The domestication of the tomato in America: round trip

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Just as there are no chihuahuas in the countryside, but wolves, in nature there are no large-fruited varieties of tomatoes, but very small-fruited plants. So much so that they measure about a centimeter in diameter. Wild tomatoes can be found from Chile to northern Mexico, although the greatest diversity is found in the coastal regions and Andean valleys of Peru and Ecuador.

The process of transforming a wild species into a domesticated species is called “domestication”. lodge in Latin, it means house, so a domesticated species would be one that is adapted to live near us. It happened to wolves, tomatoes and almost all the species that accompany us and on which we feed.

The detailed history of tomato domestication is quite complex. We lack clear archaeological evidence, but we have been able to unravel it through analysis of the genetics and morphology of current wild and domesticated tomatoes.

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From Mexico to Ceja de Montaña

It is common to read that domestication took place in Mexico as it has been one of the most popular hypotheses since the 1950s. However, determining where it actually happened is not easy.

Two areas are concerned: Mesoamerica (Mexico and Central America) and Ceja de Montaña, the area between the foothills of the Andes and the Amazon in Peru and Ecuador. Tomatoes from these two remote regions are genetically very similar. It is striking. One might expect Ceja de Montaña tomatoes to be more like, given their geographical proximity, the wild tomatoes of the Ecuadorian and Peruvian coast, but this is not the case.

The evidence regarding domestication is not yet definitive because inferring the morphology of fruits thousands of years ago from present-day plant genetics is not trivial. The archaeological remains could decide the question but, unfortunately, they are very rare. The tomato does not keep very well in humid regions and, moreover, it has always been a secondary crop, used mainly for sauces.

Despite this, the available evidence suggests that some wild plants came from Mesoamerica to Ceja de Montaña and once there were domesticated. The most diverse population of cultivated tomatoes in the world is found precisely in this region of Peru and Ecuador.

This extraordinary diversity may be due, at least in part, to the fact that this is where domestication took place. Additionally, traditionally grown Mexican tomatoes, or at least those that have been genetically analyzed, are not descended from wild Mexican plants, but rather domesticated Ecuadorian and Peruvian plants. That is to say, they were imported.

Tomato, a round trip

It seems that the tomato has made a round trip between Mesoamerica and Ceja de Montaña. It probably came down south as a weed and came back as a crop. The genetic evidence for this return to cultivation is very clear.

This behavior of wild tomatoes like weed it’s not strange either. Today, these plants are still common in human-affected environments, such as agricultural fields or roadsides. This is the case in all subtropical regions.

In the Canary Islands, for example, tomatoes very similar to wild Mexican tomatoes can be found in open fields or along roads and parks. Moreover, we know with certainty that the agricultural cultures of Ceja de Montaña, for example that of Mayo-Chinchipe (between 3000 and 2000 BC), had imported domesticated maize from Mexico. It is possible that the tomato lined up in the exchanges that took place between these distant agricultural cultures.

In any case, domestication was not the last stage of human modification of the tomato, just the first. After this first change, in each place and each time, we created new varieties adapted to our tastes and our needs. First in America, then in Spain and Italy and finally, and only from the 19th century, in the rest of the world.

Tomato Genetics

The tomatoes that can be found in Ceja de Montaña are not simply descended from those that arrived from Mesoamerica, but rather hybrids with the wild ones located on the coasts of Peru and Ecuador.

The genetic signals of this ancient hybridization are still very clear in the genomes of current plants in Ceja de Montaña. In these genomes, fragments very similar to those of Mexican tomatoes alternate with sequences almost equal to those of wild plants from the Peruvian and Ecuadorian coasts.

The genomic regions that have been incorporated into tomatoes arriving from Mesoamerica include genes related to flowering and light response, among others, and were likely selected to adapt the newly arrived plants to an equatorial seasonal regime. .

The plants need to adapt their flowering season to the climate and the latitude and, probably, this hybridization with wild Ecuadorian plants allowed the plants arriving from the north to adapt to the new latitudes.

That wasn’t the only change. Any process of domestication implies a genetic modification of the species: the descendants of domesticated organisms are different from wild ones. Genetics is nothing more than the study of biological inheritance, so if we change inherited traits, we are doing genetics.

How did the ancient farmers manage to make these changes?

On the one hand, they were looking for mutants. For example, they chose plants with unusual colors or fruit shapes. In addition, crosses have been made and new varieties have been selected.

It’s basically the same thing that modern breeders do who create the varieties that we can buy in the markets. The difference is that ancient farmers practiced these practices intuitively, without knowing the underlying genetics, whereas today’s professionals do it in a conscious, controlled, systematic and much faster way.

For example, while a great knowledge of the plants they grew enabled ancient farmers to breed new mutants and spontaneous crosses, modern breeders are able to plan crosses to produce new varieties that incorporate desirable traits from different parents.

In both cases, the objective pursued is the same: to improve the culture and adapt it to our needs and our tastes. The strategy for achieving this is essentially the same, although the modern approach is much faster and more efficient.

Our current genetic knowledge has allowed us to discover many modifications made by ancient farmers. For example, among the mutants that changed the shape and size of the fruit in Ceja de Montaña is “fas”. This mutation of the CLAVATA3 gene, located on tomato chromosome 11, produces larger fruits with irregular shapes.

Genetics, history and culture

Determining the location of the region in which the tomato was domesticated, although a matter of academic and historical importance, is not the main lesson to be learned.

The most important thing is to remember that the tomato is no exception. Like the rest of domesticated species, it contains a complex history that combines genetics and various agronomic and cultural practices. Today’s tomato, like all other crops, is the result of the contact and mixing of very different peoples and it is a fundamental part of our common history and our future.

Character font: José Blanca / Joaquin Cañizares / THE CONVERSATION

Reference article: https://theconversation.com/tomato-domestication-in-america-a-round-trip-trip-174844

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