|Authors: ||J.B. Ristaino, Chia-Hui Hu|
|Keywords: ||Phytophthora infestans, oomycete, phytopathology, population genetics|
Previous work with modern isolates of Phytophthora infestans had shown that a single clonal lineage (US-1), characterized by mtDNA haplotype Ib, was present across the globe in the 1970s leading to the hypothesis that one strain of the pathogen had spread from a single source, probably Mexico where it was known to be more variable.
But mitochondrial DNA sequences from archival materials have identified the Ia mtDNA haplotype of P. infestans in 19th century samples from Europe, the US and Ireland and not the Ib haplotype.
Multilocus sequence data from nuclear and mitochondrial loci support an Andean origin of P. infestans and also suggest that the source for the potato famine epidemics in Ireland was the Andean region rather than the Mexican highlands.
Closely related, novel species including Phytophthora andina, have evolved in the Andean region of South America and share a common ancestor with P. infestans. More recently, we studied historic samples of P. infestans from Asia.
DNA was extracted from 41 archival samples from 5 herbarium collections.
Early samples from Asia, Russia and Australia were all the Ia mtDNA haplotype (China, 1938, 1940; Japan, 1901, 1930, 1931; India, 1913; Peninsular Malaysia, 1950; Nepal, 1954; The Philippines, 1910; Australia, 1917; Russia, 1917; Latvia, 1935; and the Ukraine). In contrast, only later samples included the Ib mtDNA haplotype (China, 1952, 1954, 1956, 1982; India, 1968, 1974; Thailand, 1981). Modern Chinese populations of P. infestans contain all four mitochondrial haplotypes suggesting more recent origins of the IIa and IIb mtDNA haplotypes.
All the samples analyzed for mitochondrial haplotypes so far indicate that colonization of North America, Europe and Asia was first, by type Ia and subsequently by type Ib and that this haplotype migrated from the Andes rather than from Mexico.
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