Today's markets are full of tomatoes of different types and prices, but even this diversity pales before the enormous diversity of varieties that have been formed throughout history. There are red, yellow, pink, rounded, flattened, large, small, ribbed, smooth... However, wild tomatoes are almost all red, spherical and very small.
How, from wild plants with uniform fruit, have farmers and breeders managed to create this enormous agricultural diversity?
The importance of diversity. Diversity is not only used to satisfy our palates and decorate our tables: it is also key to our future. A uniform species will hardly be able to adapt to overcome unexpected problems, such as the appearance of new diseases. Selection, both natural and artificial, works by choosing the most suitable varieties for each situation. Without diversity there is no possible evolution.
It is paradoxical that this cultivated species enjoys such a wide variety of fruit types
The tomato, like other crops, enjoys a great morphological and agronomic diversity. However, their genetic diversity, the set of genetic changes that can be found in plants, is very low. This represents a problem for the crop. During the 20th century, for example, breeders had to search for disease-resistance genes among wild species because the cultivated species had lost them during domestication.
Furthermore, it is paradoxical that this cultivated species enjoys such a wide diversity of fruit types despite having lost most of its genetic diversity over time.
The weight of the story. The tomato was domesticated between the agricultural cultures of Peru, Ecuador, Mexico and Central America. This domestication, together with the travel associated with it, involved a significant loss of genetic diversity.
Later, in the 16th century, a small representation of the existing varieties in America were brought to Europe. Once there, the tomato was only cultivated by the popular classes of Spain and Italy. The ruling classes of the old continent considered it an unhealthy food and a mere botanical curiosity.
It was precisely these humble farmers who managed, starting from the meager diversity that came from America, to generate the great diversity of European types. Within the framework of the Traditom project, financed by the European Union, this genetic change has been studied by analyzing 1,254 traditional European varieties.
This study has shed light on the paradox of how European farmers managed to generate a great diversity of types from a negligible genetic diversity.
The tomato grown in the Canary Islands has made the leap to Europe. /
For the most part, the tomato genome has very little diversity. If we choose two plants at random we will see that they have almost identical genetic sequences. However, in this genomic desert there are a handful of oases, small pieces of genome, in which different varieties show very different types of sequences. In addition, it has been possible to verify that many of these regions of high diversity are associated with characters of interest, for example, with genes that control the shape or size of the fruit.
Both the great desert and the small oases were sculpted by the selection carried out by farmers. When a farmer selects from among the available plants those that he prefers, in general, he reduces the diversity of the genome, since he is discarding all the genetic variations of the plants that he stops cultivating.
However, when you choose to keep small-fruited varieties and, at the same time, other large-fruited, or red and yellow-fruited ones, you are indirectly preserving the diversity of a few regions of the genome, those that control those traits. The latter is called balancing selection.
In the analysis of the sequences of European varieties, 298 highly variable positions have been found and it has been seen that many of them are associated with morphological characters selected by farmers.
In addition, European farmers were also attentive to the appearance of new mutations. For example, the 'de penjar' varieties from Catalonia, Valencia and the Balearic Islands or the 'da serbo' varieties from southern Italy include the nor mutation. This genetic alteration made it possible to generate varieties whose fruits endure without rotting for months.
The long labor of humble Italian and Spanish farmers turned these two Mediterranean regions into secondary centers of diversity. Numerous varieties adapted to local climatic and agronomic preferences and conditions were developed in these two areas. Most of these traditional varieties are no longer grown today, or are produced for very local markets, but their legacy lives on today.
Almost all the varieties currently cultivated in the world have their origin in those present in one of these two regions or in some cross between them.
The industrial Revolution. We have commented that since the 16th century tomato cultivation was limited to the poor people of Italy and Spain, but this changed in the mid-19th century. Little by little, the old medical theories that accused fruits and vegetables of being unhealthy were abandoned. However, this factor alone did not make the crop popular in the rest of Europe and North America.
It must be borne in mind that, before the appearance of greenhouses, growing tomatoes in cold climates was not easy and, furthermore, the fruit, once harvested, could not be kept for a long time. To these problems it was necessary to add that the crop was seasonal, so its consumption was limited to a few months.
However, the industrial revolution changed the landscape. New ships and steam trains made it possible to transport the fruits quickly. At the end of the 19th century, for example, the Canary Islands became a center for the production of tomatoes for the English market.
In addition, during that century, new methods of preservation were investigated. The first notable success was that of Nicolas Appert, who at the beginning of the 19th century investigated a method of preservation in glass jars sterilized in a water bath. This advance was followed by the development of canned preserves. All this made possible the creation of large export canning industries in Italy and the United States.
On the other hand, from the end of the 18th century, seed production became more professional and the new seed houses began to create varieties in a controlled and systematic way.
For example, many of the varieties grown for export in the Canary Islands came from American and British seed companies. At the beginning of the 20th century these breeders adopted the newly discovered Mendelian genetic knowledge and decided to use wild diversity to alleviate the problems that the small underlying genetic diversity of the cultivated tomato was causing.
The main result of these advances was the incorporation of numerous disease resistance genes from different wild species.
These innovations, surely unintentionally, have also ended up being introduced into traditional varieties. In the Traditom project, for example, it has been found that 25% of the traditional plants studied contain modern disease resistance genes introduced by professional breeders into commercial varieties.
Facing the future by learning from the past. Farmers and breeders have always been looking for the best varieties and to get them they have used the knowledge and materials available at each time.
Agriculture is currently facing a major problem: sustainably feeding a growing population in the midst of climate change that is becoming more evident every day. Faced with these challenges, we must use any technology that allows us to improve crops, while reducing their enormous environmental impact. The alternative that we have unfortunately adopted in Europe, discarding tools for ideological reasons, implies hunger and unnecessary destruction of natural resources.
José Blanca is Professor of Genetics, Polytechnic University of Valencia; Joaquin Cañizares is Professor of Genetics, Polytechnic University of Valencia