Biology Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2749
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Item GENOMIC AND REPRODUCTIVE CONSEQUENCES OF SELF-FERTILITY IN CAENORHABDITIS NEMATODES(2019) Yin, Da; Haag, Eric S; Behavior, Ecology, Evolution and Systematics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The evolution of a new reproductive strategy is expected to be reflected in an organism's genome and impact mating-related traits. Several species of Caenorhabditis nematodes have evolved the ability to self-fertilize from their outcrossing ancestors. Comparisons of species with different reproductive strategies may therefore reveal consequences of transition to self-fertilization. We compared chromosome-scale genome assemblies for the outcrossing nematode Caenorhabditis nigoni and its recently self-fertile sister species, C. briggsae. C. nigoni genome resembles that of outcrossing relatives but encodes 31% more protein-coding genes than C. briggsae. C. nigoni genes lacking C. briggsae orthologs were disproportionately small and male-biased in expression, including the male secreted short (mss) gene family that encodes sperm surface glycoproteins conserved only in outcrossing species. Sperm of mss-null males of an outcrossing species failed to compete with those of wild-type males, despite having normal fertility in non-competitive situations. Restoration of mss to C. briggsae males was sufficient to enhance sperm competitiveness. These results reveal the pervasive influence of sex on genome content that can be used to identify sperm competition factors. Further I found the fitness of mss+ genotype was influenced by mating system and population subdivision. Specifically, mss+ is sufficient to increase male frequency and depress population growth in genetically homogenous androdioecious populations. Using experimental evolution, I demonstrated that when mss+ and mss-null (i.e. wild-type) genotypes compete, mss+ is positively selected in both mixed-mating and strictly outcrossing situations, though more strongly in the latter. I suggest that the lack of inbreeding depression and the strong subdivision thought to characterize natural Caenorhabditis populations impose selection on sex ratio that makes loss of mss adaptive in self-fertile species.Item NATURAL SELECTION, POPULATION GENETICS, AND TRAIT DIVERSIFICATION OF SILENE STELLATA AND ITS POLLINATING SEED PREDATOR HADENA ECTYPA(2017) Zhou, Juannan; Dudash, Michele R; Fenster, Charles B; Biology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)My dissertation explores four aspects of the interaction system consisting of the hermaphroditic plant Silene stellata and its pollinating seed predator Hadena ectypa in a community context. My overarching goal is to deepen our understanding of the selection dynamics influencing floral evolution of hermaphroditic plants. First, I characterized the mating system of S. stellata to evaluate its role on floral evolution of S. stellata and the Silene-Hadena interaction. Second, I compared the spatial genetic structures of S. stellata and H. ectypa to evaluate any discrepancy in their dispersal abilities. Third, I addressed whether selection pressures on floral traits of S. stellata differ between sexual functions and between pollinator types. Last, I quantified the genetic basis of the Silene floral traits to predict evolutionary response under complex selection scenarios. In Chapter 1, I found the study S. stellata population to be predominantly outcrossing with short pollen dispersal distance. The lack of effect of pollinator types (specialized seed predator and other nocturnal copollinating moths) on S. stellata mating system parameters suggests that the dual pollinator type relationship with S. stellata is stable and perhaps contributes to the persistence of the plant species. In Chapter 2, I found no genetic differentiation among the Hadena populations, while the Silene populations showed strong spatial structure. This suggests that pollen flow between Silene populations rarely co-occurs with moth movement. This asynchrony in gene flow could potentially stabilize the interaction dynamics and prevent strict local coadaptation. In Chapter 3, I found conflicting selection pressures between male and female reproductive functions of S. stellata. Strong selection through female function was detected to avoid fruit predation, while competition for mates through male function provides a counterbalancing force potentially contributing to the long-term maintenance of this interaction. In Chapter 4, I found intermediate heritability and prevalent positive genetic correlations between Silene floral traits, suggesting the Silene population is capable of responding to phenotypic selection on its floral design, while the abundant genetic correlations could also pose certain constraints on trait divergence. My results suggest that floral evolution is governed by complex, interdependent processes and that the Silene-Hadena interaction could be maintained through the dynamical balance between various opposing evolutionary forces.