The Biologist Is In: Genetic Assimilation.

Thursday, November 27, 2014

Genetic Assimilation.

Tomatoes sporadically produce fruit with horns, fleshy extensions adjacent to the calyx. Do a web search for, "Devil Tomato" and you will find several like the one in image #1. Generally, there is no evidence for these being the result of a genetic mutation. Rather, they represent the sort of thing that can happen when the normal development program of the fruit is disrupted in some way. Seeds taken from such a horned fruit will be no more likely to produce a plant that has similar fruit than seeds taken from any other fruit on the plant.

2. [source]

There is a related species, Solanum mammosum, that has multiple such horns (image #2). (Though, there are example plants without horns.) The fruit of S. mammosum are rather toxic, so it wouldn't be a great idea to try and make a hybrid between the species and domesticated tomatoes.

3. [source]

Because there is the developmental potential for horns to be generated in tomatoes, there is the potential for a mutation to emphasize the trait. In the Tomato-TILING project, a few such mutations turned up (image #3). I'm not a professional plant developmental biologist, so I don't expect to get access to these interesting mutant seed lines any time soon.

I like the idea of looking for something that everyone else is trying to avoid. Every tomato breeder I've come across has been trying to breed away from a horned tomato, to produce a more "perfect" fruit shape, so I instead want a tomato that is all horns. I have the mental image of a tomato covered in fleshy projections featuring on a counter in some new science fiction movie.

As the previous examples have certain difficulties as a source for this trait, I've been looking for tomato lines which show a higher rate of these "deformations" to use as starting material in a project to breed a tomato that has the trait more consistently.

A rarely studied evolutionary model called "Genetic Assimilation" describes the process where an aberrant trait produced as the result of some stress is selected for and eventually becomes genetically fixed even without the presence of the stress. This mechanism sounds like Lamarckian evolution, except that it relies on the natural selection and the developmental plasticity of organisms… rather than the personal experiences and intention of the organism that was favored by Lamark. It works because every trait is impacted by the genetic background, the combination of many subtle influences from other genes throughout the genome.

I frequent the Tomatoville forums, including the "Crosstalk: Tomatoville Research and Development™" forum. I started doing so because people there have a tendency to post lovely photos of the interestingly colored and patterned tomatoes they have been growing. Recently, a user was posted images from the results of a complex cross (["Pink Furry Boar" x "Ananas Noir"] x "Bosque Green Cherry") that they were working with. One of the diverse progeny they grew (image #4) had horns on 4 of the 20 fruit. 20% is a far higher rate than I'd otherwise come across, so I asked for a few seeds.

In a few years, I'll have a better idea of where this project is going. The good thing is that I can eat all the rejects along the way.

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