// Twitter Cards // Prexisting Head The Biologist Is In: Evolution and Tomatoes

Friday, March 21, 2014

Evolution and Tomatoes

Mutations occur all the time and are random in nature. Most mutations have minimal impact, but some are dramatic. A mutation isn't intrinsically negative or positive, but becomes so in the context of how it impacts the survival and reproduction of an organism in the specific environment it finds itself.

Selection, the biased survival of certain genotypes over others, trims the random mutations to those that work. 'Artificial' selection is what we call it when we make choices about which organisms continue to reproduce. 'Natural' selection is what we call it when it is the undirected interactions with the environment and other organisms that determines which organisms continue to reproduce. There is no real difference between 'artificial' and 'natural' selection. They both push organisms around, changing how they work and what they do, in ways that are to the advantage of the organisms themselves.

The plants and animals that we have 'artificially' selected have had incredible increases to their populations. There are far more dogs than there are wolves. There are far more wheat/corn plants than there ever were of the wild species from which they were derived. You could describe the development of our domesticates as their independent evolution into a very profitable niche…  us.

I like colorful and tasty tomatoes. Given that tomatoes represent ~$2 billion in farm revenues per year in the USA, I'm not the only one. Wild tomatoes and early market varieties develop with a darker green shoulder (at right) that ripens to the final red a bit slower than the rest of the fruit. By the time the green shoulder has ripened completely, the bottom of the fruit is already beginning to get soft. For home-grown tomatoes, this isn't a problem, since you're going to be eating it soon after plucking it from the plant at its most vibrant red ripeness. For commercial tomatoes, by the time the top has fully ripened, it has become too soft at the bottom for shipping. The result is that commercial tomatoes were picked and shipped before they were ripe and would appear partially green in the stores.

At some point before the 1930s, a farmer in the USA noticed a plant producing tomatoes that didn't have green shoulders (at left) and that ripened uniformly from top-to-bottom. The tomatoes could be picked when they started to go red and would ripen evenly all over by the time they reached the grocer.

In the market those tomatoes that still had the green-shoulders didn't sell as well as the all-red neighbors. They didn't look as ripe and couldn't compete. Commercial plant breeders saw the trait as the boon it was and soon the uniform-ripening trait (u) was bred into most commercial tomatoes everywhere. In the environment of the home garden, the uniform-ripening trait didn't provide any advantage to growers and the trait remains rare in the (heirloom) tomato varieties passed down along family lines.

Tomatoes still went soft on the way to stores, costing the industry a fortune in lost revenue. In the 1960s, a tomato was found by Henry Munger (a professor of plant breeding at Cornell) that showed strongly delayed ripening. This ripening-inhibition (rin) trait, when bred into commercial tomatoes, resulted in plants which produced fruit that remained hard for a longer time. Because they didn't go soft during ripening, they could be shipped with fewer losses. This made it cheaper to get a tomato to market and soon the trait had been bred into most commercial varieties. The ripening of these tomatoes is sped up by exposure of ethylene gas (a common plant development hormone), so they can be perfectly ripe upon arrival at the store.

In 1974, researchers realized that the ripening-inhibition (rin) trait resulted in changes in the chlorophyll and carotenoid composition of the fruit, in addition to the obvious inhibition of ripening. Several years later in 2002, the rin trait was identified as being caused by a mutation which impacted two adjacent transcription factors central to the development of fruit and flowers in tomatoes.

In 2012, other researchers realized that the uniform-ripening (u) trait was also caused by a mutation impacting a transcription factor. The broken transcription factor resulted in reduced production of carbohydrates and carotenoids in the fruit, in addition to the apparent change in ripening.

That transcription factor changes were found in both important-market traits is interesting and highlights their potential importance in other traits of interest.

At some point during this process, people began to notice that the tomatoes bought from the store didn't compare to those they or their friends grew at home. This realization has contributed to the recent resurgence of interest in heirloom tomatoes (and other crops) that have been maintained within families who saved seeds over the years. In the case of tomatoes, many of the varieties originated well before the 1930s when the story I tell above began.

The combination of mutation and selection is a very creative and powerful process, but our interests can only direct this process if we're involved in it. If we let others do it for us, it is their interests that will direct the process, as is now evident in the case of tomatoes.

An important aspect of my gardening philosophy is that anyone who grows vegetables should also save seeds. This keeps the interests of the gardener involved in the evolution of the crops they grow and, over the longer term, ensures the vegetables remain worthwhile (by diverse definitions) to grow.

Now that we know how the market-oriented selection of tomatoes has led to their decline, there is the potential to breed varieties which have the better flavor we associate with the heirloom varieties and the traits that make a tomato a market-sucess. However, even if market tomatoes are returned to the glory they should have always been, it will remain important for home growers to keep saving seeds if their interests are to remain involved in the plant's future.

  1. uniform-ripening (u)
  2. ripening-inhibition (rin)