Production of Hybrid Seed
The two requirements of commercial hybrid seed production are:
Both these factors are largely governed by the floral structure and the natural mode of reproduction of the crop species in question. Example, emasculation in maize, due to its peculiarly favourable floral structure, consists of a very simple operation of detasseling, i.e., removal of the entire tassel (the male inflorescence) from the plant before pollen is shed. Thus one simple operation emasculates the whole plant. The production of hybrid maize until the 1960s was essentially based on detasselling. However, male sterility and self-incompatibility offer the means for genetic emasculation, that is, preventing self-fertilization by manipulating the genotype of the plant, and are the basis of hybrid seed production in many crops. Pollen dispersal is often satisfactory in most cross-pollinated species since it is their natural mode of reproduction. But in self-pollinated species, satisfactory pollen dispersal is often the limiting factor in hybrid seed production. Hybrid seed may be produced in one of the following several ways.
Cytoplasmic – Genetic Male Sterility
This system is the most widely used method in hybrid seed production. It is commercially used in maize, bajra, jowar, onions (A.cepa) and sugarbeets (B.vulgaris). The system is based on a cytoplasm that produces male sterility and on a gene that restore fertility in the presence of the male sterile cytoplasm. The use of this system in hybrid seed production is outlined below.
Production of single Cross Hybrid Varieties
For the production of a single cross, a male sterile line is used as female, and the male parent is a restorer. The seed set on the female parent (the male sterile line) is the hybrid seed, while that produced on the male parent is a selfed seed. The resulting hybrid is male fertile since it has received the restorer gene from the male parent. Generally, two rows of the male fertile inbred (the male parent) are planted after every two rows of the male-sterile parent (the female parent). But when the male inbred produces sufficient pollen, 2 rows of the male inbred may be planted after every 3 or 4 rows of the female parent. The present single cross hybrid varieties of maize are produced by planting male and female inbreds in the ratio of 2:4.
Production of Double Cross Hybrid Varieties
Double cross hybrid varieties are produced by crossing two single crosses, one male sterile and the other male fertile. The male-sterile single cross is produced by crossing a cytoplasmic male sterile line with a non-restorer male fertile line. The male fertile single cross may be produced in one of the two ways. First, a cytoplasmic male sterile line is crossed with a restorer line. The double-cross in this case has both males sterile and male fertile plants in the ratio 1:1. In the second method, two restorer lines are crossed together; one of the restorer lines serves as female and is detasselled manually. All the plants in double-cross would be male fertile.
Genetic Male Sterility
Genetic male sterility is ordinarily governed by a single recessive gene, but dominant genes governing male sterility are also known, e.g., in safflower. Male sterility alleles arise spontaneously or may be artificially induced. A male sterile line may be maintained by crossing it with heterozygous male fertile plants. Such a mating produces 1:1 male sterile and male fertile plants. In India, it is being used for hybrid seed production of arhar (C.cajan) by some private seed companies.
Cytoplasmic Male Sterility
This type is male sterility is determined by the cytoplasm. Since the cytoplasm of a zygote comes primarily from the egg cell, the progeny of such male-sterile plants would always be male sterile. Cytoplasmic male sterility is known in many plant species, some of which is a crop plant. Cytoplasmic male sterility may be transferred easily to give strain by using that strain as a pollinator (recurrent parent) in the successive generations of a backcross programme. After 6-7 backcrosses, the nuclear genotype of the male-sterile line would be almost identical to that of the recurrent pollinator strain. The male-sterile line is maintained by crossing it with the pollinator strain used as the recurrent parent in the back-cross programme since its nuclear genotype is identical with that of the male-sterile line. Such a male fertile line is known as the maintainer line or B line as it is used to maintain the male sterile line. The male-sterile line is also known as the A-line. Cytoplasmic male sterility may be utilized for producing hybrid seed in certain ornamental species, or in species where a vegetative part is of economic value. But in those crop plants where the seed is the economic part, it is of no use because the hybrid progeny would be male sterile.
Cytoplasmic-Genetic Male Sterility
This is a case of cytoplasmic male sterility where a nuclear gene for restoring fertility in the male sterile line is known. The fertility restorer gene, R, is dominant and is found in certain strains of the species, or maybe transferred from a related species, e.g., in wheat. This gene restores male fertility in the male sterile line, hence it is known as the restorer gene. The cases of cytoplasmic male sterility would be included in the cytoplasmic-genetic system as and when restorer genes for them would be discovered. It is likely that a restorer gene would be found for all the cases of cytoplasmic male sterility if a thorough search were made. This system is known in maize, jawar, bajra, sunflower, rice and wheat. The cytoplasmic – genetic male sterility is used commercially to produce hybrid seed in maize, bajra and jowar.