DNA Study Sheds Light on Irish Potato Famine

John Roach
for National Geographic News
May 5, 2004
In the mid-19th century, a fungus-like disease that turned potatoes into black, inedible mush led to the fatal starvation of approximately a million people in Ireland. A team of DNA sleuths now believes they know the true identity of the killer disease.

The mystery began unraveling three years ago, when the researchers presented DNA evidence from samples of 150-year-old potato leaves. The scientists said the findings exonerated the previous prime suspect behind the Irish potato famine: a strain of the pathogen Phytophthora infestans known as the Ib haplotype. (The pathogen causes a plant disease known as late blight.)

Now the team, led by Jean Ristaino, a plant pathologist at North Carolina State University in Raleigh, says a different strain of P. infestans, the Ia haplotype, was the culprit. The scientists trace the pathogen's probable roots to South America's Andes Mountains.

Late blight continues to devastate potato and tomato crops around the world. In 2001, an epidemic wiped out Russia's potato crop, a major food source for many of the country's poor. Smaller outbreaks regularly occur in Mexico, Ireland, Ecuador, and the United States.

Ristaino and her colleagues hope to conclusively pinpoint the strain of P. infestans behind the Irish potato famine and trace the strain's place of origin. Doing so, the researchers say, will help scientists to control the plant disease.

"If you know the particular strain and where it came from, it will be of use in understanding how modern strains have evolved and aid in the search for sources of host resistance, which should lie in the center of origin," Ristaino said.

The researchers will describe their findings in the May issue of the science journal Mycological Research.

In addition, the scientists are compiling evidence that suggests South America was the probably source of the strain. The researchers plan to publish those results next year.

Greg Forbes, a research scientist with the International Potato Center in Lima, Peru, said the DNA research by Ristaino and her colleagues helps scientists to better understand the history of the late blight pathogen.

"The study of historical specimens is a novel and useful endeavor," Forbes said. "It is our only chance to actually get a glimpse of the past. Although we must remember that these analyses are only tiny snapshots from a large and complex series of events."

DNA Sleuthing

Ristaino's research is based on isolating and analyzing mitochondrial DNA from the P. infestans pathogen preserved in infected potato leaves. While on sabbatical in 1997, the scientist collected preserved leaves from university and museum collections in Europe and the Americas.

The leaves dated from 1845 to 1982, and included specimens gathered during late blight epidemics in Ireland, continental Europe, and the United States.

Based on her analysis, Ristaino and her colleagues discovered three years ago that the Ib strain of P. infestans did not cause the Irish potato famine. The culprit had to be one of three different strains.

In their new study, the scientists conducted further analysis of 186 potato plant leaves from the 19th and 20th century. The researchers found that 90 percent were infected with the P. infestans pathogen and 86 percent were infected with the pathogen's Ia strain.

The pathogen's Ib strain, however, was only detected in leaves from more modern samples gathered in Central and South America.

The finding led the scientists to conclude that the Ia strain was the culprit behind the Irish famine. "Work is now progressing to figure out where it actually came from," Ristaino said.

Stephen Goodwin, a plant pathologist at Purdue University in West Lafayette, Indiana, said the data identifying the Ia strain of P. infestans as the culprit behind the Irish potato famine are surprising but convincing.

"It's certainly not what I would have predicted, and it doesn't make sense with other data, but … it seems quite clear that Ia was the most common haplotype in those samples," he said.

South American Origin?

Earlier research by Goodwin based on analysis of nuclear DNA markers from modern strains suggests that the P. infestans pathogen is associated with the US-1 genotype, or type species. The US-1 genotype originated in Mexico and is associated with the Ib strain of P. infestans.

However, Ristaino says her recent findings makes it unlikely that Mexico was the source of the plant disease behind the Irish potato famine.

Ristaino and her colleagues are working on a theory that suggests the Ia strain of P. infestans originated in the South American Andes—the birthplace of the potato—and traveled to Europe and North America via exports of potato seed on steamships.

"In the 1840s, [farmers] were not growing commercial potatoes in Mexico," she said. But they were growing them in South America and shipping them to Europe and North America."

According to Ristaino, the Ib strain of the pathogen likely evolved from the Ia strain in the Andean region. The Ib strain subsequently dispersed to the U.S., Europe, and the rest of the world during the past century.

As evidence, Ristaino cites historical documents and letters of Spanish colonizers that refer to the blight of potatoes in regions that are now Colombia and Ecuador. Indigenous peoples were also familiar with the disease, she notes.

"The diversity of the potato germplasm in the Andean region probably kept the pathogen in check, unlike modern fields where [the] potato is grown as a monoculture and susceptible varieties are widely planted," Ristaino said.

Goodwin, meanwhile, said his analysis of the same historical records indicates there is no evidence for late blight in South America prior to the Irish potato famine. "The earliest one that could possibly be late blight is from about 1890," he said. Goodwin holds to his hypothesis that P. infestans originated in Mexico.

Forbes, of the International Potato Center, said that he has not seen any information that indicates a clear migration pathway from Mexico or South America. "It seems plausible that there would be more chance of introducing the pathogen from an area where potatoes were grown as a domesticated crop," Forbes said.

Ristaino's research is supported by a grant from the National Geographic Society's Committee for Research and Exploration. Additional support comes from the U.S. Department of Agriculture, the North Carolina State Agricultural Research Service, and North Carolina State University.

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