Genetic variation across VNTR loci in central North American Taraxacum surveyed at different spatial scales

Dandelions (Taraxacum officinale Weber (sensu lato); Asteraceae) have been introduced to North and South America with human migration from Europe. While potential source populations have both sexually and obligate asexually (agamospermous) reproducing lineages, apparently only the latter have successfully colonized the Americas. The consequences of obligate agamospermy on dandelion population genetic diversity in North America remain little explored. Here we use four different synthetic DNA probes that reveal genetic markers at multiple variable-number-tandem-repeat (VNTR) loci to examine patterns of genetic variation among plants collected along three different central North American transects with plants (21 to 22 individuals per transect) separated by: 1) > 2 m and < 60 m (short transect); 2) > 5 km and < 30 km (medium transect); and 3) > 30 km and < 340 km (long transect). The mean number of VNTR markers revealed per plant was 59.3. Co-clonal individuals (proportion of bands shared exceeding 90%) were found in each transect, with the index of clonality (the percent of co-clonal individuals detected in a transect) ranging from 34.12% for the short transect to 18.65% for the long transect. Co-clonal individuals were separated by up to 200 km. With redundant examples of co-clonal individuals removed, mean similarity (proportion of band sharing) of distinct genotypes within transects was 0.426, and no statistical differences in level of similarity between transects, nor indication of genetic differentiation between transects, was detected (mean F-st between transect levels with all individuals included=0.05). These results indicate: 1) that dandelion genetic diversity of colonizing lineages in central North America is moderately high and does not reflect extreme bottleneck effects shown by some colonizing species; and 2) that dandelion seed dispersal can be very effective in maintaining similar levels of genetic diversity at the different scales of sampling in this study, with certain clones maintaining numerous, widespread individuals. Evidence that VNTR mutation is detectable within dandelion clonal lineages is presented, demonstrating that "clonal families" with lines increasingly differentiated from one another will continually evolve, and that Muller's rachet is, in all likelihood, turning for asexual lines.