Genetic evidence for founder effects in the introduced range of houndstongue (Cynoglossum officinale)

Phenotypic differentiation can occur between the native and introduced ranges of a species as a result of novel selective pressures, or by neutral processes and historical events. Our aim was to determine how underlying patterns of genetic diversity and potential population origin might have contributed to phenotypic differentiation between the native and introduced ranges of an herbaceous weed. We combined data from microsatellite markers from 16 native and 16 introduced populations of Cynoglossum officinale, a noxious weed of the western US, with previously published phenotypic data from common gardens to investigate genetic diversity in both ranges and relate population structure to phenotypic differentiation. Several lines of evidence suggest loss of genetic diversity during the introduction of C. officinale. Despite reduced diversity, introduced plants out-performed natives in a common garden in one environment. We found little evidence that population-level variation in diversity contributed to phenotypic variation (e.g. through inbreeding depression). Our results suggest that establishment, spread, and potentially adaptation of a species to a new range is not prevented by reductions in genetic diversity of the magnitude we observed. Further, we suggest that non-random filtering or biased introduction at the point of emigration may contribute to phenotypic divergence between ranges.