Molecular Evolution of Phytophthora infestans (Mont.)de Bary, The Late Blight Pathogen

Abstract

Phytophthora infestans (Mont.) de Bary causes late blight of potato and tomato and is one of the world's most devastating plant diseases. P. infestans left its footprint in human history when, in the 19th century, it was responsible for the Irish Potato Famine. Nuclear and mitochondrial DNA variability was used to examine the population history of P. infestans. DNA sequence data from three nuclear regions (Intron Ras, Ras, and β-tubulin) and two mitochondrial regions (P3 and P4) were obtained from ninety isolates from various locations including Brazil, Bolivia, Ecuador, Peru, Costa Rica, Mexico (Toluca Valley), the USA and Ireland. Population summary statistics show that the Mexican population from the presumed center of origin of P. infestans, harbored less nucleotide and haplotype diversity than South American populations, and was genetically differentiated from other populations, particularly at the mitochondrial loci. Coalescent-based genealogies of mitochondrial (rpl14, rpl5, tRNAs, cox1) and nuclear (Intron Ras+Ras) loci were congruent and demonstrated the existence of two lineages leading to the present day haplotypes of P. infestans associated with potatoes. A third lineage, associated with a group of isolates from Solanum tetrapetalum collected in the Andean Highlands of Ecuador was also found. In the mitochondrial genealogy the two potato lineages corresponded to the mitochrondrial haplotypes Type I and Type II described elsewhere. Mitochondrial haplotypes were associated with different nuclear backgrounds. Haplotypes found in the Toluca Valley population were derived from only one of the two lineages in both mitochondrial and nuclear genealogies, whereas haplotypes found in South American populations (Peru and Ecuador) were derived from both lineages. Haplotypes found in USA and Ireland populations were also derived from both lineages and these populations were not genetically differentiated from the Peruvian populations, suggesting a common ancestry among these populations. Evidence for recombination was found for Mexican and USA populations. Solanum tetrapetalum isolates were highly polymorphic within the regions analyzed and may be a new species. The results support a South American origin of P. infestans and are discussed in relation of previous hypotheses regarding the geographic origin of this plant pathogen.