The Basic Concept of Hybrid Seed Production using Cytoplasmic Male Sterility
Before Getting Started!!
This content applies only to bisexual flowers. If it is not a bisexual flower, there is no reason to use cytoplasmic male sterility for seed production. It is recommended to read the post below first.
□ Sexual Classification of Plants
■ Male Sterility: the condition in which plants are unable to produce functional pollen grains or male gametes, which is necessary for sexual reproduction. ■ Male Fertility: the ability of male reproductive structures (stamens) to produce viable and functional pollen that can fertilize the female reproductive structures (pistils) and result in the production of viable seeds.
If a plant breeder develops variety A using mother and father materials (parents), the male gamete of the father must be transmitted to the female reproductive organ of the mother to create variety A. This results in the formation of a seed that can produce variety A. Since this seed exhibits hybrid vigor (F1), it is used as a commercial seed because the genetic traits that will appear in the next generation are unknown after creating variety A. In other words, farmers should purchase seeds every year to grow crops instead of using seeds from the previous year’s crops due to the risk of not knowing which traits will appear.
For example, if an onion plant breeder discovers that the market demands a muscular, short, brown-haired, chiseled-jawline, and handsome onion variety, they might decide to create a new variety that includes these traits, which could be called “Bradley Cooper”.
To introduce this new onion variety to the market, thousands of Bradley Cooper onion seeds need to be produced. However, living organisms cannot be mass-produced in a factory like inanimate objects. So, how can we produce the new onion variety Bradley Cooper? One approach is to produce thousands of Bradley Cooper parents. The plant breeder has already created the variety; Bradley Cooper, so they already have the parents (referred to as “materials” in breeding). By crossbreeding these parents, the breeder can produce a large number of Bradley Cooper onion seeds, which can then be sold in the seed market.
Now, we need to mass-produce the parents of Bradley Cooper to produce a large number of Bradley Cooper onion seeds. In plants, the process of self-pollination, where the male and female reproductive parts of a flower are fertilized by its own pollen, is called ‘selfing.’ However, if a plant is self-pollinated only, a new variety cannot be developed because the offspring will be identical to the parent. For example, if the pollen from the stamen of an ABC plant fertilizes its own pistil, the resulting seed will produce another ABC plant. Therefore, the new onion variety, Bradley Cooper cannot be produced by repeatedly selfing the parents.
Usually, autogamous plants prioritize self-pollination and have less than 3% pollination rate from outcrossing. Even if heterozygous occurs through outcrossing, it will disappear over time.
Let’s take an example. Wheat variety A has the genotype aa, and the variety B has the genotype AA. Let’s suppose crossing A and B resulting in heterozygous Aa in the next generation. Then, this heterozygous wheat (Aa) will self-pollinate again in the following generation.
selfing Aa x Aa = AA Aa Aa aa
What would happen if each of the genotypes (AA, Aa, Aa, aa) continued to self-pollinate over time? The heterozygous (Aa) would decrease to (1/2)n where n is the generation number.
In the first generation, Aa was 50% (AA Aa Aa aa; Aa=50%). It means it was (1/2)1, which is 50%. If each of the genotypes (AA Aa Aa aa) self-pollinates and becomes the next generation, what would be the ratio of heterozygous individuals? It would be 25% because it is (1/2)2. After only six generations, the proportion of heterozygous individuals would become 1% [(1/2)6 = 0.015]. This means that it would become completely extinct.
So, how can we mass-produce “Bradley Cooper onion seeds”?
To produce Bradley Cooper, who is the hybrid offspring, the mother of Bradley Cooper (maternal parent) needs to mate with the father of Bradley Cooper (paternal parent). However, the maternal parent also has its own pollen and therefore has a greater chance of self-pollination.
Which case has a higher chance of self-pollination in the maternal parent with pollens between case ① and case ② in the above picture? Of course, it is case ①. If case ① continues, Bradley Cooper cannot be produced. That is, it is self-pollination of the maternal parent of Bradley Cooper, not the result of crossbreeding by the father of Bradley Cooper.
To produce “Bradley Cooper” onion seeds, we need to remove the stamen from the mother plant of Bradley Cooper. Then, we can transfer pollen from the father plant’s stamen to the mother plant’s stigma to produce Bradley Cooper through cross-fertilization. However, manually removing the pistil function from thousands of mother plant flowers to produce multiple “Bradley Coopers” is not practical.
Instead, breeders have developed male sterility (MS), which prevents pollen from being produced in the mother plant’s flowers. As a result, the mother of Bradley Cooper functions only as a female flower (with no pollen produced), and when crossed with the father plant, Bradley Cooper is produced. In other words, the term “mother of Bradley Cooper” refers to a plant that has lost the function of the pistil due to male sterility. The plant itself has both male and female flowers, but depending on its role in the breeding process, we refer to it as the mother or father of Bradley Cooper. This designation is based on the breeder’s chosen combination.
If we consider the scenario where the mother plant of Bradley Cooper has been made male sterile (MS), it would be unable to produce seeds without pollen from other male flowers. However, to produce a large quantity of Bradley Cooper, it is necessary to mass-produce the mother plant. But since the mother of Bradley Cooper has lost its function as a male flower, how can seeds be produced? To address this issue, breeders have created “maintainer lines” that can transfer pollen to the mother plant of Bradley Cooper, enabling it to produce seeds through fertilization. This technique is used solely for the purpose of seed production and does not alter any other genetic traits.
Here an important concept arises!!
Cytoplasmic inheritance is the important concept here, which means that genetic traits of cell organelles are inherited through the maternal lineage. In humans, during fertilization, sperm only provide the nucleus, while the remaining organelles come from the egg. Similarly, in plants, organelles in the cytoplasm (mitochondria, chloroplasts) follow maternal inheritance. This means that the genetic traits of Bradley Cooper’s variety are contributed by the maternal lineage. The paternal lineage plays a role in transferring pollen to the maternal lineage. Therefore, even if the mother of Bradley Cooper receives pollen from the maintainer line and produces seeds through fertilization, the resulting seeds will also be male-sterile (due to cytoplasmic inheritance following the maternal lineage, which was male-sterile in the case of Bradley Cooper’s mother).
If you look at the picture below, it should be easy to explain.
For the watermelon A that we usually purchase in the market, here are pictures of A’s mother and father. A’s mother looks similar to the watermelon we eat, but the father’s shape is unusual. When these mother and father plants are crossed (with the A father’s pollen delivered to the A mother’s female flower), the traits that result will be similar to those of A’s mother. While the father can transmit certain genetic traits, watermelon A will follow cytoplasmic inheritance, so most of the variety’s characteristics will come from A’s mother.
Therefore, the process for obtaining Bradley Cooper’s seeds is:
A (Bradley Cooper's mother) + B (maintainer lines as pollen donor; only delivers pollen to A for fertilization) = increased A (Bradley Cooper's mother)
You may not understand why Bradley Cooper’s mother increased so rapidly after crossing with the pollen donor, so let’s take watermelon as an example. When a watermelon mother plant (like Bradley Cooper’s mother) is fertilized with pollen from a donor plant for seed production, each watermelon can contain hundreds of seeds, all of which are replicas of Bradley Cooper’s mother.
When these seeds are then crossed with Bradley Cooper’s father,
A (Bradley Cooper's mother / still male sterility) + C (Bradley Cooper's father having pollen function) = A+ (Bradley Coopers)
and when these Bradley Coopers are planted, they will produce the desired variety, Bradley Coopers, which is in high demand in the market. This is a hybrid seed production process that utilizes cytoplasmic male sterility.
In technical terms, the breeder created the first generation of Bradley Coopers by developing A’ (the mother plant), B (maintainer), and C (the father plant), which are referred to as “breeder or foundation seeds“. The resulting offspring from these breeder seeds, including the mother, father, and maintainer, are known as “stock seeds“. These propagated stock seeds are distributed both domestically and internationally, and are crossbred between the mother and father plants of the variety to produce and sell the F1 hybrid seeds.
If it ends like this, it will be a happy ending. But what would be the consequences if the cytoplasmic male sterility trait fails during the process of seed production?
A (Bradley Cooper's mother having pollen function / Selfing) + C (Bradley Cooper's father having pollen function) = ???
If pollen comes out of Bradley Cooper’s mother and selfing occurs in Bradley Cooper’s mother itself without crossing with C (Bradley Cooper’s father), it is not Bradley Cooper who appears, but the clone of Bradley Cooper’s mother. The reason is that if selfing occurs in Bradley Cooper’s mother first, it will become the mother of Bradley Cooper, and if it is crossed with the father first, Bradley Cooper will be produced. No one knows the ratio of this occurrence, only nature knows.
This phenomenon of pollen production in male sterile (MS) individuals is called male fertility (MF). Even in MS varieties, there are cases where pollen is produced. In such cases, all flowers that produce pollen in that variety must be removed to prevent selfing and to ensure that normal crossing and fertilization occur, resulting in the production of seeds with normal variety characteristics.
In the case of these onions, they belong to the MS lineage, but 1-2% of pollen is still produced in the maternal parent. Therefore, flower removal is being carried out in the maternal parent.
