Although polyploidy is a common feature of agricultural weeds and natural area invaders, there are few studies comparing related diploid and polyploid exotics. It is unclear what ecological and genetic factors favor the establishment of weedy polyploids, features fundamental to the understanding, and thus control, of many invaders that pose threats to biodiversity and cause economic losses. My doctoral dissertation combined molecular and cytological studies with common gardens and fieldwork to address this problem in English ivy (Hedera spp. Araliaceae), a polyploid complex of woody vines native to much of Asia, Europe, and North Africa.
Loved for its attractive foliage and hardiness, ivy is widely cultivated in North America and has become invasive in forests on the east and west coasts. Despite its popularity among horticulturists and research groups interested in the systematics and ploidy variation among the species, the evolutionary relationships among the ivies were largely unresolved when I began my thesis. I designed a molecular approach combining the cloning and sequencing of a low-copy nuclear gene with a large-scale chloroplast sequencing effort to increase the phylogenetic resolution within this group. This also provided evidence that genome duplication had occurred independently several times within Hedera and demonstrated no evidence of a history of hybridization among different species, both of which are controversial ideas in the literature.
Combining field collections with sequencing and cytological analysis, I discovered that although both diploids and polyploids are invasive, virtually all invaders originate from a closely related clade within Hedera. Furthermore, I found that diploids are the primary invader on the east coast while polyploids comprise the majority of the west coast invasion, suggesting ecological filtering, anthropogenic effects, or a combination of the two. My work also uncovered the existence of naturally-formed triploid ivy for the first time. These plants exhibit attractive foliage and a vigorous growth habit and, once their sterility is confirmed, will be useful in areas sensitive to invasion via sexual recruitment.
This work is featured in the following publications:
Green, A., T.S. Ramsey and J. Ramsey. 2013. Geographic origins and taxonomic affinities of North American invasive ivy (Hedera spp., Araliaceae). Biological Invasions. 15: 2219-2241. (PDF)
Green, A., T.S. Ramsey and J. Ramsey. 2011. Phylogeny and biogeography of ivies (Hedera spp., Araliaceae), a polyploid complex of woody vines. Systematic Botany 36: 1114–1127. (PDF)