Cytoplasmic male sterility

Cytoplasmic male sterility is total or partial male sterility in plants as the result of specific nuclear and interactions. Male sterility is the failure of plants to produce functional anthers, pollen, or male gametes

The first documentation of male sterility was by Joseph Gottlieb Kölreuter, who 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 by the absence of seeds. Male-sterile plants may be propagated, since they can still set seed, while female-sterile plants cannot. Male sterility can arise 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 extranuclear genome (mitochondria or chloroplast) and shows maternal inheritance. Manifestation of male sterility in CMS may be controlled either entirely by cytoplasmic factors or by interaction between cytoplasmic and nuclear factors.

Cytoplasmic male sterility, as the name indicates, is under extranuclear genetic control (under control of the mitochondrial or plastid genomes). It shows non-Mendelian inheritance, with male sterility inherited maternally. In general there are two types of cytoplasm: N (normal) and aberrant S (sterile) cytoplasms. These types exhibit reciprocal differences.

While CMS is controlled by an extranuclear genome, nuclear genes may have the capability to restore fertility. When nuclear restoration of fertility genes (“Rf”) is available for a CMS system in any crop, it is cytoplasmic–genetic male sterility; the sterility is manifested by the influence of both nuclear (with Mendelian inheritance) and cytoplasmic (maternally inherited) genes. There are also restorers of fertility (Rf) genes that are distinct from genetic male sterility genes. The Rf genes have no expression of their own unless the sterile cytoplasm is present. Rf genes are required to restore fertility in S cytoplasm that causes sterility. Thus plants with N cytoplasm are fertile and S cytoplasm with genotype Rf- leads to 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 that N cytoplasm with Rfrf is best for stable fertility.

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