Cytoplasmic male sterility
Encyclopedia
Cytoplasmic male sterility is 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.
, 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 arised 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–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.
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. The male-sterile line is maintained by crossing with a maintainer line carrying the same nuclear genome as the MS line but with normal fertile cytoplasm.
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 detasseling
. In the early 1970s 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, in contrast to genetic, restoration occurs when certain environmental stimuli signal the plant to bypass sterility restrictions and produce pollen anyway..
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 defines protein families across all plant species and facilitates comparative functional genomics. This nomenclature accommodates functional RF genes and pseudogenes, and offers the flexibility needed to incorporate additional RFs as they become available in future. [4
Botany
Botany, plant science, or plant biology is a branch of biology that involves the scientific study of plant life. Traditionally, botany also included the study of fungi, algae and viruses...
as the result of specific nuclear
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
and mitochondrial
Mitochondrion
In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...
interactions. Male sterility is the failure of plants to produce functional anthers, pollen
Pollen
Pollen is a fine to coarse powder containing the microgametophytes of seed plants, which produce the male gametes . Pollen grains have a hard coat that protects the sperm cells during the process of their movement from the stamens to the pistil of flowering plants or from the male cone to the...
, or male gametes.
Background
The first documentation of male sterility was by Joseph Gottlieb KölreuterJoseph Gottlieb Kölreuter
Joseph Gottlieb Kölreuter , also spelled Koelreuter or Kohlreuter, was a German botanist.Kölreuter was born the son of a pharmacist in Karlsruhe, Germany, and grew up in Sulz. He studied medicine at the University of Tübingen under physician and botanist Johann Georg Gmelin, receiving his PhD in...
, 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
Sporophyte
All land plants, and some algae, have life cycles in which a haploid gametophyte generation alternates with a diploid sporophyte, the generation of a plant or algae that has a double set of chromosomes. A multicellular sporophyte generation or phase is present in the life cycle of all land plants...
and gametophyte
Gametophyte
A gametophyte is the haploid, multicellular phase of plants and algae that undergo alternation of generations, with each of its cells containing only a single set of chromosomes....
are less protected from the environment than the ovule
Ovule
Ovule means "small egg". In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: The integument forming its outer layer, the nucellus , and the megaspore-derived female gametophyte in its center...
and embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
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
Carmine
Carmine , also called Crimson Lake, Cochineal, Natural Red #4, C.I. 75470, or E120, is a pigment of a bright-red color obtained from the aluminum salt of carminic acid, which is produced by some scale insects, such as the cochineal beetle and the Polish cochineal, and is used as a general term for...
, lactophenol or iodine
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
), 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 arised 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
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.Cytoplasmic-genetic male sterility
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
Genotype
The genotype is the genetic makeup of a cell, an organism, or an individual usually with reference to a specific character under consideration...
. 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 prevent a plant from producing pollen so that it serves 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 this male-sterile line cannot self-pollinate, 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. The male-sterile line is maintained by crossing with a maintainer line carrying the same nuclear genome as the MS line but with normal fertile cytoplasm.
Cytoplasmic male sterility in hybrid maize breeding
Cytoplasmic male sterility is an important part of hybrid maizeMaize
Maize known in many English-speaking countries as corn or mielie/mealie, is a grain domesticated by indigenous peoples in Mesoamerica in prehistoric times. The leafy stalk produces ears which contain seeds called kernels. Though technically a grain, maize kernels are used in cooking as a vegetable...
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 detasseling
Detasseling
Detasseling corn is removing the pollen-producing flowers, the tassel, from the tops of corn plants and placing them on the ground. It is a form of pollination control, employed to cross-breed, or hybridize, two varieties of corn....
. In the early 1970s 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, in contrast to genetic, restoration occurs when certain environmental stimuli signal the plant to bypass sterility restrictions and produce pollen anyway..
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 defines protein families across all plant species and facilitates comparative functional genomics. This nomenclature accommodates functional RF genes and pseudogenes, and offers the flexibility needed to incorporate additional RFs as they become available in future. [4
External links
- Biological approaches to preventing gene flow - Co-extra research project on coexistence and traceability of GM and non-GM supply chains