Genetic divergence of Cirsium scariosum in eastern and western Canada

Cirsium scariosum is a native western North American thistle that is disjunct in the Mingan Archipelago of Quebec. The presence of these Mingan individuals is thought to be the result of either an eastern migration during the Pleistocene or of a contemporary anthropogenic introduction. Our molecular analysis of Mingan and western individuals of C. scariosum showed that the Quebec individuals harbour the highest percentage of unique alleles and the lowest percentage of polymorphic loci. They differ from western individuals by a total of 2(-4) base pairs in the ETS/ITS combined region. Western C. scariosum is more similar to C. hookerianum from the same region than it is to Mingan C. scariosum. These data favour a Pleistocene origin hypothesis for the Mingan populations and suggest either that hybridization among western species has had a homogenizing effect or that Mingan C. scariosum originated from C. scariosum populations genetically different from those found in Alberta.