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The genetics of potatoes (Solanum tuberosum) and how they have adapted to European conditions have been studied through reconstruction from historical genomes, including specimens from the Museum's Sloane Herbarium.
Originating from the highlands of the Andes, South America, potatoes were introduced to Europe in the sixteenth century. They were initially popular in Spain because they provided cheap sustenance for the poor. It wasn't long before they were in demand throughout the continent as a food that was easy to grow and highly nutritious. Today, potatoes are a staple for people all over Europe.
Sandy Knapp, a Museum life sciences researcher, worked with colleagues from the Max Planck Institute in Tübingen, Germany, (MPI Tübingen) to trace the history of genetic change in potatoes after their introduction to Europe.
'We sequenced the DNA of 88 specimens ranging in date from 1650 to the present,' says Sandy. 'These included modern potatoes and historical samples from the Museum, as well as herbarium collected by Darwin during his voyage of the Beagle.' A herbarium is a collection of plant species - many of the six million specimens in the Museum's botanical collections are found in herbaria.
The potatoes eaten today are actually swollen underground stems called tubers. In the wild, they are starch storage organs for the plant, allowing it to grow from year to year.
People in the Andes changed potatoes over time, gradually cultivating larger tubers that were easier to eat.
The first potatoes seen in Europe had tiny tubers the size of peas or cherries. This was because the formation of tubers was regulated by the length of day.
Being close to the equator, the Andes experience days and nights of equal length. The first potatoes brought to Europe experienced the shock of the European growing season, where the length of night and day varied through the year, with the longest days occurring during summer. Tuberisation - the development of larger tubers - did not occur in the long European summer days until later.
Scientists had earlier found that tuberisation in Andean potatoes was regulated by a gene called St-CDF1 (St for Solanum tuberosum, CDF for Cycling DOF Factor). Adaptating to the European seasons involved key changes in this gene.
So where did this adaptation to long days come from? The origin of these long-day adaptive variants could have come from three sources:
· variation in Andean potatoes that were brought to Europe
· a new mutation (change) in the gene that occurred once the potato was grown in Europe
· a mixture of Chilean potatoes (that grow in long-day climates in southern South America) with Andean potatoes in Europe
The team, led by MPI Tübingen, found that the earliest potatoes in Europe did not have the CDF1 gene variant, neither did modern potatoes from the Andes, nor the historical Chilean samples.
This made the team think that the genetic change occurred once the potatoes were in Europe. Because it allowed tuberisation in long days, potatoes with the gene were preferred by people and rapidly spread around the area.
'Samples from modern Chile all had the European variant of CDF1, showing that these plants were mixed with European potatoes,' says Sandy.
'So if we had assumed that the modern Chilean samples were genetically the same as the historical ones, we would have been wrong.
'The diversity of potatoes in Europe depends not only on the variation in the plants originally brought here, but in later mixture of genotypes from Chile and even later mixture of wild species genes through twentieth-century plant breeding.'
This research shows how combining genetic analysis of contemporary and historical samples allows us to begin to understand the complex history of crop adaptation to new environments.
It also highlights how human activity has contributed to the widespread geographical expansion of potato crops.
The study is a good example of how valuable specimens housed in places like the Museum are key to solving not only mysteries of how plants have adapted to their environments, but also helping to solve modern-day problems such as food shortages.
'Within Europe, potatoes have kept evolving to adapt to this new environment,' says Sandy.
'Future efforts to help them along in the face of climate change are really important. This research adds to this collective endeavour.'