Origins and invasion history of the French broom complex in California

Invasive species pose major environmental and economic threats, but they also provide unique opportunities to study the ecological and evolutionary factors that facilitate the establishment of species in new areas. Most of the woody invasive plants in California were introduced as ornamental plants. These horticultural invasions create ideal study systems to test fundamental questions about how species are introduced, establish, and expand their ranges. Such horticultural introductions are often complex, as multiple closely related species or cultivars may have been introduced from both the native range and cultivated plantings within the invaded range, and may hybridize in the invaded range. For my dissertation research, I utilized molecular phylogenetic and population genetic methods to investigate the origins and population history of the complex French broom invasion in California. French broom was introduced into California as an ornamental and is native to the Mediterranean region. Invasive populations have been hypothesized to include: 1) Genista monspessulana, 2) G. stenopetala and G. canariensis, 3) ornamental sweet broom, and/or 4) hybrids between any of these species. In my first chapter, I examined the evolutionary relationships among invasives, ornamentals, and native-range broom species using sequence data from chloroplast and nuclear regions in a phylogenetic framework. Phylogenetic analyses revealed that, although the majority of invasive French broom in California is G. monspessulana, hybridization with individuals from the ornamental sweet broom group likely occurs in populations throughout the state. In my second chapter, the origins and introduction history of the invasive French broom complex in California were inferred by characterizing the genetic diversity and population structure of invasive and horticultural plants in the invaded range and of G. monspessulana in its native Mediterranean range, using nuclear microsatellite markers developed specifically for this study. Significant differentiation between most native G. monspessulana and invasive French broom in California was found and Bayesian clustering analysis results suggested multiple introductions. An approximate Bayesian computation analysis was used to assess the ornamental origins of the invasive French broom complex, which supported the hypothesis that some invasive French broom is derived from an unsampled population that coalesces with ornamental sweet broom. Overall, my dissertation research suggests that multiple introductions and hybridization have contributed to the establishment of the complex French broom invasion in California.