Evolutionary biologists have long been interested in understanding how populations of organisms become different from one another, because it is a necessary precursor to speciation. Variation in population sex ratio (for example, the number of females vs. males, or females vs. hermaphrodites) provides an excellent model for investigating mechanisms of population differentiation. Because the population sex ratio determines the identity and number of available mates, it affects how genetic variation is distributed within and between populations. This project will investigate why the proportion of females vs. hermaphrodites in a flowering plant (Lobelia siphilitica) are higher in small populations and at warmer sites. We will test whether female frequencies vary because of natural selection, or if other evolutionary forces, such as genetic drift and gene flow, are preventing many populations from reaching an equilibrium sex ratio. Distinguishing the effects of these different evolutionary mechanisms is important because they produce distinct patterns of population genetic structure. This research will provide a framework for evaluating how breeding systems influence the ability of species to modify their range in response to climate change, particularly global warming. If female L. siphilitica plants are more common in areas with higher annual mean temperatures because of natural selection, then populations should become more female-biased in response to global warming, which could affect migration rates and species persistence.
PI: Andrea Case (Kent State University); Co-PI: Christina Caruso (University of Guelph) Evolutionary biologists have long been interested in understanding how populations of organisms become different from one another, because it is a necessary precursor to speciation. Variation in population sex ratio (for example, the number of females vs. males, or females vs. hermaphrodites) provides an excellent model for investigating mechanisms of population differentiation. Because the population sex ratio determines the identity and number of available mates, it affects how genetic variation is distributed within and between populations. This project investigated why the proportion of females vs. hermaphrodites in a flowering plant (Lobelia siphilitica) are higher in the southern and central portion of its range in eastern North America. We specifically tested whether female frequencies vary because of natural selection, or if other evolutionary forces, such as genetic drift and gene flow, are preventing some populations from reaching an equilibrium sex ratio. Distinguishing the effects of these different evolutionary mechanisms is important because they produce distinct patterns of population genetic structure. Our findings indicate that natural selection, rather than other evolutionary forces, have shaped the distribution of females within Lobelia siphilitica. However, our results indicate that selection is not acting on the fitness of females relative to hermaphrodites, as might be expected. Instead, natural selection appears to be operating directly on the sex determining genes. Plants with females and hermaphrodites, such as L.siphilitica, do not have simple sex determination (i.e., they do not have X and Y chromosomes). Many genes are responsible for sex determination, and consequently plants of the same sex don’t all carry the same sex-determining genes. This makes sex determination, and the study of variation in population sex ratio, more complex. In our studies, we have found that not all combinations of sex-determining genes are equally fit in natural populations. Specifically, selection among different genotypes of hermaphrodites may be largely responsible for maintaining variation in the frequency of females. In other words, the maintenance of females within populations does not reflect selection on females, but selection among hermaphrodites. Populations with many females contain hermaphrodites that vary considerably in their pollen viability, while populations with few females contain hermaphrodites that all have relatively high pollen viability. Evolutionary theory suggests that the cause of this variability in pollen viability among populations reflects a fitness cost of sex determination. Our research is the first to provide evidence of this cost in natural populations, and the first to indicate that the magnitude of this cost is variable among populations. The next step in our research will be to understand the agents of natural selection that affects the cost of sex determination, and exactly why this cost varies among populations, and why cost is highest in the south and central portion of the species range of L. siphilitica. If female L. siphilitica plants are more common in areas with higher annual mean temperatures because of natural selection, then populations should become more female-biased in response to global warming, which could affect migration rates and species persistence. Therefore, more generally, this research will provide a framework for evaluating how breeding systems influence the ability of species to modify their range in response to climate change, particularly global warming.
