(Male Sterility)A male sterile plant cannot pollinate other plants or itself. However, the female reproduction structures of the flower are still fertile. A male sterile plant can therefore set seed if they are pollinated by a different (male fertile) plant. This genetic anomaly is used in hybrid seed production since it guarantees cross-pollination on male sterile plants.
There are 3 male sterile situations(Male Sterility):
- Genetic Male Sterility
- Cytoplasmic Male Sterility
- Genetic Cytoplasmic Male Sterility.
- Transgenic Male Sterility
- Chemical Induced Male Sterility
Genetic Male Sterility
It is governed by nuclear genes, in most of the cases by single gene.
The gene causing male sterility are ordinarily recessive (ms) and rarely dominant (e.g. safflower).
In the system, there are A and B line.
A line (mm) is genetic male sterile line. B line is heterozygous male fertile line (Mm). A line is maintained by crossing it with B line, the cross produces male sterile and male fertile lines in 1:1 ratio.
Application of Genetic Male Sterility in Plant Breeding
It is applicable in production of hybrids in both, vegetatively propagated crops and crops important for fruit or seed yield.
Examples – Wheat, maize, barley, sorghum, lucern cotton, sunflower, tomato and cucurbits
Cytoplasmic Male Sterility
Cytoplasmic male sterility is governed by cytoplasmic or plasma genes.
Progeny of male sterile plant is always male sterile, as its cytoplasm is derived entirely from female gamete
In the system, there are A and B line.
The male sterile line is also known as A line. The line used to maintain male sterile line is male fertile, B line. The A line is maintained by crossing it with B line (pollinator strain used as recurrent parent in the backcross program), as its nuclear genotype is identical with that of the A line. The restorer line only can provide fertility in F1.
It is stable i.e. not influenced by environmental factors.
There are certain drawbacks for the method like it can not be used in asexually propagated crops, to produce hybrids in crops where improvement in seed / fruit is considered.
Application of Cytoplasmic Male Sterility in Plant Breeding
It is applicable in production of hybrids in ornamental crops and vegetatively propagated crops, overall, where grain or fruit is not the economic product.
Examples – Observed in maize, wheat, rice, sorghum, cotton
Cytoplasmic-Genetic Male Sterility
It is governed by both nuclear and cytoplasmic genes. Here, nuclear genes for fertility restoration (Rf) are available.
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.
There are commonly two types of cytoplasms, normal (N) and sterile (S). There are restorers of fertility (Rf) genes, which are distinct from genetic male sterility genes. The Rf genes do not have their own expression of any kind unless the sterile cytoplasm is present. Rf genes are required to restore fertility in sterile cytoplasm which contains genes causing sterility.It is used in commercial production of hybrid seeds in maize, sorghum and bajra.
Examples – It is observed in maize, sorghum, bajara, sunflower, rice and wheat. The one with cytoplasmic male sterility would be included in the cytoplasmic genetic system as and when restorer genes for it discovered.
Transgenic Male Sterility
When the male sterility is induced by the techniques of genetic engineering, it 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).