Nowadays, many of us are driving hybrid cars. Hybrid cars combine the best of two worlds into a single vehicle. The same principle applies when we consider hybrid vegetable varieties.
Seed is the product of sexual reproduction
Plants produce seed through sexual reproduction: the pollen from the male plant fertilizes the ovum of the female plant, which then develops into a seed that contains the embryo of a new plant. This new plant carries genetic traits from both the male and female parent.
The exchange of pollen between plants of the same species occurs at random in the wild. Pollen is distributed by wind or by insects. This unregulated distribution of pollen is called “open pollination”, and it results in random distribution of genetic traits. There can be quite a bit of variation between the individuals that make up the population, with variation in vigor, strength, height, fecundity (ability to produce seed), root development, stress tolerance, etc.
Plant breeding started in antiquity
Humans have taken advantage of this large amount of variability within wild populations of plant species to select plants that produce more food, more fiber, better nutrition, better storability, etc. By saving the seeds of plants that produce more grain, have better flavor, that outperform, humans have created more productive varieties for thousands of years.
It does indeed take a long time to improve varieties, especially when you would like to create plants and crops that have multiple useful traits. Because of the random distribution of genetic traits, it is not easy to find individual plants that have the ideal combination of traits, such as large seeds AND disease resistance AND good storage AND good flavor. You have to be very lucky, and work with a very large number of plants to find that one special individual that has (most of) it all.
How to make a hybrid variety
Plant breeders have learned a lot since Gregor Mendel first worked out the basic principles of genetics working with peas in his monastery’s garden. The technique known as hybridization provides a process for bringing desirable traits together into a single individual. Here is how it works:
Imagine that you have an open-pollinated population of carrots. Some individuals in the population make really nice carrots, but they are still susceptible to foliar diseases. This means that these plants do not perform well in wet seasons. In the same population there are also individuals that do seem to have better tolerance to foliar diseases, but their roots are only average. As much as we try, we cannot find plants that have both nice roots AND have good tolerance to foliar diseases.
Let’s now select plants with good roots and keep them separate from the rest of the carrots. We let these plants interbreed, and over the course of several years we keep selecting the plants with the best roots and discard the rest. Eventually we will end up with a population of plants with mostly good roots.
We can do the same with plants that seem to have good tolerance to foliar disease. Through selection of the healthiest plants we end up with a group of plants with strong and healthy leaves, even under wet conditions.
We call the population of plants with good roots a breeding line, and the population of plants with healthy leaves is another breeding line. Now we let these two lines make flowers in the same field and let them exchange pollen and make seed. This seed grows into hybrid individuals that produce both good roots AND have healthy foliage. Hybridization combines the traits of the parent populations, or breeding lines, into a new hybrid population, or hybrid variety. (In Latin this hybrid variety, which is the first generation of offspring from the two breeding lines, is called Filial 1. This is where the abbreviation F1 comes from).
Advantages of hybrid varieties
The strong selection and inbreeding of the parent lines makes these parent lines more uniform. By combining two highly uniform breeding lines we create hybrids that are also quite uniform. This means that the crop tends to grow predictably and matures evenly. This has advantages in crop treatments and at harvesting.
The combination of selected genetics into a hybrid often gives the hybrid extra vigor (the hybrid has more vigor than the vigor of the two parent lines combined). This hybrid vigor allows the plants to grow strongly and produce larger crops.
Maintaining hybrid varieties
Gregor Mendel demonstrated that the combination of traits of hybrid individuals will be broken up when these hybrid plants are allowed to interbreed through a process that we know as segregation. This is a result of the random rearrangement of genetic material through sexual reproduction. So, in order to be able to continue to produce the same hybrid variety we have to maintain the two parent breeding lines. Every time we cross the two parent lines, we will re-create the hybrid variety that carries the desirable traits from both parents.
What about Seed Savers?
Sometimes we hear criticism of hybridization that points out that growers cannot save their own seed and have to buy new seed of the hybrid variety every year from the seed company. Yes, this is certainly true. But hybridization is not the exclusive domain of commercial seed companies. Hobbyists, gardeners and commercial growers can apply the same principles and make their own hybrids. It is just a lot of work, and it takes a lot of time and a well-organized team to maintain breeding lines, produce hybrids and produce clean and healthy seed. Gardeners and growers who rely on the advantages of good and productive hybrids agree this yearly purchase is well worth the money.
Hybrids are natural
Hybridization makes use of genetic variation that exist in natural populations. Through selection we can make multiple different breeding lines in which traits of our choosing are combined. By mixing and matching different breeding lines we can combine these traits in a variety of combinations and create a range of new hybrid varieties.
Hybrids are not GMO
There are multiple techniques that can facilitate the development of inbred parent lines and hybrid varieties, such as hand-pollination (instead of relying on insects or wind) and naturally occurring self-incompatibility mechanisms or male sterility. Regardless, hybridization works with the genetic material in one species, and does not involve techniques that modify the species genome by introduction of DNA from non-crossable species.
Hybrids are not clones. A hybrid population is made up of a group of individuals that share some major characteristics, but there still is a considerable amount of genetic variation. After all, the parent lines are not completely identical, just strongly selected (typically for about 6 – 7 generations). Clones are individuals that are produced by vegetative reproduction (potatoes, garlic, fruit trees, grapes, etc.), while hybrids are the product of sexual reproduction.
Plant Breeding never stops
Modern plant breeding is assisted by increasing knowledge of the plant genome. We now have laboratory equipment that can help us quickly figure out the genetic content of an individual, and allows us to determine in an early stage of plant growth which plants have the trait that we are interested in. This is the same kind of equipment that is being used to study viruses and develop vaccines, by the way.
The rapid expansion of our knowledge of the plant genome and the availability of automated analytical equipment will help develop new varieties that can thrive under stressful environmental conditions, have greater nutritional content and better flavor, and produce well with only minimal inputs.
The thoughtful and considerate application of these techniques will benefit us all.
