Roadmaps to the origins of potato
David Spooner is a US Department of Agriculture (USDA) taxonomist who collects wild and cultivated potatoes in order to study their species boundaries and relationships. His recent discoveries have helped re-write much of what we thought we knew about the origin and evolution of the cultivated potato.
In 2005, you co-authored a paper that overturned previous ideas about the evolutionary history of the potato. What were your findings?
"What we demonstrated was that the cultivated potato has a single place of origin, in Peru. Up until then, all publications had hypothesized that the early Andean and Chilean cultivated forms of potato had evolved from different progenitors, much like beans were domesticated in both Southern and Meso-America. For the potato, the domestication hypotheses had suggested complex hybrid or multiple independent origins from what is known as the Solanum brevicaule complex, a group of 20 morphologically very similar tuber-bearing wild taxa broadly distributed from central Peru to northern Argentina. Now, as part of a study of the taxonomy of that complex, I and a group of scientists from the Scottish Crop Research Institute were analysing accessions of potato landraces when we made a surprising discovery – at the molecular level, the accessions all group together, not in separate places on a 'phylogenetic tree' with different wild species, as would be expected with many separate origins. Based on those initial results, we broadened the study to analyse DNA markers in 261 wild and 98 cultivated potato varieties, and our data indicated that those early cultivars originated from a single ancestral line in the 'northern' component of the S. brevicaule complex in central or southern Peru."
And how important is that finding?
"For Peru, of course, it was great news, and a source of national pride. But that aside, the purpose of taxonomy is to help determine what is a species and to classify species into related groups, providing other scientists with a roadmap to guide them down proper research paths. If the taxonomy is bad, research goes awry. In fact, another of our findings was that what were considered member 'species' of the S. brevicaule northern group were poorly defined, and that further studies might reduce them to a single species."
Another of your recent papers has broken new ground by reclassifying the cultivated potato into four species...
"That was from a study done with the International Potato Center [CIP] in Peru. We carried out one of the largest molecular marker studies ever done on crop landraces, covering 742 landraces of all cultivated potato species and eight closely related wild species progenitors. Until that paper, there were many different ideas about the number of cultivated potato species – the widely used classification of [the British plant geneticist] J.G Hawkes identified seven species and seven subspecies, while Russian taxonomists recognized as many as 21 species. But, in combination with the findings of earlier morphological analyses done with CIP, our analysis found just four: Solanum tuberosum, divided into Andean and Chilean cultivar groups, and three hybrid cultivated species of 'bitter potato'. We also found that consistent and stable identification of the other purported 'species' was impossible, and only created confusion."
Are there likely to be further discoveries about the number of cultivated potato species?
"Well, I like to say there's as few as one, and as many as 20, and we've found four."
Now, to your third recent discovery, published in 2008, about the introduction of Chilean germplasm in the modern potato. What was at issue there?
"All modern potato cultivars have predominantly Chilean germplasm. To explain that, Russian investigators proposed that the potatoes introduced to Europe were Chilean landraces, while British investigators thought they came from the Andes but were killed off in the late blight epidemics of the 1840s and replaced by introductions from Chile. My student Mercedes Ames and I addressed this question by examining 49 European herbarium specimens collected between 1700 and 1910 for a DNA marker that distinguishes Andean from Chilean landraces. The results showed that, yes, Andean potato predominated in Europe in the 1700s and persisted until 1892 – long after the late blight epidemics – while the Chilean potato first appeared in Europe in 1811 and became predominant long before the late blight epidemics."
This may be an unusual question. Having collected potato throughout the Americas every year since 1989, have you formed a "spiritual connection" to the plant?
"I'm not driven primarily by potatoes. I am directed by USDA to work on potato, but my real motivation is the intellectual exercise of finding answers to the complex taxonomic and biological questions posed by the potato. What makes this job so fascinating for me is the great infrastructure that is available for potato research and makes those answers possible."