Male Sterility

Male Sterility

Male sterility is defined as an absence or non-function of pollen grain in plant or incapability of plants to produce or release functional pollen grains. The use of male sterility in hybrid seed production has a great importance as it eliminate the process of mechanical emasculation.

Types of Male Sterility:

The male sterility is of five types 1) Genetic male sterility, 2) Cytoplasmic male sterility, 3) Cytoplasmic genetic male sterility, 4) Chemical induced male sterility and 5) Transgenic male sterility.

1) Genetic Male Sterility:

The pollen sterility, which is caused by nuclear genes, is termed as genic or genetic male sterility. It is usually governed by a single recessive gene ms or ‘s’ with monogenic inheritance, but dominant gene governing male sterility are also known E.g Safflower. The male sterility alleles may rise spontaneously or it can be induced artificially and is found in several crops viz. Pigeon pea, castor, tomato, limabean, barley, cotton, etc. A male sterile line may be maintained by crossing it with heterozygous male fertile plant, such a mating produces 1:1 male sterile and male fertile plants.

Utilization in Plant Breeding:

Genetic male sterility is usually recessive and monogenic hence can be used in hybrid seed production. It is used in both seed propagated crops and vegetatively propagated species. In this progeny from crosses ( msms X Msms) are used as a female and are inter planted with homozygous male fertile ( MsMs) pollinator. The genotypes of msms and Msms lines are identical except for the ‘ms’ locus i.e. they are isogenic and are known as male sterile A) Maintainer B) Line respectively. The female line would      
Therefore contain both male sterile and male fertile and male fertile plants, the later must be identified and removed before pollen shedding. This is done by identifying the male fertile plants in seeding stage either due to the pleiotrophic effect of ms gene or due to phenotypic effect of closely lined genes.

In this rouguing of male fertile plant from the female is costly operation and due to this cost of hybrid seed is higher. Therefore, GMS has been exploited commercially only in few crops by few countries. E.g. In USA used in castor while in India used for hybrid seed production of Arhar (cajanus cajan).

2) Cytoplasmic Male Sterility:

The pollen sterility which is controlled by cytoplasmic genes is known as cytoplasmic male sterility (CMS). Usually the cytoplasm of zygote comes primarily from the eggs cell and due to this progeny of such male sterile plants would always be male sterile.

CMS may be transferred easily to a given strain by using that strain as a pollinator (recurrent parent) in the successive generation of backcross programme. After 6-7 backcrosses the nuclear genotype of male sterile line would be almost identicle to that of the recurrent pollinator strain. The male sterile line is maintained by crossing it with pollinator strain used as a recurrent parent in backcross, since the nuclear genotype of the pollinator is identicle with that of the new male sterile line. Such a male fertile line is known as maintainer line or ‘B’ line and ‘male sterile line is also known as ‘A ‘ line. Cytoplasmic male sterile is not influenced by environmental factor and it resides in maize in mitochondria.

Utilization in Plant Breeding:

CMS has limited application. It cannot be used for development of hybrid, where seed is the economic product. But it can be used for producing hybrid seed in certain ornamental species or asexually propagated species like sugarcane, potato, and forage crops.

3) Cytoplasmic Genetic Male Sterility:

When pollen sterility is controlled by both cytoplasmic and nuclear genes is known as cytoplasmic and nuclear genes is known as cytoplasmic genetic male sterility. Jones and Davis first discovered this type of male sterility in 1944 in onion.

This is the case of cytoplasmic male sterility, where a nuclear genes restoring fertility in the male sterile line is known. The fertility restore gene ‘R’ is dominant and found in certain strains of the species. This genes restores male fertility in the male sterile line, hence is known as restores gene.

This system includes A, B, and R lines. A line is a male sterile line, B is similar to ‘A’ in all features but it is a male fertile and R is restore line it restore the fertility in the F1 hybrid. since B line is used to maintain the fertility and is also referred as maintainer line. The plants would be male sterile line in the presence of male sterile cytoplasm if the nuclear genotype is rr, but would be male fertile if the nucleus is Rr or RR. New male sterile lines may be developed following the same procedure as in the case of cytoplasmic system, but the nuclear genotype of the pollinator strain used in transfer must be the fertility would be restored. Development of new restorer strain is somewhat indirect. First a restorer strain (R) is crossed with male sterile line. The resulting male fertile plants are used as the female parent in repeated backcrosses with the strain (C) used as the recurrent parent to which transfer of restorer gene is desired. In each generation, male sterile plants are discarded and the male fertile plants are used as female for back crosses. This acts as selection device for the restores gene R during the backcross programme. At the end of back cross programme a restorer line isogenic to the strain ‘C’ would be recovered.

Utilization in Plant Breeding:

Cytoplasmic genetic male sterility is widely used for hybrid seed production of both seed propagated species and vegetatively propagated species. It is used commercially to produce hybrid seed in maize, Bajara, cotton, rice, sunflower, jowar, etc.