Parasites and protists: the dynamics of species with facultative sexual reproduction

Facultative sexual reproduction is common in nature, and those species that have occasional sex may be more common than those that are obligately sexual. Monogeneans are a common and diverse group of hermaphroditic ectoparasites infecting nearly all aquatic vertebrate species, but they primarily infect fish species. The connections between parasitism and the consequences of sexual reproduction have been debated for nearly half a century. Sexual reproduction remains one of the great mysteries of the biological sciences, because of the postulated two-fold cost of dioecious sexual reproduction. While sexual reproduction has been studied extensively, demonstrations of its explicit benefit have not been fully explored experimentally. The wide variety of sexual systems adds to the complexity of understanding sexual reproduction’s potential benefits. Facultative sexual reproduction, occasional sex, or conditional sex all refer to the benefit of being able to reproduce as an individual either clonally or through self-fertilization. The purpose of this dissertation is to explore the evolutionary benefits and/or disadvantages of facultative sexual reproduction and explore trends in the dynamics of host and parasite communities. Parasite community structures are complex and depend on a variety of host, environmental, and parasite traits. Chapter 1 aims to elucidate factors affecting parasite component communities (all parasites of all species in an ecosystem) and infracommunities (all parasites of all species on or in a host). I collected bluegill sunfish (Lepomis macrochirus) from three reservoirs in New Jersey, U.S.A. with similar environments and recorded host characteristics including sexual morphotype, sex, age, and size. I dissected the fish to describe their gill ectoparasitic infracommunities and analyzed differences at the component (all parasites in all hosts in a single environment) and infracommunity level among measure host traits. I found that host sex and sexual morphotype played a significant role in parasite infracommunity composition and that communities differ significantly from lake to lake. I also found specific patterns on host gills, where total community abundance was higher on outermost gills and decreased steadily to innermost gills. In Chapter 2, I investigated relationships between parasite body size, infracommunity density, and reproductive output. Body size may offer some advantages in reproduction. The Allee effect, or the increase in fecundity with increases in population density, arises when low population densities are common among a sexually reproducing species. This often corresponds to increases in body size as a proxy for fitness. I found that while there was a significant positive relationship between body size and density this did not result in an increase in reproductive output by the parasite community. Chapter 3 used protists that were isolated for decades in laboratory culture (and presumably from major environmental changes) to examine the population dynamic consequences of sexual and asexual reproductive strategies of protists. I subjected populations of sexually and asexually reproducing Paramecium multimicronucleatum to novel abiotic (extreme temperatures) environmental changes through a series of experiments to determine when and if sex provided a fitness advantage. I found that the benefits of sex, especially under such conditions of presumed limited genetic variability, offers little to no benefit to populations and may in fact be detrimental in variable conditions where initial genetic variation may be low.Ph.D.Includes bibliographical reference