Cytoplasmic male sterility

Cytoplasmic male sterility is total or partial male sterility in plants as the result of specific nuclear and mitochondrial interactions. Male sterility is the failure of plants to produce functional anthers, pollen, or male gametes. Cytoplasmic male sterility is total or partial male sterility in plants as the result of specific nuclear and mitochondrial interactions. Male sterility is the failure of plants to produce functional anthers, pollen, or male gametes. Joseph Gottlieb Kölreuter was the first to document male sterility in plants. He reported anther abortion within species and specific hybrids. Cytoplasmic male sterility (CMS) has now been identified in over 150 plant species. Male sterility is more prevalent than female sterility. This could be because the male sporophyte and gametophyte are less protected from the environment than the ovule and embryo sac. Natural selection on cytoplasmic genes, which are only maternally inherited and are thus not involved in pollen production, could also lead to male sterility. Male-sterile plants can set seed and propagate. Female-sterile plants cannot develop seeds and will not propagate. Male sterility can arise spontaneously via mutations in nuclear genes and/or cytoplasmic genes. Male sterility is easy to detect because a large number of pollen grains are produced. Pollen grains can be assayed through staining techniques (carmine, lactophenol or iodine). Male sterility can be either cytoplasmic or cytoplasmic–genetic. In this case, the trigger for CMS is in the extranuclear genome - (mitochondria or chloroplast). The extranuclear genome is only maternally inherited. Manifestation of male sterility in CMS may be controlled either entirely by cytoplasmic factors or by interactions between cytoplasmic factors 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. Cytoplasmic–genetic male sterility systems are widely exploited in crop plants for hybrid breeding due to the convenience of controlling sterility expression by manipulating the gene–cytoplasm combinations in any selected genotype. 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. Hybrid production requires a plant from which no viable male gametes are introduced. This selective exclusion of viable male gametes can be accomplished via different paths. One path, emasculation is done to prevent a plant from producing pollen so that it can serve only as a female parent. 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. Since a male-sterile line cannot self-pollinate, seed formation is dependent upon pollen from another male line. Cytoplasmic male sterility is also used in hybrid seed production. 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 (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 genes. The male-sterile line is maintained by crossing with a maintainer line carrying the same nuclear genome but with normal fertile cytoplasm.