Male sterility – absence of pollen grain in plants
Male sterility is the 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 because it eliminates the process of mechanical emasculation.
- Prevents self-pollination, permits cross pollination.
- Leads to heterozygosity.
- Female gametes function normally.
- Assayed through staining techniques.
- In nature, occur due to spontaneous mutations.
- Can be induced artificially.
The male sterility is of five types
- Cytoplasmic male sterility (CMS) – governed by cytoplasmic genes.
- Genetic male sterility (GMS) – governed by nuclear genes.
- Cytoplasmic-Genetic male sterility (CGMS) – governed by both nuclear and cytoplasmic genes.
- Transgenic male sterility – induced by the technique of genetic engineering.
- Chemical induced male sterility – induced by the use of chemical.
Cytoplasmic male sterility (CMS)
The pollen sterility which is controlled by cytoplasmic genes is known as cytoplasmic male sterility (CMS). The cytoplasm of zygote comes 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 program.
- After 6-7 backcrosses the nuclear genotype of male sterile line would be almost identical 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.
- 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.
- Examples – Observed in maize, wheat, rice, sorghum, cotton.
Genetic male sterility (GMS)
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.
- 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.
- It is used in both seed propagated crops and vegetatively propagated species.
- Examples: Wheat, maize, barley, sorghum, lucern cotton, sunflower, tomato and cucurbits.
Cytoplasmic Genetic Male Sterility
When pollen sterility is controlled by both 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 and is widely used in breeding program.
- This is the case of cytoplasmic male sterility, where a nuclear gene 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 gene 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 restoring line it restores the fertility in the F1 hybrid. Since B line is used to maintain the fertility and is also referred as maintainer line.
- Cytoplasmic genetic male sterility is widely used for hybrid seed production of both seed propagated species and vegetatively propagated species.
- Examples: It is observed in maize, sorghum, bajara, sunflower, rice and wheat.
Transgenic Male Sterility
When the male sterility is induced via the techniques of genetic engineering is called as transgenic male sterility.
- It is heritable and basically comes under genetic male sterility.
- In this system, the two kinds of genes are involved. One gene causes male sterility (integrated with genome of A line) while the other suppresses it (in R line).
Chemical Induced Male Sterility
The chemical which induces male sterility artificially is called as male gametocide.
- It is rapid method but the sterility is non-heritable. In this system A, B and R lines are not maintained.
- Some of the male gametocides used are gibberellins (rice, maize), Sodium Methyl Arsenate (rice) and Maleic hydrazide (wheat, onion).
- Hybrid seed industry
Increase in crop yield is due to the process of heterosis or hybrid vigor in which F1 hybrid plants generated by crosses between two genetically diverse parents. Cross pollination is essential for the production of hybrids. In normally self-pollinating crop plants (for example, Brassica sp., rice and wheat), one of the parents needs to be male sterile to facilitate cross pollination.
Plant sterility can improve the economics of wood and pulpwood production because phosphorus and nitrogen are not “wasted” in the production of pollen and seed. This is particularly applicable to pine and eucalyptus.
- In some crops, sterile cytoplasm has adverse effect on yield.
- Unsatisfactory restoration of fertility.
- Break down of male sterility because of some reasons like, certain environmental conditions which leads to some pollen production by the male sterile lines and cytoplasm contribution (though small) by the sperm in some cases.
- Difficult to identify line with GMS.
- Unsatisfactory or poor pollination.
- Modifiers or modifying genes may affect cytoplasmic male sterility.