Production
Production
of Hybrid Seed
The
two requirements of commercial hybrid seed production are:
- Easy emasculation
of the female parent, and
- Effective pollen
dispersal from the male parent to ensure a satisfactory seed set in
the female parent.
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 detaselling, 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 till 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.
- Cytoplasmic male
sterility.
- Genetic male
sterility.
- Self-incompatibility.
- Manual emasculation
and/or pollination.
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 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 male 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 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 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 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 may be
transferred from a related species, e.g., in wheat. This gene restores
male fertility in the male sterile line, hence it is known as 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, jowar, bajra, sunflower, rice and wheat. The
cytoplasmic – genetic male sterility is used commercially to produce
hybrid seed in maize, bajra and jowar. |
Ag.
Technologies
(Hi-Tech Agriculture)
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