More than 300 isolates of the Irish potato famine fungus, Phytopkthora iufesns, colc In 20 countries on five ontits, were analyzed for genetic variation at the mating type and two allozyme loci. A subset of more than 200 islates was also analyzed for DNA "fingerpint" variation. A surprisng result was that a single conal ineage dominated most populaton worldwide. AD of the variation within this loneage appeared to have arisen by mitotic embinat or by mutation. In addItion to the most common cal lineage, a number of diferent, but a rtiy closely related, lineages occurred in the United States and Canada. The low levels of gene dIverty In the derived poltins comped to the presumed ancestral pution in central Mexico indicate that P. infestas went throb extreme genetic bottlenecks during its diperal. The genetic data are consistent with the hypothesis that the initial migrt-ion of P. ifesta in the 184s was from Mexico to the United States and that only a single genetic individual was t ra ed to Europe and subsequently to the rest of the world. If this hypothesis is correct, then the Irish potato famine was caused by a singe clonal genotype of P. -festns.

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