The Science of Food

On the face of it a tomato and a potato are two very different things. One a fruit¹, soft and bursting with juice and flavour. The other a tuber that grows underground, hard but packed with starch ready to become a delicious source of nutrients after some judicious cooking. Despite this some new research shows that potatoes are the result of a hybridisation event between a species of tomato plant and another plant called Etuberosum. The tomato, so to speak, is the mother of the potato.

I covered plant hybridisation in my banana post where we saw that plants are able to tolerate polyploidy to a much greater extent than animals such as ourselves. Occasionally, in the wild, two different species of plant reproduce by cross-pollination producing an offspring with copies of both genomes. This seems to be what has happened with the tomato plant (Solanum lycopersicum) and a species from the Etuberosum group of plants. Hybridisation of these two species led to the potato plant (Solanum tuberosum).

What is interesting about this research is that neither of the parent species are capable of producing potatoes. Potatoes develop from thickened plant structures called rhizomes². Tomatoes don’t have rhizosomes at all and Etuberosum species have rhizosomes but they don’t develop into potatoes. If you combine the two species, though, you get the potato plant that not only has rhizosomes but it has rhizosomes that can develop into potatoes. The authors also managed to identify the genes that contribute to the potato; from the tomato plant two light signal response genes, gigantea (GI) and self-pruning 6A (SP6A), and from Etuberosum phytochrome B (PHYB) were essential for tuberisation.

To work all this out the authors sequenced the genomes of 127 wild species of potato, tomato and Etuberosum. Using this data (the sequences of A, T, G, C that makes up the genetic code) they could analyse differences between the potato genes and it’s parents and establish which were more closely related to which plant. Turns out that a potato is roughly 60% Etuberosum and 40% tomato. Using this data they could also work out that this hybridisation event likely occurred about 7 million years ago.

To be able to generate 127 genomes for a single study is an amazing thing for someone who started off when Sanger sequencing³ was the only way that we could get sequence information from DNA. As I discussed in the post on Vitamin D, this recent technology gives us the ability to personalise medicine by routinely sequencing someones genome. This could become a very powerful way to tailor treatment to the specific genetic makeup of an individual patient. It’s also really useful when trying to work out the lineage and development of some of our most important crops.

There are still questions that the researchers are interested in, the big one for me is how some potato species went back to sexual reproduction after the hybridisation event, wild potato species are a mixture of haploid and polyploid species. They also plan to do some mad scientist stuff like giving tomatoes the ability to form potatoes by transplanting the necessary genes from Etuberosum. So there should be some pretty entertaining results coming out over the next few years.

Footnotes

1. Yes, scientifically it’s a fruit ↩︎
2. Technically they form from a subset of rhizomes called stolons, that are rhizomes that grow horizontally. ↩︎
3. Sanger seqencing was the old, gel-based way of determining the sequence of DNA. Although very accurate it takes a long time, unlike more modern techniques that can operate in parallel. ↩︎