DNA Evidence Calls Irish Potato Famine Theory Into Question

It turns out they were wrong.

A new study by scientists at North Carolina State University, published in the June 7 issue of the journal Nature, finds that the strain of the pathogen blamed for the famine isn't the culprit after all.

DNA fingerprinting analysis of 150-year-old leaves, preserved from the Irish potato famine, found no trace of the widely suspected 1b haplotype (strain) of the late-blight pathogen Phytophthora infestans. Instead, the analysis points a finger at one of three other late blight haplotypes – none of which previously had been considered prime suspects – and even raises the possibility that the epidemics were caused by an unknown haplotype which became extinct or mutated into a different strain shortly afterward.

"The old theory was that the 1b haplotype was the ancestral strain, originating in Mexico. Our research refutes the first half of that theory, and calls into question the second half," said lead researcher Dr. Jean Beagle Ristaino, NC State professor of plant pathology.

The study is the first ever to extract and analyze mitochondrial DNA from historic specimens. Previous studies by other researchers, which implicated the 1b haplotype, were conducted using only DNA collected from modern-day late-blight outbreaks. "Our data emphasize the importance of using actual historic specimens when making inferences about historic populations," Ristaino said.

The study's findings have more than just historical significance, she adds, because modern forms of the P. infestans pathogen are the most serious potato pests in the world. Late blight is currently ravaging Russia's potato crop, and smaller-scale epidemics occur every year in Mexico, Ireland, Ecuador and the United States. Potatoes are one of the world's four leading food crops. Late blight poses an especially great threat in developing countries where control measures may be too costly.

"Identifying the geographic center of origin for late blight is important, because that's where you're most likely to find host resistance – potato plants that have developed natural resistance to the pathogen – that can be used to breed new, more resistant potato varieties for widespread use," Ristaino said. In their paper, she and her colleagues suggest the pathogen may have originated in South America rather than Mexico, "although much more research is needed before we can know for sure," she stressed, "because the South American collections are not as complete or well-studied as they should be."

Knowing how the pathogen has spread, and how it has mutated or evolved over the years, would also be useful, she says, because it could help scientists develop better control measures to prevent future epidemics.

She believes that forensic DNA fingerprinting techniques, similar to those she developed to study P. infestans, could be conducted on other plant pathogens that wreak havoc on food crops.

In the Nature study, Ristaino and her colleagues extracted DNA from 123 potato leaf specimens. PCR amplifications were successful from 28 of those. The specimens were collected in England and Ireland between 1845 and 1847 and stored at the Royal Botanic Gardens in Kew, England, and other herbariums. To prevent contamination, none of the research was conducted in laboratories or on equipment previously used for Phytophthora studies. For comparison purposes, the researchers also collected mitochondrial DNA from potato leaves infected with late blight in modern epidemics.

Ristaino's co-authors on the paper are Gregory Parra, an agricultural research specialist in plant pathology at NC State, and Dr. Carol T. Groves, a former NC State postdoctoral associate now at the USDA Agricultural Research Service's New England Plant, Soil and Water Laboratory in Orono, Maine.

Their study was funded by the National Geographic Society, the North Carolina State Agricultural Research Service and NC State's International Programs Office.