For many crops certain strains are developed that are called “F1 hybrids”. For example in corn, certain flowers and vegetables. The farmer or gardener sows these “F1 hybrid seeds” and will get a superior crop. The plants grown from these F1 hybrid seeds will generally do much better than traditional seeds and will have one or more desirable traits like disease resistance. They even do much better overall than their own parental plants. How is this possible? Let’s dive into the world of heterosis.
How to produce F1 hybrids
To create F1 hybrid corn seeds, you need to complete the following steps:
Find plants with the desirable traits
Inbreed these plants among themselves, select on their important traits while continuing to inbreed the plants
Interbreed two parental lines
The resulting seeds are F1 hybrids, select them on their success
This sounds very easy, but of course it’s not super easy. Especially creating the parental lines is very hard as you will need a lot of trails and errors before you have satisfactory plants.
Create the parental lines with desirable traits Let’s say you want to create a type of corn with the following traits: resistance to disease A, B and C, large seeds and fast maturing. You have a wide variety of corn plants from all over the world, they are genetically diverse and have many combinations of traits. You will test them all and find the plants that have the desired traits. You will only select them if they have one or more of the desired traits, no other criteria are needed.
Inbreed these plants among themselves Let’s say you have now 6 plants that have the desired traits. One plant is resistant to disease A and B, one plant is resistant to disease C, one plant has very large seeds and one is fast maturing. You will inbreed these plants among themselves. Every generation you will select the plants on the important trait. If the plants start to look bad, e.g. small, not growing so fast, you will ignore this. As long as the plants are viable you will continue to inbreed them until 100% of the offspring will have the desired trait. In the best case scenario you will end up with 6 different inbred parental lines, but in practice it will turn out that it is hard to do. Some traits will not breed true, for example very complex traits that involve multiple genes or have heterozygote advantage. In our example we will end up with an inbred line that is resistant to disease A and B, a different line that is resistant to disease C and one line that has large seeds. All these plants don’t look the best overall, but at least they have the traits we want.
Interbreed two parental lines Now its time to interbreed the parental lines to produce F1 hybrids. To create a F1 hybrid that is resistant to all 3 mentioned diseases, you will interbreed both resistant lines together. The resulting F1 hybrid will be resistant to diseases A, B and C. They plants will also do much better than both their parents, they will be taller and grow better as the inbreeding depression that has occurred in the parental lines is relieved. All of this is not guaranteed, that’s why there is another step.
Select the F1 hybrids on their success Now all F1 hybrids are created equal. It is not a guaranteed success: the F1 hybrid that is created could also still be susceptible to one of the diseases or would continue to look bad and grow badly. This means you will have to start all over again with new plants and new parental lines. In our case you do have more that two parental lines. You can try each of the lines on one another, trying to find a good combination. For example you will have the chance to try breeding a F1 hybrid that is resistant to disease A and B ánd has big seeds.
The benefits and disadvantages of F1 hybrid seeds
There are many benefits to F1 hybrid seeds. They perform much better, are very uniform in their traits and have really well controlled traits. For the producers they have an added benefit: you cannot produce the same quality seeds from F1 hybrid plants. The F1 hybrids will interbreed and produce F2 hybrid seeds, which are not as good as the F1 hybrid seeds. You can only produce F1 hybrids if you have both the parental lines. Seed propagators will hide their parental plants and seeds from competitors and farmers, obliging everyone to keep buying new F1 seeds for sowing. In other cases the F1 plants are sterile, meaning their seeds will not germinate and the grower is again obliged to buy new F1 seeds to sow.
The genetic basis for F1 hybrids
The underlying causes of the aspects of the F1 hybrids can be found in the genetics of these plants. When creating an inbred parental line, you are removing a lot of genetic information. The plants will lose alleles every generation, as they are being inbred. You will make sure the plants to not lose their desired traits, by only inbreeding the ones that show the desired trait. When you will get offspring that uniformly show the trait, without any that do not show it, you know you have fixed the trait in the population. Any of these plants will transmit this trait to the next generation.
When you are then combining two inbred parental lines, you are also combining the traits in the offspring. Because each of the parental line will transmit their trait, the F1 hybrid offspring will have one copy of each trait. If the trait is dominant or partially dominant, this means the trait will be visible in all F1 hybrids.
When breeding F1 hybrids among themselves, they will sometimes transmit the trait and sometimes not transmit it. This causes the F2 hybrids to be genetically diverse and not reliably have the traits you are looking for.
When inbreeding the parental lines you will weaken them by reducing their genetic diversity. You will inadvertently fixate some deleterious alleles into the line. The chance that the same deleterious alleles will be fixated in the other parental line is small. When combining these two lines again into the F1 hybrid, the deleterious alleles are complimented by their normal counterpart and will not cause problems. This is one of the mechanisms behind heterosis.