The first documentation of male sterility came in 1763 when Joseph Gottlieb Kölreuter observed anther abortion within species and specific hybrids.
It is more prevalent than female sterility, either because the male sporophyte and gametophyte are less protected from the environment than the ovule and embryo sac, or because it results from natural selection on mitochondrial genes which are maternally inherited and are thus not concerned with pollen production.
Male sterility is easy to detect because a large number of pollen are produced and are easily studied.
Male sterility is assayed through staining techniques (carmine, lactophenol or iodine); while detection of female sterility is detectable by the absence of seeds.
Male sterility has propagation potential in nature since it can still set seed and is important for crop breeding, while female sterility does not.
Male sterility can be aroused spontaneously via mutations in nuclear and/or cytoplasmic genes.
Among the two types of male sterility, genetic and cytoplasmic, cytoplasmic male sterility (CMS) is caused by the extranuclear genome (mitochondria or chloroplast) and show maternal inheritance.
Manifestation of male sterility in these may be either entirely controlled by cytoplamsic factors or by the interaction between cytoplamsic and nuclear factors.
Explanation of Cytoplasmic male sterility –
Cytoplasmic male sterility, as the name indicates, is under extra nuclear genetic control.
They show non-Mendelian inheritance and are under the regulation of cytoplasmic factors. In this type, male sterility inherited maternally.
This is not a very common type of male sterile system in the plant kingdom.
In general there are two types of cytoplasm viz.., N (normal) and the aberrant S (sterile) cytoplasms. These types exhibit reciprocal differences.
Cytoplasmic genetic male sterility –
When nuclear genes for fertility restoration (Rf) are available for CMS system in any crop, it is called as Cytoplasmic Genetic Male Sterility (CGMS).
This type of male sterility system is common in many plant species across plant kingdom.
The sterility is manifested by the influence of both nuclear and cytoplasmic genes.
There are commonly two types of cytoplasms, N (normal) and S (sterile).
There are also restorers of fertility (Rf) genes, which are distinct from genetic male sterility genes.
The Rf genes do not have any expression of their own unless the sterile cytoplasm is present.
Rf genes are required to restore fertility in S cytoplasm which causes sterility.
Thus a combination of N cytoplasm with rfrf and S cytoplasm with Rf- produces fertiles; while S cytoplasm with rfrf produces only male steriles.
Another feature of these systems is that Rf mutations (i.e., mutations to rf or no fertility restoration) are frequent, so N cytoplasm with Rfrf is best for stable fertility.
Because of the convenience to control the sterility expression by manipulating the gene–cytoplasm combinations in any selected genotype, cytoplasmic genetic male sterility systems are widely exploited in crop plants for hybrid breeding.
Incorporation of these systems for male sterility evades the need for emasculation in cross-pollinated species, thus encouraging cross breeding producing only hybrid seeds under natural conditions.
Cytoplasmic male sterility in hybrid breeding –
Male sterile plants produce no functional pollen, but do produce viable eggs.
Cytoplasmic male sterility is used in agriculture to facilitate the production of hybrid seed.
Hybrid seed is produced from a cross between two genetically different lines; such seeds usually result in larger, more vigorous plants.
The main practical problem in producing hybrid seed is to prevent self-pollination, which would produce seeds that are not hybrid.
One breeding scheme is illustrated in Figure 2.19.
Hybrid production requires a female plant in which no viable male gametes are borne.
Emasculation is done to make a plant devoid of pollen so that it is made female. Another simple way to establish a female line for hybrid seed production is to identify or create a line that is unable to produce viable pollen.
This male sterile line is therefore unable to self-pollinate, and seed formation is dependent upon pollen from the male line.
Cytoplasmic male sterility is used in hybrid seed production. In this case, the sterility is transmitted only through the female and all progeny will be sterile.
This is not a problem for crops such as onions or carrots where the commodity harvested from the F1 generation is produced during vegetative growth.
These CMS lines must be maintained by repeated crossing to a sister line (known as the maintainer line) that is genetically identical except that it possesses normal cytoplasm and is therefore male fertile.
In genic cytoplasmic male sterility restoration of fertility is done using restorer lines carrying nuclear restorer genes in crops.
The male sterile line is maintained by crossing with a maintainer line which has the same genome as that of the MS line but carrying normal fertile cytoplasm.