Beefsteaks are large bumpy tomatoes, with many more locules (the slimy areas containing seeds) than usual. Tomatoes in the wild are tiny fruit, but some of the tomatoes taken to the west from Mexico hundreds of years ago already had the genetic mutation responsible for the beefsteak tomato’s gargantuan size, so it’s a very old mutation.

Researchers from Cold Spring Harbor Laboratory on Long Island, New York, found that plum tomatoes will convert to beefsteak tomatoes via a very simple mutation. In a regular plum tomato, three sugar molecules attach to a signalling protein called CLAVATA3, which tell the stem cells to stop proliferating. In tomatoes with the mutated gene, this chain of sugar molecules fail to attach, and the stem cells keep multiplying.

“Three sugars is normal, and you get normal stem cell control,” says lead scientist Zachary Lippman. “But lose one sugar and the [signalling] works a bit less efficiently, then lose two sugars and it works with even less efficiency. Lose all three sugars and you’ve got real problems!”

With this new understanding of how the beefsteak stem cells work, and with new DNA-editing technology called Crispr that lets you disable any gene you like, breeders could soon be fine-tuning other fruits to the same effect… Beefsteak banana anyone?