My research focuses on two main areas: evolutionary ecology of plant mating systems and conservation biology. I am interested in reproductive processes in plant populations, and there are three main questions that drive my research:
1) What are the processes that generate and maintain variation in plant mating systems?
2) How do ecological and genetic factors interact to influence the mating system as well as male and female fitness?
3) How do shifts in the pollinator community or loss of pollinator services affect plant mating patterns?
Evolution of hermaphroditism
Most flowering plants are hermaphrodites, and thus gain fitness by producing seeds (female function) as well as by siring seeds on other individuals (male function). The diversity of mates is typically only quantified through the number of mates siring seeds on maternal plants. We know very little about mate diversity from the male perspective, despite such genetic diversity having important fitness consequences.
Using paternity analyses, we quantified mate composition through male and female function in Mimulus ringens (Christopher et al. 2019, Amer. J. Botany). We found that there was almost no overlap between the two sex functions of the same flower (Fig. 1). This results in a near doubling of mate diversity, which may play a role in maintaining dual sex functions in flowering plants. Future work in this area will investigate the strength of selection on floral traits through male and female fitness.
Fig. 1. Mating networks. A) paternal individuals (left) and links to the individuals on which they sired seeds (right). B) maternal individuals (left) and the individuals who sired seeds on each maternal plant (right).
Interactions between ecological and genetic factors
Variation in the selfing rate among populations reflects the interplay between ecological and genetic factors. We investigated how ecological context, including pollinator identity and visitation rate, and genetic factors, including heritable floral traits, influence the mating system in 13 natural populations of M. ringens in northeast Ohio.
We found that the selfing rate across populations showed considerable variation (Fig. 2A; 0.13-0.58). The selfing rate increased significantly as pollinator visitation decreased and floral display size increased. The selfing rate was not correlated with floral morphology. These results highlight that the interactions between ecological and genetic factors play a critical role in maintaining a mixed mating system.
Fig. 2. A) Selfing rate and B) inbreeding depression in 13 natural populations of M. ringens in northeast Ohio.
Mating system evolution depends not only on genetic and ecological factors, but also on the extent to which inbreeding depression lowers the fitness of selfed offspring relative to outcross progeny. Thus, inbreeding depression is a strong selective force that can maintain outcrossing. We found substantial variation in the level of inbreeding depression across the 13 natural populations (Fig 2B; 0.0-0.80). Contrary to theoretical expectations, we found populations with moderate levels of selfing and moderate to high levels of inbreeding depression, suggesting that deleterious alleles have not been purged from these populations.
Continuing work in this area will explore the variation in inbreeding depression. We will grow seeds collected from the natural populations in the greenhouse under a range of environmental conditions. Using this system, we will investigate questions about how inbreeding depression is expressed: is it affected by environmental conditions? Do levels of inbreeding depression differ between the greenhouse and the field? When in the lifecycle is it expressed? How does inbreeding depression interact with the mating system and what are the evolutionary implications?
Conservation biology and pollinator communities
Pollinators provide critical ecosystem services but are threatened by habitat loss and climate change. Of course, the complete loss of pollinators from an ecosystem would be devastating, but before extinction occurs, pollinators may decrease to low frequencies such that they no longer provide adequate pollination services. Additionally, co-occurring pollinators may respond differently to habitat changes, resulting in shifts in the composition of the pollinator community. If pollinator species have different foraging behaviors, changes in the pollinator community may have significant effects on plant mating patterns. For example, data from almost two decades at the UW-Milwaukee Field Station has shown that the relative abundance of bumblebee species has changed markedly over the years (Fig 3).
Fig. 3. Bumblebee species that occur at the UW-Milwaukee field station, from left to right: Bombus impatiens, B. pensylvanicus, B. vagans, B. fervidus, B. griseocollis .
Understanding how biotic interactions are affected by habitat loss is an important aspect of conservation biology. We will examine the consequences of changes in pollination services for M. ringens. We will systematically exclude all but one bumblebee species and then identify changes in mating patterns by comparing selfing rate, mate diversity, male fitness, and female reproductive success between treatments.