Cytoplasmic Male Sterility

Cytoplasmic Male Sterility

What is male sterility?

Male sterility is the failure of plants to produce functional pollen or male gametes.
Male sterility is more prevalent than female sterility. The reason is that the male sporophyte and gametophyte are less protected from the environment than the ovule and embryo sac.
Male sterility can arise spontaneously via mutations in nuclear genes and/or cytoplasmic or cytoplasmic–genetic. The extranuclear genome is only maternally inherited.
Male-sterile plants can set seed and propagate. Female-sterile plants cannot develop seeds and will not propagate.

Cytoplasmic male sterility

CMS refers to the condition under which a plant is unable to produce functional pollen, and is widespread among higher plants.
Cytoplasmic male sterility is under extranuclear genetic control (mitochondrial or plastid genomes). It shows non-Mendelian inheritance, with male sterility inherited maternally. Generally, there are two types of cytoplasm, i.e. Normal and Sterile cytoplasms. These types exhibit reciprocal differences.
Plasmogenes– Cytoplasmic male sterility (CMS) is governed by plasmogenes located in the mt-DNA, which causes pollen abortion in higher plants.
CMS can arise spontaneously in breeding lines, as a result of wide crosses or the interspecific exchange of nuclear and cytoplasmic genomes, or following mutagenesis.
For example, CMS-WA (wild abortive) rice was developed in indicarice cultivars from a male-sterile plant of the wild rice Oryza rufipogon The well-known male-sterile Texas cytoplasm in maize arose spontaneously in a breeding line, and CMS-PET1 cytoplasm of sunflower arose from an interspecific cross between Helianthus petiolaris and H. annuus.
The CMS is useful in hybrid seed production of several important crop species.
Cytoplasmic male sterility (CMS) has now been identified in over 150 plant species.

Cytoplasmic-genetic male sterility & Fertility Restoration

As CMS is controlled by an extranuclear genome, nuclear genes may have the capability to restore fertility. When nuclear restoration of fertility genes is available for a CMS system in any crop, it is cytoplasmic–genetic male sterility. So, the sterility is manifested by the influence of both nuclear and cytoplasmic genes.
In cytoplasmic–genetic male sterility restoration of fertility is done using restorer lines carrying nuclear genes. The male-sterile line is maintained by crossing with a maintainer line carrying the same nuclear genome but with normal fertile cytoplasm.
Rf genes– In many cases, it has been found that male fertility can be restored by nuclear-encoded fertility restorer (Rf) gene. On the one hand, sterility results from mitochondrial genes causing cytoplasmic dysfunction, and on the other, fertility restoration relies on nuclear genes that suppress cytoplasmic dysfunction. Therefore CMS/Rf systems also matter in the study of interactions between nuclear and mitochondrial genomes.
The Rf genes have no expression of their own unless the sterile cytoplasm is present. Rf genes are required to restore fertility in sterile cytoplasm that causes sterility.

Importance of CMS

Hybrid seed production- CMS is useful in hybrid seed production of several important crop species like maize, sorghum, pearl millet, rice, wheat, pigeonpea and a number of vegetable crops where it eliminates the process of hand emasculation. Hybrids often exhibit heterosis or hybrid vigour.
In this case, male sterility is maternally transmitted and all progeny will be male sterile. These CMS lines must be maintained by repeated crossing to a sister line (called maintainer line) that is genetically identical.
The simple way to establish a female line for hybrid seed production is to identify a line that is unable to produce viable pollen. Since a male-sterile line cannot self-pollinate, seed formation is dependent upon pollen from another male line.
CMS is an important part of hybrid maize The first commercial cytoplasmic male sterile, discovered in Texas, is known as CMS-T. The use of CMS-T eliminated the need for detasseling.