The main goal of this line of my research is to understand the evolutionary processes that are responsible for shaping the pattern of sexual expression in plants. One intriguing question in mating system evolution is the separation of two sexes in plants. It is generally believed that dioecious plants (plants with male and female individuals) have derived from hermaphroditic ancestors. Theoretical studies have proposed several possible conditions that may facilitate such transition. However, little empirical data is available to evaluate these predictions. The approach I take to study this question is to take advantage of the existence of taxa that appear to have initiated the first step of gender specialization but have not yet completed the process. These plants consist of plants with different sexual expression. By studying the selection forces that are operating on systems that are in transition, we may be able to catch evolutionary mechanisms "in action". Currently, there are two projects in related topics.
Variation of sex ratios in Glechoma hederacea
Glechoma hederacea is a species in the Mint family (Lamiaceae) that exhibits a sex expression system called gynodioecy. Gynodioecious species consist of two types of individuals: hermaphroditic individuals that have perfect flowers with functional anthers and stigmas and female individuals that have functional stigma but sterile or nearly sterile anthers. This variation in male fertility is most likely due to interaction between mitochondrial and nuclear genomes. Gynodioecy has been suggested to be the transitional stage from hermaphroditism (combined sex) to dioecy (separate sexes) in plants, hence provides the opportunity to study the potential selection forces for the evolution of separate sexes in plants.
One of the phenomena commonly observed in gynodioecious system is the wide variation in sex ratios in natural populations. Many factors may influence the frequency of females in a population. For example, the genetic inheritance and the ecological factors in the habitat can both affect how likely a male sterility mutant can be maintained in a population. We are combining survey of plants from a wide geographic range and field experiments to examine the potential mechanism that are responsible for the sex ratio variation observed in natural populations of Glechoma hederacea.
Evolutionary genetics of the maintenance of variation in sex expression in Geranium maculatum
Watch ShuMei's 3MT (3 Minute Thesis) presentation in Spring 2014
Most of flowering plants are hermaphroditic and therefore can reproduce both as male and as female parents. Therefore, the fitness of a hermaphroditic plant should be the combination of the seeds it produces (the female fitness component) and the seeds that it sires as the pollen parent (male fitness component). Most plant studies have been using seed number as THE fitness measurement. These results might be misleading if the male and female fitness are not positively and linearly correlated. In this project, we are asking the question how or whether the pattern of natural selection differs between male and female functions in hermaphroditic plants and how these patterns influence the evolution of floral traits in the common morning glory, Ipomoea purpurea. We are using a combination of artificial selection experiments in the greenhouse and common garden experiments to characterize the patterns of selection that are specific to male or female function. I am developing both the marker techniques and the statistical methods to allow us to obtain good estimates of male fitness.
Evolution of Selfing
Though theoretical studies predict that most hermaphroditic species should be either predominantly selfing or predominantly outcrossing, most of them exhibit an intermediate level of self-fertilization (i.e. a mixed mating system). I am interested in the ecological and genetic mechanisms that contribute to maintaining this intermediate rate of selfing fertilization. The species I study is the common morning glory, Ipomoea purpurea. The average selfing rate of this species was estimated to be about 30% for a natural population. Combining the genetic marker techniques and greenhouse and field experiments, I have tested 4 possible mechanisms that might be contributing to the maintenance of this intermediate level of selfing: (1) joint evolution of inbreeding depression and selfing rate, (2) pollen discounting, (3) antagonistic pleiotropic effects and (4) gender-specific inbreeding depression. I have demonstrated that though both inbreeding depression and pollen discounting were detected in my study, neither one of the mechanisms alone was sufficient to explain the mixed mating system in this species. In contrast, a fifth mechanism, the frequency dependent male siring success, that has never been considered in theoretical studies of mating system evolution seems to be critical for the maintenance of a intermediate selfing rate in this species. Results from this project so far suggest that our bias towards using seeds as the measurement of the "lifetime" fitness may not be appropriate for some species and more attention should be paid to the reproductive success through pollen.
Current experiments in this area include (1) examining the variation in the reproductive success through pollen, (2) testing sex allocation theory using Ipomoea species and (3) further examining the gender-specific inbreeding depression hypotheses.