AFLP analysis of California and Mediterranenan populations of Seiridium cardinale provide insights on its origin, biology and spread pathways

Seiridium cardinale is regarded as the most important agent responsible for a deadly disease of cupressaceous hosts commonly referred to as Cypress canker. The fungus was first described in California, and is currently reported in all continents. A recent study based on only seven polymorphic SSR loci has suggested that California S. cardinale populations may represent the source of the severe and widespread Cypress canker epidemic in the Mediterranean. In this study, 185 AFLP markers are used on an extended sample size of 125 individuals to determine whether the Mediterranean population may indeed be derived from the California one, and to compare the reproductive modes of populations of this fungus in the two regions of the world. Additionally, AMOVA, NJ, STRUCTURE, PCA, and ?-tubulin sequence analyses are employed to infer the presence of genetic structure within and between populations from the two regions. The distribution of pairwise AFLP similarity coefficients suggests Mediterranean populations are reproducing only clonally, while California populations are reproducing both clonally and sexually. AMOVA indicates Mediterranean and California populations are currently genetically isolated, but NJ and STRUCTURE analyses both suggest ancestral Mediterranean genotypes belong to the California population. No alleles were private to either population, and the presence of identical or quasi-identical genotypes at large distances supports the notion that movement of infected cypress plants is responsible for the global spread of the disease. Surprisingly, STRUCTURE identified a second cluster of Mediterranean genotypes distinct from the basal mixed California-Mediterranean cluster. This second cluster may either have originated from the first one under the selection pressure imposed on the pathogen in the new Mediterranean environments, or may be the result of a later introduction either from California or from elsewhere in the world. Cumulatively, the evidence presented here suggests S. cardinale may be native or long naturalized in California, and that two genetically distinct groups are currently present in the Mediterranean, with obvious implications for further studies on this disease.