The effects of herbivory on plant mating systems

Abstract

Inbreeding is common in many plant species. Inbreeding reduces heterozygosity and increases the expression of recessive deleterious alleles that cause inbreeding depression. The level or intensity of inbreeding depression plays an important role in the evolution of plant mating systems. Furthermore, high levels of inbreeding depression increase the probability of extinction for small, isolated populations. The vast majority of studies that have quantified inbreeding depression have been conducted under controlled conditions. Inbreeding depression, however, is not static and can vary with environmental conditions. Thus, by altering the magnitude of inbreeding depression, environmental and ecological interactions could alter the rate and direction of mating system evolution or dramatically increase the threat of extinction of small plant populations. An important ecological interaction that affects plant fitness is herbivory. Because inbreeding alters genetic variation, it is likely that inbred offspring will have pronounced physiological and morphological changes, making them more susceptible to herbivory. Compromised defense in inbred plants may serve as the mechanism whereby herbivory increases inbreeding depression. In the first chapter of my thesis, I tested the hypotheses that herbivory by the sunflower spittlebug, Clastoptera xanthocephala (Cercopidae) affects the expression of inbreeding depression in the yellow monkeyflower, Mimulus guttatus. In addition, I tested the hypothesis that changes in morphological traits in inbred plants are responsible for changes in the interaction between spittlebugs and inbred plants. My results suggest that herbivory increases inbreeding depression by reducing plant tolerance of herbivory, but this effect varied among plant genotypes and populations. Inbreeding did not affect resistance to sunflower spittlebugs. Thus, herbivory can increase inbreeding depression and potentially alter the evolutionary dynamics of plant populations and the persistence of plant populations. The results of this and other studies, however, suggest that the effect of herbivory on inbreeding depression varies among plant genotypes and populations. The goal of my second chapter was to explain variation in the effects of selfing on plant-herbivore interactions. I hypothesized that variation in the effect of selfing on plant defense may be explained by the mating history of plant populations. Plant populations with a history of outcrossing often exhibit high levels of inbreeding depression. Plant populations with a long history of selfing typically exhibit low levels of inbreeding depression. To test this hypothesis, I correlated the level of inbreeding depression due to herbivory with flower size. I used flower size as an estimate of mating history because plants with smaller flowers are significantly more likely to inbreed, and typically come from lineages with a long history of inbreeding. Deleterious alleles are much more likely to have been purged within these lineages resulting in reduced inbreeding depression. I found that as flower size increased, so did the effects of herbivory on inbreeding depression. Corolla width was significantly correlated with inbreeding depression due to herbivory. This study adds to a growing body of work that suggests that environmental and ecological interactions can alter the expression of inbreeding depression and alter the rate and direction of mating system evolution and the persistence of plant populations. This is particularly important for conservation biology. As changes in habitats increase, populations of native plants are often isolated and reduced in size. The effects of ecological interactions on inbreeding depression may be amplified in these situations and in order to conserve these populations the effects of ecological interactions may need to be included in management plans.