Most potato tubers come from Eurasian stems, but their ‘roots’ are in South America.
We reconstructed the history of potato introduction and adaptation to the European climate at an unprecedented scale. The most exciting part was the sampling of historic herbaria that transported us back in time to European expeditions in South America that discovered treasures such as potato.
Researchers argued for a very long time about the origins of European potato. This problem was not an easy one to solve. Spud certainly arrived from South America, but this continent harbours many closely related potato species under cultivation. Additionally, Europeans were mostly agnostic to the difference between sweet potato (Ipomea batatas) and potato (Solanum tuberosum), two completely unrelated species! Until today, in English language, we use the word ‘potato’ (that is derived from the word ‘batata’ for sweet potato) to name a tuber crop that in Quechuan (and some Spanish dialects) is called ‘papa’ for potato. A huge advance in the origin debate was made in the early 20th century when large botanical expeditions, first from Russia and then from England, were organized to study plants in South America. For a long time Russian researchers concluded that potato was brought to Europe directly from Chile, while English researchers argued for the Andean origin. This discord suggested to us that potato likely had a complex story of origin, and we set out to investigate it using ancient genomics.
At the time I embarked to solve the potato riddle, a joint project with Cambridge University Herbarium got funded and we were given the opportunity to sample potato specimens that Charles Darwin collected during the voyage of the Beagle in Chiloe Islands in Southern Chile! Darwin noted the surprising diversity of potatoes on the Chilean archipelago and sent his samples to his colleague and former mentor, John Henslow, who has been cataloguing the diversity of plants at the University of Cambridge. These samples conserved beautifully, were dusted off nearly 180 years later in an effort to digitize collections from the Beagle voyage, and landed under the scalpel for tiny destructive samplings. I could not contain my excitement when those samples landed safely on my desk. It was an incredible feeling to have a piece of history in my hands.
The subsequent material we collected for our study went beyond our expectation. We had the privilege to sample herbarium specimens from the famous Sloane Herbarium (one of the oldest and most comprehensive botanical collections) at the Natural History Museum London, from the Herbarium of the Royal Botanical Gardens in Madrid, and from the university of my hometown, Warsaw. Now we had added 16 specimens spanning the years 1660 - 1891 to our earlier collection. Within those, there had to be at least a few samples that broke the record for the oldest herbarium material sequenced with whole-genome approaches. These were the crown potatoes, but without contemporary relatives, we would not have been able to investigate the process of introduction. We got lucky once again when we were put in touch with the International Potato Center (CIP) in Lima, Peru; they had the exact material we needed - the mini-core of potato diversity in South America.
Having all this amazing material on the bench felt great, but also put pressure on me to get things right. There was no ‘second chance’. Everything, starting from DNA extractions to high-throughput sequencing, had to go well. We used tiny amounts of herbarium tissue to produce and immortalize the genomic libraries, successfully securing DNA for the next generations of scientists. Potato, being heterozygous tetraploid, required very high sequencing coverage for accurate estimation of allele frequencies. But that was just the tip of the iceberg of problems arising when working with tetraploid species. Going deeper, I realized that many standard processing and analytical tools did not cope with tetraploid alleles. Ultimately I made it work by adapting their codes to analyze tetraploid potato genomic diversity.
All those moments of joy and hardship that accompanied us throughout the project produced a tightly woven story. European potato was first introduced from the Andes in 16th century. Subsequently, Chilean varieties were brought in the 19th century and intermixed with potato in Europe. At a similar time European potato acquired an adaptation that made it better suited for the European climate. To add to the complexity, during the 20th century, wild potato species were used for resistance breeding and left a huge mark on European potato genomes. I have a feeling that there might be something special about European potato propensity for hybridization. That is just a teaser, you can now read the whole story in Nature Ecology and Evolution!