Exploring the causes and consequences of variation in flowering phenology in monkeyflower (Mimulus ringens)

Abstract

Flowering phenology, the timing and pattern of flower deployment, can have a marked influence on intrapopulation gene dispersal and individual reproductive success. Traits that influence flowering phenology can be affected by genetic factors, environmental conditions, and biotic interactions. Within a population, among-individual variation in flowering phenology affects the number of prospective mates, the risk of mating with lower quality individuals, and the likelihood of self-pollination. In this dissertation, I explore variation in flowering phenology within and among nine natural populations of monkeyflower (Mimulus ringens). Monkeyflower is a hermaphroditic perennial herb occurring in wetlands throughout central and eastern North America. This species is self-compatible and exhibits a mixed-mating system. It is primarily bumblebee pollinated, and individual flowers last for only a single morning. This short floral lifespan makes monkeyflower an excellent system for studying variation in flowering phenology.In Chapter I, I use a common garden experimental design to explore the extent of intra- and interpopulation variation in flowering phenology. By raising plants in a common garden, I minimized environmental variation, allowing me to quantify variation in flowering phenology due primarily to genetic factors. Every other day throughout the flowering period, I measured the size of each plant’s daily floral display. Using these data, I quantified variation in several traits that influence flowering phenology. I discovered that flowering phenology traits varied widely within and among populations. Flower deployment patterns exhibited a sputtering pattern and frequently oscillated between large and small daily floral displays. Flowering patterns of individual plants substantially influenced the extent of flowering synchrony within populations. I also found that the extent to which individuals flower synchronously with others in the population as well as the number of days they are in bloom significantly impacted the number of potential mates (their mating opportunity). In Chapter II, I explore the association between variation in flowering phenology and molecular genetic diversity. In populations with low genetic diversity, plants possess many of the same alleles and may therefore exhibit highly synchronous flowering patterns. On a given day, plants in a population with low genetic diversity may be expected to have numerous potential mates because most plants are flowering at the same time. By contrast, higher genetic diversity may be associated with increased mismatch in the timing of flower deployment and may therefore have lower mating opportunity. Here, I test the hypothesis that standing genetic diversity is related to among-individual heterogeneity in flower deployment patterns. Using progeny genotype arrays, I estimated intra- and interpopulation genetic diversity using putatively neutral molecular markers. I then related this to mating opportunity as previously measured in the common garden. I found that two indices of genetic diversity were negatively associated with mating opportunity. I also noted substantial among-population genetic divergence that is likely due to a combination of limited gene flow as well as moderate levels of inbreeding. My dissertation presents a novel perspective on the evolution of flowering phenology. Flowering phenology traits can influence the dynamics of pollen flow and are likely to be important targets of natural selection. As the raw material on which natural selection may act, genetic diversity is necessary for populations to adapt to changing conditions. By exploring the causes and consequences of variation in flowering phenology, we can enhance our understanding of the evolution of flower deployment strategies and associated mating patterns.