Mystery solved.
Researchers at North Carolina State University have traced the origins of the Irish potato famine pathogen, Phytophthora infestans, to the South American Andes Mountains.
Through an extensive genetic analysis of P. infestans and related Phytophthora species, the study strengthens evidence that the pathogen spread from South America to North America before devastating Ireland in the 1840s, according to a press release. Today, it continues to cause late blight in potato and tomato crops worldwide.
By comparing whole genomes, researchers found striking similarities between P. infestans and two closely related pathogens — Phytophthora andina and Phytophthora betacei — both of which are exclusive to South America.
“It’s one of the largest whole-genome studies of not only P. infestans, but also the sister lineages,” said Jean Ristaino, William Neal Reynolds Distinguished Professor of Plant Pathology at North Carolina State University and corresponding author of a paper in PLOS One that describes the study. “By sequencing these genomes and accounting for evolutionary relationships and migration patterns, we show that the whole Andean region is a hot spot for speciation, or where a species splits into two or more distinct species.”
For decades, scientists have debated the origins of P. infestans, with some proposing Mexico instead of South America. However, the study highlights clear genetic differences between P. infestans and the two Mexican pathogens, P. mirabilis and P. ipomoeae, challenging the Mexico-origin theory.
“A lot of the search for resistance to this disease has focused on a wild potato species in Mexico – Solanum demissum – which was used to breed resistant potato lines that were used for the past 100 years,” Ristaino said.
“It points out the importance of looking at the center of origin where a host and pathogen have evolved together over thousands of years,” she said. “Climate change is bringing more drought to higher Andean elevations, so we could be losing some of these potatoes before we learn if they could provide resistance to late-blight disease.” Ristaino added that more research is needed to examine wild potato species from the Andes to learn more about host resistance to P. infestans.
“Our data show that there have been more migrations of the pathogen into and out of South America, and the migrations into and out of Mexico are small in comparison,” said Allison Coomber, a former NC State graduate student researcher and lead author of the paper. “We did find there was gene flow from the Andes to Mexico, and also in reverse, because there’s a big Mexican potato breeding program and potatoes have gone into the Andean region in more recent times. But in historic times it was the other way around.”
“Historic P. infestans – the samples collected from 1845-1889 – were the first to diverge from all other P. infestans populations, with modern South American and Mexican populations both showing shared ancestry derived from historic P. infestans,” Ristaino said. “Modern global trade appears to contribute to mixing together the pathogen populations in South America and Mexico.