greater variation in infection relative to parthenogenetic individuals. The research outlined above should provide important insights into how epidemic diseases spread in organisms with limited amounts of genetic variation. Two important groups of organisms that may have limited genetic variation are inbred lines of agricultural crops and small populations of endangered plants and animals.
