How Genomics Solved The Mystery Of Ireland's Great Famine

May 22, 2013

An international group of plant pathologists has solved a historical mystery behind Ireland's Great Famine.

Sure, scientists have known for a while that a funguslike organism called Phytophthora infestans was responsible for the potato blight that plagued Ireland starting in the 1840s. But there are many different strains of the pathogen that cause the disease, and scientists have finally discovered the one that triggered the Great Famine.

And although so many Irish emigrated to the U.S. to escape the devastation, little did they know they were only getting closer to the source of the pathogen in the Americas.

A team of researchers from the U.K., Germany and the U.S. sequenced the genomes of some herbarium samples — dried plants preserved by botanical gardens and museums. They compared the pathogens in 11 of these potato samples from more than 100 years ago with 15 modern strains and found the one responsible for Ireland's devastating blight. The culprit's name? They dubbed it HERB-1.

This uprooted the previous theory that another strain, called US-1, was behind the blight. But the two are closely related, says Sophien Kamoun, a co-author of the study, which appears in the journal eLife. With this information, he and the other scientists deduced a possible timeline for Phytophthora infestans.

Everything began in Mexico's Toluca Valley, the center of the pathogen's diversity. "The Spanish introduced Europeans to the South American staple crop potato shortly after their conquest of the New World," the authors write, "but for three centuries Europe stayed free of P. infestans."

They think that it wasn't until the start of the 19th century that a strain of the pathogen left Mexico. Wherever it ended up — the researchers suspect farther north in North America — it then separated into HERB-1 and US-1, and each strain set off to take over the world, one potato at a time.

By the summer of 1845, HERB-1 had made it to Europe and spread quickly across the continent. Because of Ireland's high dependence on potatoes and its fragile socioeconomic status, losing much of the crop to the blight was catastrophic. A million Irish died and another million left the island before the famine lifted in 1852.

In the early 20th century, people began breeding potatoes that would be more resistant to pathogens. "We have some data from the genetics that suggests that HERB-1 wouldn't have been able to cope with those resistance genes," Kamoun says. "Of course, we can never really know for sure, but it looks like HERB-1 is most likely extinct at this point." Even the heartier US-1, which dominated the globe for 150 years and is still around today, was overshadowed by more aggressive strains in the 1970s.

But the scientists weren't merely sleuthing for the sake of history. Though the pathogen behind the Irish potato famine is no longer a threat, related strains, they say, are still a menace.

"Potato blight is still a huge problem worldwide," Kamoun says. "[It's] the third most important food crop in the world, and potato blight is the major constraint for growing potatoes." One 2012 estimate says the amount of potatoes lost each year to blight would be sufficient to feed at least 80 million people — and perhaps hundreds of millions more.

So plenty of scientists are trying to breed new potato varieties either through classical breeding or through genetic modification, but they're up against the "amazingly adaptable" Phytophthora infestans. "Understanding the pathogen dynamics and how the pathogen has evolved through time is really important for helping us predict future changes in pathogen populations," Kamoun says.

And to him, the millions of herbaria stored in museums are hidden treasures just waiting for researchers to find. "We can go back and not just look at the leaves or identify the species by looking at the shape of the flowers. ... We can actually get to the DNA," he says. "This is a great example of how we can use all those treasures in museums and use the latest technology to exploit information that's in those museums."

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