Polyploidy and invasion of English ivy (Hedera spp., Araliaceae) in North American forests

Polyploidy is a common feature of agricultural weeds and natural area invaders. There are few studies comparing related diploid and polyploid exotics, however, and it is unclear what ecological and genetic factors favor the establishment of weedy polyploids. This research characterizes the geographic distribution and phenotypic characteristics of diploid Hedera helix and tetraploid Hedera hibernica, European species that are invading North American forests. To confirm the taxonomic affinity of invasive plants, we sequenced five non-coding cpDNA regions for 108 individuals (105 populations) as well as reference samples representing all species in the genus Hedera. Because diploid H. helix and tetraploid H. hibernica are poorly distinguished by morphology and DNA sequence, we used flow cytometry to determine their distribution (585 individuals). More than 90 % of sampled plants had cpDNA sequences identical or similar to H. helix sensu lato and H. hibernica. Diploid H. helix was dominant on the U.S. east coast (78.5 % of sampled plants) while tetraploid H. hibernica was dominant on the U.S. west coast (72.2 % of sampled plants), mirroring the species’ occurrence in maritime versus continental climates of Europe. Moreover, for sympatric occurrences in the Pacific Northwest, H. hibernica was larger and more frequently reproductive than H. helix. In a 2-year garden experiment, tetraploid H. hibernica had substantial architectural differences compared to diploid H. helix, including larger (but less numerous) leaves and thicker (but less branched) stems. Field experiments are needed to evaluate “pre-adaptation” (directional ecological filtering) and other factors mediating the invasion of H. helix and H. hibernica.