Uncovering the Mechanisms that Lead to Spatial Patterning of Population Sex Ratios in Gynodioecious Plants

Environmental variation is an important determinant in the geographic distribution of species. Observing phenotypic variation in geographic space also hints at the environment differentially affecting phenotypes. In gynodioecious species, among population variation can range between 0-100% females. There appears to be a geographic pattern of variation in female percentage observed across populations of Lobelia siphilitica L., as high-female populations are more common in the south-central part of the species range. The long-term persistence of these populations comes into doubt, as increasing percentage of females leads to less pollen supply from the fewer and fewer hermaphrodites. In my dissertation, I first confirm a geographic pattern, and then, I explore potential environmental causes for the structuring of these high-female populations. Chapter I introduces the mechanisms that have been confirmed for female maintenance in these populations and where geography and environmental variation potentially affect female maintenance. In Chapter II, I present the findings from ecological niche models used in predicting female percentage across the species range. Variable importance metrics indicate that high-female populations are associated with warmer climates in nutrient-limited soils. Chapter III details findings from a field survey and greenhouse experiments. Female percentage is positively correlated with soil clay content and, consequently, negatively associated with soil sand content. Female percentage is also positively correlated with concentrations of magnesium, copper, calcium, and potassium. The greenhouse experiments document soil conditions altering leaf and soil production, but with a lack of flowering, I could not draw conclusions about relative differences between females and hermaphrodites. Chapter IV explores gynodioecy from a species perspective. High-female populations are associated with what are considered more stressful conditions. I used distribution modeling between four Lobelia species to look at species differences and sexual system differences (gynodioecy vs. hermaphroditism). Although niche overlap metrics indicate greater than 50% shared geographic and environmental space between species and sexual systems, when focusing strictly on the most important variables, there is some separation observed among species and between sexual systems. The gynodioecious sexual system appears to be more of a generalist, while the hermaphroditic sexual system appears more specialized. Chapter V validates predictions from a digital soil map, SoilGrids, with soil cores collected in the field. Sand and silt have the best fit scores, while pH and clay are more intermediate. Raw predictions from SoilGrids for carbon and nitrogen should not be used, as field data was not significantly correlated with the predictions. My dissertation explores the many correlations between the environment and population sex ratios in L. siphilitica to demonstrate that female percentage is potentially affected by temperature and soil physical and chemical characteristics.