The tomato source thickens: evolutionary mysteries go deep

A team of US researchers has identified the “syndromes,” or collections of traits, which gave tomatoes their colour, flavour and taste.

Tomatoes have a surprisingly complicated evolutionary history, and there isn’t much systematic data on what eats – or spreads – wild tomato plants.

“Have you ever held a fresh tomato in your hand and wondered why it looks good, smells good and tastes delicious?” asks Jacob Barnett, a graduate student in organismic and evolutionary biology at the University of Massachusetts Amherst, US.

Barnett, who is lead author on two recent papers in Plants People Planet and the American Journal of Botany, has wondered a lot about this.

“These two studies are the first to look at fruit traits across all species in the entire tomato group,” says Barnett’s advisor, Professor Ana Caicedo, a biologist at the University of Massachusetts Amherst.

“We have been able to tell a comprehensive story of how wild tomatoes compare to each other and to our modern, cultivated varieties.”

The researchers looked at 13 wild species related to modern tomatoes to understand how the common variety (Solanum lycopersicum) evolved.

They grew each of these species in a common garden, from seeds provided by the C.M. Rick Tomato Genetics Resource Center, US. This centre is based around a collection of South American seeds gathered by a US botanist during the 1950s and 1960s.

The wild species are very different to lycopersicum. “For one thing, they’re tiny,” says Barnett, “about the size of a blueberry. And most of them are green when ripe. Many smell like apples, melons or even cucumbers, and a number of them taste terrible.”

Once grown, the researchers examined fruit of each species for colour, shape, sugar and acid content, DNA, and volatile organic compounds which cause tomato smell.

They found an “honest signal” among the tomatoes: a link between the fruit’s appearance and  nutritional content.

They also found two clear syndromes: clusters of traits linked to one fruit or another.

“One observed cluster consists of fruits that are red/orange/yellow, high in glucose and fructose, and low in sucrose and malic acid,” write the authors in their Plants People Planet paper.

“[These are] traits thought to be preferred by birds, which are expected to be the main dispersers of the coloured-fruited tomatoes because most birds have excellent colour vision that they use to locate food, and some birds are unable to digest sucrose.

“The other cluster includes fruits greenish and lighter in colour, high in sucrose and malic acid but low in glucose and fructose – traits generally thought to be more in line with fruits primarily dispersed by mammals.”

So how did humans end up preferring the bird-favouring fruit? We still don’t know, but the research team now has a better framework for figuring out tomato evolution.