XVI International Botanical Congess

It is often assumed that parasites cospeciate with their hosts because the host is the primary habitat for parasites. The assumption of cospeciation, and the fact that the parasitological lifestyle tends to cause morphological convergence, has often led to the practice of pathogen species being described on the basis of host association. As a result, morphologically based phylogenies may inflate cospeciation. I examined the coevolution of the flower mimicking crucifer rusts and their hosts from the western US. I compared the host phylogenies (based on both chloroplast and nuclear ITS sequences) with that of their rust pathogens (based on ITS sequences). There was little cospeciation, but there was a strong pattern of geographical association, suggesting that host shifts have occurred more often than cospeciation.