The Science of Crop Breeding – Creating Better Hybrid Tomatoes (FSFS221)

It’s tomato season once again, but we won’t be talking about growing tomatoes—we’ll be talking about creating new, successful hybrid tomatoes.

We have Bulgarian tomato breeder Nikolay Georgiev on the show to talk about the how’s and why’s of hybrid tomatoes.

What are your thoughts on integrated plant protection? (2:35)

We can’t solve all plant problems—proper cultivation, pest protection, diseases. We should look at other ways to protect our crops. One example is integrated plant protection which is a system that considers multiple factors such as soil microbiome, the insects surrounding the surface, etc. These factorsmake up IPM, or integrated plant management.

Let’s take thrips for example. Thrips are pests in Europe and the Mediterranean and are vectors for viruses, the most problematic being the tomato spot wilted virus (TSWV).

To prevent thrips from attacking plants, we can try putting a physical barrier such as a fine net. But using a fine net would stop the airflow which can even lead to fungal problems. Using the principles of IPM, you can opt to use bioagents, meaning introducing beneficial insects to prey on the thrips.

How far can genetics get you when it comes to protecting plants? (6:25)

Currently, genetics can only get you so far—it cannot protect you 100%.

A lot of work is being done to find a variety of tomato that has more trichomes which are hairs on the stem, leaves, and fruit. Trichomes act as a natural protection because they prevent insects from landing on the plant. Despite being effective barriers against insects, there is still a lot of work to be done to introduce this trait to commercial varieties.

We have such genetic material in our breeding program, and it’s super interesting. It’s a crazy-looking plant, completely covered in many, many tiny hairs that make the entire plant look almost white. Even the surface of the fruit.

The hairs are so short and close together that insects can land on the surface of the trichomes, but not on the plant itself. Because of that, insects can’t penetrate it, infect or harm it.

It’s one way of approaching the pest issue using genetics, and we’re working on introducing this trait into a tasty, high-yielding variety.

Say you have a trichome-covered plant, but it’s not enjoyably edible. What science goes into transferring the trichome trait to a commercial variety that has good flavor? (10:30)

There are several approaches— genetic engineering, which is not allowed in Europe, and conventional breeding, which is what I do. Conventional breeding is taking the variety with the desired trait and integrating it into the gene pool (the collection of all the genetic material you work with).

The first step is to cross and make a hybrid between one of our commercially successful lines and observe which plants inherited the trichome trait. The best-case scenario would be that the F1 hybrid would have hairs on the leaves, stem and fruit. This means the genes from the wild plant are dominant. 

Next, we self-pollinate the F1 hybrid. From the resulting plants, we’ll only get the ones with the trichome trait and discard the rest. From those with trichomes, we will find ones that are tasty, the others will be discarded. Then we’ll look for more and more characteristics that we like, and the process goes on and on in the succeeding generations until we get a trichome-covered plant that is commercially viable.

If you’re a farmer who saves seeds from your best-looking tomatoes every year, how much are your tomatoes changing from generation to generation? (20:00)

Since the tomato plant is a self-pollinating crop, we assume that cross-pollinating is possible but very highly improbable. If you save your seeds from your best tomatoes yearly, plant them the next year and continue the process, you’re creating Something perfectly stable—a line. By definition, lines are homozygous (genetically uniform), and each generation produces the exact, same fruit.

“You’re basically reproducing the same plant.”

If you created a stable line, does it rule out the possibility of getting a locally adapted crop? (23:00)

I think some mutation may have occurred. Looking at it on a scientific standpoint, the only ways to change a DNA that can be inherited into the next generation are by (1) mutation, or (2) cross-pollination. Excluding cross-pollination, if you grow a crop for, say, 20 years, for sure some mutation occurred that made your plant more adaptable to your environment.

But in choosing the best crops, you might be missing some things. For example, you have a plant that mutated to have a favorable trait like drought resistance. But since you had a great year with ample sun and rain and you chose seeds from the ones that looked the best, you skipped getting seeds from this one plant that had drought resistance because you didn’t see it exhibit that trait. That way, you immediately eliminated this favorable mutation that might have improved the plant for the following years.

Just from taking the seeds from the best plants each year doesn’t guarantee you’re getting all the favorable traits.

If I see one of my stable heirloom plants producing bigger fruit than the rest, can I assume that a mutation occurred that made the seeds make bigger fruit? (28:00)

The trait you mentioned—bigger fruit—is a complex trait which means it affects multiple genes, not just one. And usually, mutations would only affect a single gene.

For example, the size of a fruit is controlled by, say, 50 genes. In order to have this result from a stable, genetically uniform heirloom line, all 50 genes should have mutated at the same time. And the chances for this to happen is 0.

I think the reason why this plant had bigger fruit is purely environmental—better soil, better drainage, more sun, more nutrients. Saving seeds from the plant with the biggest fruit will not guarantee your yield the following year would be just as good.

What are other traits are controlled by many genes that a non-scientific breeder has little chance of changing? (30:45)

Usually in breeding, the number one goal is yield. This is the most complex, large-scale trait that breeders try to improve. Yield consists of fruit size, number of fruit per cluster, number of clusters, distance between each cluster. All these different traits make up what we call yield.

Another trait controlled by multiple genes that a home breeder would find difficult to improve would be nutritional value.

I have a good tomato—good size, good taste, but it looks boring. How do I start to affect the color without affecting everything else? (33:30)

Let’s say you want to make your perfect tomato orange. The best way to do this is to find an orange tomato variety with a lot of similar traits with the red tomato and use a breeding program called back-crossing.

Say your neighbor grows an orange tomato that shares similar characteristics with your perfect red tomato. The first thing to do is to cross the two tomato lines. Your red tomato will be the recurrent parent, which means we’ll be crossing this parent with the succeeding progenies to transfer more and more of the original traits of this red tomato.

During the entire process, we’ll select the plants with the traits that we’re looking for and discard the rest. Eventually, after about 6 generations, we will end up with an orange tomato that is the same size and taste as your original tomato.

Why aren’t F1 hybrids stable? Why do people have to buy those seeds every year? (44:05)

I think it’s important to mention why F1 hybrids are used in the first place. Their main purpose is to combine two favorable traits from two different lines that would be otherwise be extremely difficult to put into one single line.

You’re basically taking 2 stable lines and a plant that exhibits half of the characteristics of each plant.

Say I have F1 sun-gold cherry tomatoes hybrids. What will I see if I save the seeds and replant them? (47:20)

You will see characteristics from the parents of the original sun-gold cherry tomato hybrids.

For example, the F1 seeds are produced by crossing a parent that produces ½-ounce tomatoes and another plant that produce 1-ounce tomatoes. The F1 hybrids exclusively produce uniform ¾-ounce tomatoes.

If you replant seeds from your F1 hybrids, you will see plants that are ½-ounce, 1-ounce, and ¾-ounce.

Is there any reason to believe that better-looking seeds might result in a better plant? (49:40)

Based on my experience, that is not the case. I’ve had many tiny seeds that the sorting machine would throw out, and they would germinate better than the bigger, plumper seeds. There is no scientific proof that says bigger seeds guarantee better plants.

Given unlimited resources, is there a limit to how far you can push genetics using traditional breeding? (51:40)

Many, many times I imagined myself in this scenario—unlimited resources in terms of time, money, and space—and thinking of the best strategy to create the perfect watermelon-sized fruit.

I think the quick answer is yes, there is a limit. I don’t think we’ll see a tomato larger than a watermelon any time soon using traditional breeding methods. I think the reason behind it is that there’s been a bottleneck in the genetics of tomatoes which has really reduced the amount of genetic diversity available to work with.

If I had unlimited resources, I would spend 90% of it travelling to the places where tomatoes originated to find wild types. In my opinion, integrating wild types is one of the best ways to enhance your genetic pool and increase your diversity. Only with large genetic diversity can you produce something different than what we already have.

Are all resistances created equal? Is there a standard to be met in order to list disease resistance? (57:40)

In Europe, we have this leaflet by the European Seeders’ Association called the Code for Resistance Harmonization.

It has a table that lists all the bacteria, viruses, fungi and nematodes. At the end of the table there’s a column that says which plant variety exhibits resistance. That variety is the standard for resistance. There are extensive protocols for evaluating disease resistance, but having said this, it just means that they all meet the minimum standard for resistance. Past that, some varieties could have better disease resistance than others.

Why can’t we develop plant immunity? Is it because the disease is mutating too much that we can’t corner it? (1:06:55)

Yeah, exactly! When talking about fungi and bacteria, they mutate a lot. And even if a plant has genetic resistance, the pathogen can mutate and overcome the resistance especially if another stain appears. With viruses, I cannot think of another way to create resistance against viruses apart from injecting every plant you have with a vaccine.

If you expose a young plant to a pathogen, not enough to kill the plant but just enough to elicit an immune response, would the plant build resistance? (1:09:50)

That’s a topic that’s still being discussed in the scientific world. As far as I know, plants have a systemic acquired resistance (SAR). In principle, it works the same as vaccines in humans and animals. It activates a plant’s defense response during an initial pathogen attack. This works as a sort of memory mechanism—it tells the plant how to respond to future attacks by a specific pathogen.

We’re not sure if this whole acquired systemic resistance works for all pathogen types, but it may be possible that a small biotic stress to the plant at an early age could strengthen its defense in the future. This topic is still being researched, and I haven’t heard of any specific treatments that implement this.

Why aren’t there more resistant varieties of crops to late blight, say extreme resistance vs. minor resistance? (1:13:40)

Let me start by saying that most of the tomato breeding in the industry is done by the big companies because they have the resources to invest in R&D. Their main market are huge farmers with the high-tech greenhouses that produce a lot of tomatoes meant for supermarkets and big establishments. In those high-tech, climate-controlled greenhouses, late blight is not a problem they encounter.

If big companies wanted to make late blight-resistant crops, they can definitely do it within the next ten years. But big growers would not be impressed with such a variety. In my part of the world, big growers would prefer something resistant to TSWV or something that gives higher yields or larger, more transportable fruit.

To close this episode, I want to bring up taste. People usually associate heirlooms with great taste and hybrids with lesser taste. What’s your experience with that? (1:23:35) 

Here in Bulgaria, people associate F1 hybrids with firm, bad-tasting tomatoes. I think that’s because that’s what we get in the market. We import tomatoes picked a week before reaching the market. Understandably, they have to be picked green, and they won’t be tasty at all. That’s just how the market works.

People who sell at supermarkets thought, why buy local produce that are fresher but more expensive when we can buy elsewhere cheaper. So people associate what they see in the market with hybrids.

But in reality, hybrids can be better tasting than heirlooms, and they can also have a lot of other favorable characteristics such as disease resistance, yield, etc. it all depends on breeders that create these varieties.

Taste is becoming more and more valued lately, but big growers still prefer to grow something firm rather than something tasty because they can sell it easier.


Learn more about tomato breeding from Nikolay Georgiev on!  


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