Cytoplasmic male sterility is the total or partial male sterility associated with plant biology as the result of specific nuclear and mitochondrial interactions. Male sterility is the failure of plants to produce functional anthers, pollen, or male gametes.
The first documentation of male sterility came in Joseph Gottlieb Kölreuter observed anther abortion within species and specific hybrids.
Cytoplasmic male sterility has now been identified in over 150 plant species. 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 grains 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.
Male sterility can be either cytoplasmic or cytoplasmic-genetic. Cytoplasmic male sterility (CMS) is caused by the extra nuclear genome (mitochondria or chloroplast) and shows maternal inheritance.
Manifestation of male sterility in CMS may be either entirely controlled by cytoplasmic factors or by the interaction between cytoplasmic and nuclear factors.
Cytoplasmic male sterility, as the name indicates, is under extra-nuclear genetic control (under the control of the mitochondrial or plastid genomes). They show non- Mendelian inheritance and are under the regulation of cytoplasmic factors.
In this type, male sterility is inherited maternally. In general there are two types of cytoplasm: N (normal) and the aberrant S (sterile) cytoplasms. These types exhibit reciprocal differences.
Cytoplasmic-Genetic Male Sterility
While CMS is controlled by an extra nuclear genome often times nuclear genes can have the capability to restore fertility.
When nuclear restorations of fertility genes (“Rf”) are available for CMS system in any crop, it is cytoplasmic-genetic male sterility; the sterility is manifested by the influence of both nuclear (Mendelian inheritance) and cytoplasmic (maternally inherited) genes.
There are also restorers of fertility (Rf) genes, which are distinct from genetic male sterility genes. The Rfgenes 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 N cytoplasm is always fertile and S cytoplasm with genotype 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 Rfrfis best for stable fertility.
Cytoplasmic-genetic male sterility systems are widely exploited in crop plants for hybrid breeding due to the convenience to control the sterility expression by manipulating the gene–cytoplasm combinations in any selected genotype.
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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
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 cytoplasmic-genetic 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.
Cytoplasmic Male Sterility in Hybrid Maize Breeding
Cytoplasmic male sterility is an important part of hybrid maize production. The first commercial cytoplasmic male sterile, discovered in Texas, is known as CMS-T. The use of CMS-T, starting in the 1950s, eliminated the need for DE tasseling.
In the early 1970’s plants containing CMS-T genetics were susceptible to southern corn leaf blight and suffered from widespread loss of yield. Since then CMS types C and S are used instead.
Unfortunately these types are prone to environmentally induced fertility restoration and must be carefully monitored in the field. Environmentally induced restoration is when certain environmental stimuli signal the plant to bypass sterility restrictions and produce pollen anyway.
Environmentally induced restoration differs from genetic restoration in that it is rooted in external signals rather than genetic DNA.
The systematic sequencing of new plant species in recent years has uncovered the existence of several novel RF genes and their encoded proteins.
A unified nomenclature for the RF extended protein families across all plant species, fundamental in the context of comparative functional genomics.
This unified nomenclature accommodates functional RF genes and pseudogenes, and offers the flexibility needed to incorporate additional RFs as they become available in future.
In summary, male sterility is the failure of plants to produce functional anthers, pollen, or male gametes. Cytoplasmic male sterility is the total or partial male sterility as the result of specific nuclear and mitochondrial interactions.
It is prevalent in males because the male sporophyte and gametophyte are less protected from the environment than the ovule and embryo sac, and the natural selection on mitochondrial genes which are maternally inherited and are thus not concerned with pollen production. Cytoplasmic male sterility is used in hybrid seed production.
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