Irish potato famine sparked 180-year evolutionary arms race

The Irish potato famine of 1845–52 resulted in more than a million deaths. Another million people were displaced, and Ireland’s population was reduced by about 25%. The cause of widespread potato rot that devastated the staple crop was a fungus-like microbe that no one could see—an oomycete called Phytophthora infestans, more commonly known as potato blight. Since then, breeders have worked tirelessly to develop newer varieties of potato that are resistant to blight. Now, new research has shed light on the 180-year-long evolutionary arms race between potatoes and their pathogen (Nat. Comm. 2024, DOI: 10.1038/s41467-024-50749-4).

Jean Ristaino, a plant pathologist at North Carolina State University who led the research group, acknowledges that many factors aside from potato blight contributed to the famine. They include “an overreliance on subsistence of a single food crop, absentee landlords, and slow food relief policies by the British, that led to widespread hunger.” So even though potato blight was present in the US and other countries, Ireland was uniquely affected. Yet the role of the potato blight pathogen itself shouldn’t be underestimated.

The widespread loss of potato crops prompted growers to think about how to control potato blight through “cultural practices or using chemicals” and, some years later, by breeding resistance into potatoes, Ristaino says. That sparked what plant pathologists commonly refer to as an “evolutionary arms race,” in which breeding led to the evolution of R resistance genes in potatoes to protect against infection and the corresponding evolution of effector genes in potato blight to overcome the crop’s defenses.

To study this plant-pathogen coevolution over time, Ristaino’s group took samples from potato specimens preserved in historical plant research collections, called herbaria, and extracted DNA from them. The team was then able to sequence the R genes in those potatoes and the effector genes from the potato blight pathogen cells that had infected those historical samples.

This approach allowed the group to see just how quickly potato blight was evolving and diversifying to overcome resistance in potatoes. In fact, the variant that caused the 19th-century famine had evolved an effector gene that could overcome an important potato R gene long before that R gene was bred into crops from a resistant, South American species of wild potato. Over time, the potato blight pathogen expanded its genome and responded directly to breeding programs that introduced additional R genes into the crop.

Sorrel Tran, a plant pathologist at the University of New Mexico, says that the work is important because “a lot of scientists are going back to these older populations, looking for these specific R genes, and seeing if they confer any resistance to some of the diseases that are popping up now.”

But the herbaria housing these historical collections are being threatened, Ristaino and Tran say. “Many universities have eliminated herbaria altogether,” Ristaino says. Those include Duke University, which announced earlier this year that it would close its herbarium, could join them soon. Herbaria “are worth preserving, and this study reflects just how important these collections are.”