Genetic diversity and mating system estimates for Verbascum thapsus L. (scrophulariaceae)
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Invasive species represent a threat to biodiversity and to the composition of natural communities. While most research has concentrated on ecological aspects of biotic invasions, the evolutionary potential of invasive species has received relatively little attention. In this study I used allozyme data in a two year study to determine levels of genetic diversity, the distribution of genetic diversity within and among populations, and the mating system for the ruderal weed Verbascum thapsus. Further, since V. thapsus populations are often small and ephemeral I determined the effect of population size on genetic diversity and the mating system. Genetic diversity at the species and within population levels was higher than mean estimates previously reported for plant species. Population size had little effect on levels of genetic diversity. Mating system estimates show that V. thapsus progeny are predominately the result of outcross mating events. Although there was no effect of population size on outcrossing estimates, the effective number of outcrossed pollen donors significantly decreased as population size increased. As the number of flowering individuals increased, pollinators may spend more time within the population which may skew male reproductive success and lower the effective number of pollen donors. Thus, smaller populations receive more genetically diverse pollen. Taller individuals experienced higher levels of outcrossing than shorter individuals during both years. The top of the infructescence section of short plants had significantly higher outcrossing than the middle or lower section. However, in taller plants the middle section of the infructescence had higher outcrossing than the top and lower sections. The effective number of outcrossed pollen donors within populations was higher for taller plants. These results indicate that taller plants may have larger and more genetically diverse pollen loads while shorter individuals experience some pollen limitation leading to increased selfing. Finally, significant fine-scale genetic structure (FSGS) was observed at shorter spatial scales (fij ranged from 0.148 to 0.047). There was variation among the three populations for the scale and magnitude of FSGS. This variation may be attributed to differences in the disturbance regime among populations and to variation of adult density among populations (i.e. more or less overlap of seed shadows).