Please find my most recent publications on my Google Scholar page.

The evolution of sexual dimorphism in the Salicaceae

Sexual dimorphism in flowers of Populus balsamifera.

The vast majority of plants are hermaphrodites, yet a small proportion do have individuals with discrete sexes (dioecy). The trees and shrubs of the family Salicaceae, and especially the genera Populus and Salix, are predominantly dioecious. Dioecy appears to have evolved prior to the split of Populus and Salix from their common ancestor, and yet the genomic position, genetic mechanism, and pattern of heterogameity appear to be evolutionarily labile.

Differential gene expression in male and female flowers of Populus balsamifera . The color gradient represents variation in natural-log-transformed counts per million reads of gene expression. Each row is a single gene and each column is an individual male (M1-M5) or female (F1-F5) tree. Figure from a forthcoming manuscript.

The goal of my current research project is to map the sex determination locus in novel species of Populus and Salix. We will use a combination of phylogenomics, population genomics, and chemical ecology to determine how sex has evolved in these species, and what ecological mechanisms have driven these patterns. This project is a collaboration with Matt Olson, Steve DiFazio, Larry Smart, and Ken Keefover-Ring.

The role of sex ratio as a context for selection in Silene vulgaris

Mapping transects in a wild Silene vulgaris population near Simmonsville, VA.

Traits involved in social or sexual interactions are ubiquitous in nature. The evolution of these interacting phenotypes involves both direct selection on the trait of the individual, as well as indirect selection arising from the trait values of social partners. Selection on these traits can be measured in natural populations, but the social and ecological interactions that give rise to this selection are often complex. In my dissertation research I explored whether and how selection arising from the traits of social partners drives the evolution of primary sexual characters in the gynodioecious plant Silene vulgaris. This research combined quantitative genetic analyses of social selection in wild populations and experimental gardens, with ecological observations of pollinator recruitment and behavior.

Social selection in wild Silene vulgaris populations

Sex ratio and density of seven wild populations of S. vulgaris around MLBS. Red is the frequency of female plants, blue is the frequency of hermaphrodites.

Silene vulgaris (Caryophyllaceae) is a weedy, perennial, gynodioecious plant. For several years I have been censusing wild populations of S. vulgaris around the Mountain Lake Biological Station in Giles County, VA. Sex ratio and density varies greatly among these populations, which provides an excellent natural experiment to test predictions of the effects of social environment on an individual’s fitness.

Sex ratio variation within populations

Locally-experienced hermaphrodite frequency for all individuals in two wild populations at different spatial scales. Each point represents and individual, and the shading of the point represents the subjective hermaphrodite frequency at that scale. Figure from Sanderson et al. (2016).

In addition to the large variation in sex ratio among populations, there is also continuous variation at a fine scale within populations of S. vulgaris. This form of continuous variation in a social context generates the opportunity for social selection to create differences in fitness functions among individuals. I found that given the fine-scale population structure present in these populations, the variation in locally-experienced sex ratios among individuals could be sufficient to overcome even a two-fold female fitness advantage through fruit set, under a model of social selection.

Pollinator recruitment and behavior

Eagerly anticipating pollinators

Because plants are sessile, the potential fitness consequences of social context will depend on the behavior of pollinating insects. If pollinators exhibit strong preferences for the sex of plants, or the sex ratio of aggregations of plants, then those preferences can constrain or accelerate the effects of the phenotypes of social partners on individual fitness. I used experimental plots of Silene vulgaris to test the prediction that the frequency of hermaphrodites in the plot would affect the recruitment and behavior of pollinators. S. vulgaris is pollinated both by bees and flies during the day, and also by moths during the night.

LS means and 95% CIs for the probability an insect visited an individual (A & B), the number of insects that visited an individual (C & D), and the duration of visits to that individual (E & F) during morning and evening observations. Figure from an forthcoming manuscript.

We found that, although there were differences between daytime and nighttime pollinators, the sex ratio of the plot did not have an effect on the recruitment or behavior of insects to individual plants. This suggests that the fitness effects of fine-scale variation in sex ratio in plant populations are unlikely to be mitigated through a behavioral response of the pollinating insects.