In genetics, sometimes one plus one equals zero. Genes that should work in concert with each other, each magnifying the other’s effects, can in occasionally cancel each other out instead. A study released Thursday in the journal Cell Press looks at the genetic mutation that gave us the modern, domesticated tomato, shedding some light on how these cancellations happen —and how we can use them to create a more productive tomato.
Modern tomatoes are the result of a mutation that likely arose with the spread of agriculture some eight to ten thousand years ago. Ancient tomatoes were smaller, darker, and more berry-like. The mutation allowed for the development of larger fruits that would lead to the voluminous tomatoes that we’re familiar with today. Basically all tomatoes—even the heirloom varieties—have this mutation.
In the 1950s a second mutation, called “jointless”, was discovered in the mmm mmm good fields of the Campbell’s Soup Company. The mutant attracted attention because it eliminated an elbow-like bend in the stem leading to the tomato. This had two benefits. First, a tomato with this mutation would be less likely to break off and fall to the ground where it could be bruised, or rot and thus made inedible. Secondly, it would make it easier to harvest tomatoes with mechanical pickers.
If breeders could merge tomatoes with the jointless mutations with the earlier mutations that allowed for big, bountiful tomatoes, they figured they’d end up with a more productive tomato plant.
But when breeders merged the two, they did not get the big, beautiful tomatoes with seamless joints they expected. Rather, the union created a plant that lacked joints, yes, but with a lot more branches and flowers—far more than the plant had the energy to turn into fruit. The end result was fewer tomatoes.
“The genes that determine traits are quite frequently members of gene families that in a normal plant are working together, moving towards their trait determination,” says study author Zachary Lippman, a plant biologist at Cold Spring Harbor Laboratory. If you have a mutation in one gene that affects growth and development of a trait, a second mutation can arise that contributes to that same trait. Instead of improving the trait, however, it makes it worse.
The influence that genes have on each other is known as epistasis. This sort of canceling out that Lippman is describing, which was on display when the jointless tomato merged with the original tomatoes, is known as negative epistasis—and it’s not limited to tomatoes.
“There’s some great examples from back in the twenties and thirties where people would cross tobacco plants to make new hybrid varieties,” says Lippman. “The hybrids, instead of showing a vigor effect which is typically what you expect and hope for when you make hybrids, they actually saw an autoimmune response. The plants would start to wither and die basically as if they were being attacked by some pathogen…