Yi-Hsin Erica Tsai

Dissertation project:
The post-glacial colonization history and present-day population dynamics of the parasitic plant, Epifagus virginiana

The goal of this project is to understand how parasites invade and persist in host populations. In particular, how parasite migration history and population structure are controlled by forces such as limited dispersal, host density effects, and local adaptation will be studied. Understanding how the migration patterns of parasites are constrained by their hosts is the first step in determining how closely associated species can influence each other’s spread. The study’s approach is historical and compares the post-glacial migration of a parasitic plant, Epifagus virginiana (beechdrop; Orobanchaceae), with its host, Fagus grandifolia (American beech; Fagaceae). Broad scale population structure analyses for Epifagus will be performed to identify any patterns of co-migration, common glacial refugia or similar migration corridors between the parasite and host. At a fine scale, host density effects on parasite invasion and correlations between genotypes of host-parasite pairs will be examined as potential host mechanisms that limit parasite establishment. This research will produce a comprehensive migration history of this parasite with an emphasis on what factors constrained its invasion and how the colonization patterns of host and parasite relate. These results will be applied to forecasting how assemblages of species (i.e. communities) respond to climate change.

I. The post-glacial colonization patterns of the parasitic plant, Epifagus virginiana, based on cpDNA

We use cpDNA sequence data and phylogeographic methods to describe the population structure, locate glacial refugia, and identify migration routes of Epifagus. By comparisons to the host’s migration history, we draw conclusions about the parasite’s ability to invade new host populations and about what forces may limit its range expansion.

II. Host density and dispersal limits to parasite infection at a local scale

The broad scale phylogeographic patterns found in chapter I reflect local scale processes (e.g. host density effects, limited dispersal). What controls parasite infection at a local scale? First, we model the relationship between host density, proximity to host, and infection rate. Second, we look at the population structure and distribution of parasite genotypes at a local scale to calculate gene flow and dispersal parameters.

III. The post-glacial colonization patterns of the parasitic plant, Epifagus virginiana, based on nuclear DNA

This chapter’s goal is similar to chapter I: to identify the migration routes of Epifagus. However, the methodology and approach are different. We use nuclear microsatellite markers and Bayesian clustering methods to elucidate the broad scale colonization patterns. This nuclear dataset contains additional information on pollen flow, is more variable, and is likely to identify finer scale patterns than the cpDNA dataset. These results will be compared with the host’s colonization history and with those from chapter I.

This material is based upon work supported by the National Science Foundation under Grant No. 0608310. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation (NSF).