Invasive exotic species may pose threats to native congeners through the process of genetic assimilation, where asymmetrical hybridization and subsequent hybrid backcrossing results in the loss of pure native populations. I am studying the threats of genetic assimilation by the exotic Taraxacum officinale into populations of the native alpine dandelion, Taraxacum ceratophorum. Using expected mendelian ratios based on allozyme variation, I demonstrated that, unlike the apomictic exotic T. officinale, populations of T. ceratophorum on Pennsylvania Mountain (Park County, CO) reproduce sexually. Initial surveys determined that ecological conditions required for hybridization between T. ceratophorum and T. officinale occur for sympatric populations in the Colorado Rocky Mountains. On Pennsylvania Mountain, native and exotic dandelions exhibit broad phenological overlap in flowering during 1999 and 2000. In mixed arrays, insect visitors demonstrated no preference for native inflorescences over exotic inflorescences. Furthermore, insects move pollen analogues from exotics to natives and between natives at the same frequency. Hand-pollination of T. ceratophorum with T. officinale pollen produced hybrid seeds. To test for reproductive barriers to the formation of hybrids, I performed three hand pollination treatments on native inflorescences—a control of native pollen, native and exotic pollen applied at the same time, and exotic pollen preceding native pollen by 15 minutes. Hybrids were consistently produced at low frequencies only when exotic pollen preceded native pollen by at least 15 minutes. Despite the exotic pollen disadvantage, other mechanisms may promote the establishment of hybrids formed in natural populations. Germination rates for parental and hybrid seeds sown into natural habitats on Pennsylvania Mountain varied significantly with genotype. Hybrids had a 66% selective advantage over the native species at this initial life history stage, which may eventually facilitate backcrossing with the native species.

Key words: genetic assimilation, hybridization, Taraxacum ceratophorum (Asteraceae), Taraxacum officinale (Asteraceae)