|Grunwald, Niklaus - Nik|
Submitted to: Molecular Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/8/2008
Publication Date: 3/2/2009
Citation: Goss, E.M., Carbone, I., Grunwald, N.J. 2009. Ancient isolation and independent evolution of the three clonal lineages of the emerging sudden oak death pathogen Phytophthora ramorum. Molecular Ecology. 18(6):1161-1174. Interpretive Summary: Phytophthora plant pathogens are some of the most destructive pathogens affecting agricultural and native plants and are responsible for a number of new infectious diseases of plants. Sudden oak death is a new disease caused by the pathogen P. ramorum and has caused extensive mortality of oaks and tanoaks in Northern California as well as economic losses to U.S. and European nurseries due to its infection of common ornamental plants. We examined the evolutionary history of P. ramorum from nuclear DNA sequence data using a method of analysis that allowed us to determine the relative timing of changes in the DNA back through the history of the species. In the United States and Europe, P. ramorum occurs in three distinct genetic groups. We found that these groups have been separated for at least 11 percent of their history, an evolutionarily significant amount of time estimated to be from 165,000 to 500,000 years. There was also evidence that the ancestors of P. ramorum were sexually reproducing, whereas known populations are clonal. Our data suggest that the three genetic groups were introduced from three different geographic locations that were sufficiently isolated from each other to prevent genetic mixing of the groups over a long period of time. Therefore, this disease probably arrived in North America and Europe by three separate introductions.
Technical Abstract: The genus Phytophthora includes some of the most destructive plant pathogens affecting agricultural and native ecosystems and a number of recent emerging and reemerging infectious diseases of plants. Sudden oak death, an emerging disease caused by the exotic pathogen P. ramorum, is responsible for extensive mortality of oaks and tanoaks in Northern California as well as economic losses to U.S. and European nurseries due to its infection of common ornamental plants. In its introduced range, P. ramorum occurs as three distinct clonal lineages. The two common lineages are opposite mating types, yet mating has produced nonviable progeny in lab tests and has not been documented in the field. We inferred the evolutionary history of P. ramorum from nuclear sequence data using coalescent-based approaches. We found that the lineages have been diverged for at least 11% of their history, an evolutionarily significant amount of time estimated to be on the order of 165,000 to 500,000 years. There was also strong evidence for historical recombination between the lineages, indicating that the ancestors of the P. ramorum lineages were members of a sexually reproducing population. Due to this recombination, the ages of the lineages varied within and between loci, but coalescent analyses suggested that the European lineage may be older than the North American lineages. The divergence of the three clonal lineages of P. ramorum supports a scenario in which the three lineages originated from different geographic locations that were sufficiently isolated from each other to allow independent evolution prior to introduction to North America and Europe. It is probable that the emergence of P. ramorum in North America and Europe was the result of three independent migration events.