Dalia Ventura
BBC News World
About nine million years ago, in what would later be called South America, while the Andes were still rising, vegetation was wild and humans did not exist – and two plants lived side by side.
"More precisely, two populations of plants," says Dr. Sandra Knapp, a botanist at the Natural History Museum in London.
"They were the ancestors of what we know today as the tomato (Solanum lycopersicum) and the group of plants we know as Solanum etuberosum, the three current species of which can be found in Chile and the Juan Fernandez Islands," she adds.
As you might have noticed from their names, they were related and interbred with each other.
“It was a gene rearrangement that created something completely new,” says Dr. Knapp, which “allowed it to thrive in the cold, dry habitats of the Andes.”
Experts call this interspecific hybridization, and it happens frequently, sometimes with unfortunate results.
A mule, for example, is born from an intimate relationship between a female horse and a male donkey.
Although a successful hybrid, valued since ancient times, it cannot reproduce on its own.
In the plant kingdom, says Dr. Knapp, crossbreeding between species happens frequently – so frequently that it's how we get our garden plants.
This can happen naturally or through human intervention, resulting in plants that are a mixture of both parents.
"Sometimes they are sterile, so they don't develop into a new population," she says.
But when the combination of circumstances is ideal, the result of the union can surpass all expectations.
That's exactly what happened here.
From that chance encounter millions of years ago between two species of the Solanaceae family, the potato was born.
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"It's fascinating that something as everyday and important to us as the potato has such an ancient and unique origin," says Dr. Knapp.
"Tomato is the mother, and etuberosum is the father," said Sanwen Huang, a professor at the Chinese Academy of Agricultural Sciences who led the international study published in the journal Cell in July.
Solving a long-standing mystery
The starting position was something already familiar.
Although when you see it at the market, a tomato – firm and starchy – doesn't look much like a tomato – red and juicy – "they are very, very similar," notes Dr. Knapp, who also participated in this research.
According to this scientist, their leaves and flowers are very similar, and the fruit of the potato plant even looks like a small green tomato.
"Beyond what we can see, we have known for a long time that potatoes, tomatoes and etuberosum are very closely related," she says.
"What we didn't know was who was closer to the potato, because different genes were telling us different stories."
Scientists spent decades trying to solve the enigma of the origin of the popular tuber, but they encountered difficulties: the potato's genetics were unusual.
Although most living species, including humans, have two copies of chromosomes in each cell, potatoes have four.
To resolve this paradox, the team analyzed more than 120 genomes (the complete set of genes and genetic material present in a cell) of dozens of different species, including potato, tomato and E. tuberosum.
The potato genomes they sequenced were roughly evenly split between tomato and E. tuberosum.
The ancestor "wasn't one or the other; it was both," Dr. Knapp points out.
That's how researchers discovered this love affair from the foothills of the South American mountains millions of years ago.
It was a successful community “because it produced genetic combinations that allowed this new lineage to flourish in the newly created high-altitude habitats in the Andes,” explains Dr. Knapp.
This was largely due to the fact that, although the potato plant looked very much like its parents above ground, there was something hidden that neither of them possessed: tubers.
Having tubers is like having a lunchbox at all times: they store energy, which helps to survive winter, drought, or any other unfavorable growing conditions.
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'Genetic lottery'
Scientists also discovered something fascinating: the plant that produced the tubers did so by winning the genetic lottery.
It turns out that each of its ancestors had a gene crucial for the formation of tubers.
Neither was enough on its own, but in combination, they set in motion a process that transformed the underground stems into delicious potatoes.
The Chinese team that Dr. Knapp collaborated with even managed to prove it.
"They did a lot of very elegant experiments, they turned off these genes to confirm their hypothesis," she says, "and without them, the tubers didn't form."
The hybridization that created the potato was more than a lucky coincidence – it created a new organ.
And that organ, the potato, represents an evolutionary feat.
Its mere existence allowed the plant to reproduce without the need for seeds or pollinators.
It has managed to adapt to a wide range of altitudes and weather conditions, which has led to an explosion of diversity.
Even today, “there are more than 100 wild species that can be found only in the American continent, from the southwestern United States to Chile and Brazil,” says Dr. Knapp.
Vulnerabilities
However, this ability to reproduce asexually has also harmed potatoes.
"To grow them, you plant small pieces of potatoes, which means if you have a field with only one variety, they are effectively clones," explains Dr. Knapp.
When you are genetically uniform, it means that no potato plant will be able to defend itself against new diseases, for example.
And that brings us to the reason why scientists conducted this research in the first place.
According to Dr. Knapp, the Chinese team wants to create potatoes that can reproduce from seed and be genetically modified.
They hope that introducing genes from wild species can create variants that better tolerate environmental challenges.
"The other evolutionary biologists who participated in this study and I, in turn, wanted to find out who the closest relatives of the potato are and why they are so diverse," she says.
"And so we approached the research from very different perspectives and were able to ask questions from each of our perspectives, which is why it was so much fun to participate in and work on this study."
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