Investigating the 19th Century Irish Potato Famine with Ancient DNA

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Potato infestation

This map shows the proposed map of spreading P. infestans lineages HERB-1 and US-1. Image courtesy of Yoshida, K. et al

Forensic Plant Pathologists Used Ancient DNA to Investigate the Great Famine

Dr Hernán Burbano was part of an international team who sought to answer these questions. He told Decoded Science that the project aimed “to bring to the plant and plant pathogen field the techniques developed in the aDNA field in the last years.” Ancient DNA (or “aDNA”) is a term used to describe DNA from museum specimens. The specimens could be bones or preserved soft tissues from animals, or dried and pressed herbarium specimens of plants.

There has been much excitement about extracting DNA from dinosaur and mammoth tissues but much less coverage of the use of ancient plant specimens by forensic plant pathologists.

The team studied pressed herbarium samples of potato leaves collected from Europe and North America at the time of the Great Famine. They sequenced the ancient DNA from the infected leaves to provide information on P. infestans, and examined modern strains of the pathogen.

The researchers’ work shows that all the strains from the period 1845-1896 were from a single lineage called HERB-1, responsible for the 19th Century pandemic of the disease. HERB-1 persisted for over 50 years, but the researchers didn’t find it in any of the modern specimens. The team also discovered that although US-1 is not a direct descendant of HERB-1, it is a closer relative than the strains which are dominant today.

Decoded Science asked Dr. Burbano if the team were surprised by the absence of HERB-1 from modern samples. He replied:

I wasn’t surprised given the developments of 20th Century agriculture (pesticides, breeding) that might have been a big selective force for P. infestans. However, it might be possible that HERB-1 is still around in some isolated populations that we have not sampled yet.

The paper also proposes a new scenario for the global spread of Late Blight. They suggest that a single strain of the disease travelled from Mexico to North America in 1842 or 1843. From there it spread to Europe and the rest of the world, and caused disease and destruction for over a hundred years. This strain had become rare (or possibly extinct) by the late 1970s, but not before it had given rise to the US-1 strain which became such a problem for farmers around the world.

Other Uses of Forensic Plant Pathology

This is not the first time herbarium samples have been used to examine historical disease levels. In 2005, Bearchell et al (that’s yours truly) used the Broadbalk Archive to study a devastating disease of wheat: Septoria. The archive contains samples taken almost every year since 1844 and gave us a time series of the disease over a 160 year period.

We compared the disease levels to various environmental factors and discovered a close relationship between disease levels and atmospheric sulphur dioxide levels. Just like the team who studied Late Blight, we also had to overcome issues of DNA preservation after such long storage periods; working with ancient DNA is not an easy task. We used information from studies on museum specimens to inform our techniques.

Septoria disease on wheat is related to atmospheric sulphur dioxide levels. Image from Bearchell et al (2005)

Septoria disease on wheat is related to atmospheric sulphur dioxide levels. Image from Bearchell et al (2005)

Herbaria and archive collections are expensive and time-consuming to collect and maintain. However, they provide us with a wonderful window into the past of more than just the plants they were originally intended to preserve.

Dr Stephen Harris, Druce Curator of the Oxford University Herbaria, told Decoded Science  “Maintaining natural history collections for future generations is important. Past collectors could not have conceived of the information we can extract from accessible, well-maintained and catalogued specimens. The same is true of today’s collectors.”

Is the Great Famine Pathogen Still Out There? Is Another Devastating Epidemic Just Around the Corner?

The strain which caused the Great Famine might still be lurking in a small population of relatively wild potatoes. However, today we have more weapons against such diseases. Research has shown us the conditions which cause Late Blight to spread and infect so that farmers can apply a pesticide at exactly the right time. There is also a range of fungicides which farmers can use, and a wider variety of resistant varieties for planting, all reducing the probability of a pandemic.

Resources

Yoshida et al. The rise and fall of the Phytophthora infestans lineage that triggered the Irish potato famine. (2013). eLife 2:e00731. doi: 10.7554/eLife.00731. Accessed July 4, 2013.

Bearchell et al. Wheat archive links long-term fungal pathogen population dynamics to air pollution. (2005). PNAS 102: 5438-5442. Accessed July 4, 2013.

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