Population genetic differentiation, mating system, and effective population size of the tuliptree (Liriodendron tulipifera L.) in the Mid-Atlantic United States

Abstract

Continuous natural habitats are increasingly transformed into anthropogenic landscapes —mosaics of natural habitats fragments surrounded by heterogeneous mixtures of different land use. Anthropogenic habitat transformation is particularly evident in the clearing of forests. Forests that have reached a great age with little or no anthropogenic disturbance (old-growth) are increasingly rare. At the same time, urbanization —one of the most readily apparent human-mediated effects on the landscape— continues to expand, and as a result, an increasing percentage of forests are becoming part of urban landscapes and ecosystems. The aims of this thesis were to investigate population genetic effects of anthropogenic landscapes on long-lived forest trees at different time scales: immediate, through the estimation of the mating system; short-time, through the estimation of effective population size using linkage disequilibrium based methods; and long-time, through the estimation of population genetic differentiation. To accomplish these aims, I used the tuliptree (Liriodendron tulipifera L.), a tree in the family Magnoliaceae native to Eastern North America that has a wide geographic distribution in the Southeast and mid-Atlantic United States and occurs in diverse habitats, including both old-growth remnants and forest patches embedded in urban areas where it reproduces and recruits naturally.

Chapter 1 describes the development and validation of novel L. tulipifera genomic microsatellite (SSR) genetic marker loci using high-throughput sequencing and bioinformatics methods. These new SSR markers facilitated empirical study of population genetic patterns in L. tulipifera. Chapter 2 compares the population genetic differentiation and the mating system among old-growth forests and urban forest patches. L. tulipifera population genetic differentiation in the mid-Atlantic U.S. was characterized by a pattern of isolation by distance (IBD), suggesting that urbanization has not had a major impact on regional genetic differentiation patterns which have evolved over hundreds of generations in the past. Despite marked environmental differences between old-growth and urban forest, all populations shared a fully outcrossed mating system without any variation in self-fertilization that can accompany loss of pollinators. Chapter 3 compares the strength of random genetic drift estimated from the effective number of breeders (Nb) from seedlings and from mixed-age reproductively mature trees (Ne {mixed ages}). These two sampling methods showed similar and small effective population sizes, suggesting genetic drift is a major determinant of polymorphism within populations and of genetic differentiation among populations.