sterility.
There are three kinds of pollen sterility – nuclear, cytoplasmic, and cytoplasmic‐genetic – which define the types of true male sterility as follows:
Genetic Male SterilityGenetic (nuclear, genic) male sterility is widespread in plants. The gene for sterility has been found in species including barley, cotton, soybean, tomato, potato, and lima bean. It is believed that nearly all diploid and polyploidy plant species have at least one male sterility locus. Genetic male sterility may be manifested as pollen abortion (pistillody) or abnormal anther development. Genetic male sterility is often conditioned by a single recessive nuclear gene, ms, the dominant allele, Ms, conditioning normal anther and pollen development. However, male sterility in alfalfa has been reported to be under the control of two independently inherited genes. The expression of the gene may vary with the environment. To be useful for application in plant breeding, the male sterility system should be stable in a wide range of environments and inhibit virtually all seed production. The breeder cannot produce and maintain a pure population of male‐sterile plants. The genetically male‐sterile types (msms) can be propagated by crossing them with a heterozygous pollen source (Msms). This cross will produce a progeny in which 50% of the plants will be male‐sterile (msms) and 50% male‐fertile (Msms). If the crossing block is isolated, breeders will always harvest 50% male‐sterile plants by harvesting only the male‐sterile plants. The use of this system in commercial hybrid production is outlined in Figure 5.9.Markers linked to genetic male sterility have been identified in some crops (e.g. bright green hypocotyls in broccoli and potato leaf shape and green stem in tomato). Molecular markers (including SCAR, STS, RAPD) associated with male sterility have also been found in some plant species. Male sterility may chemically be induced by applying a variety of agents. This is useful where cytoplasmic male sterility (CMS) genes have not been found. However, this chemical technique has not been routinely applied in commercial plant breeding, needing further refinement.
Cytoplasmic Male SterilitySometimes, male sterility is controlled by the cytoplasm (mitochondrial gene) but may be influenced by nuclear genes. A cytoplasm without sterility genes is described as normal (N) cytoplasm, while a cytoplasm that causes male sterility is called a sterile (s) cytoplasm or said to have cytoplasmic male sterility (CMS). CMS is transmitted through the egg only (maternal factor). The condition has been induced in species such as sorghum by transferring nuclear chromosomes into a foreign cytoplasm (in this example, a milo plant was pollinated with kafir pollen and backcrossed to kafir). CMS has been found in species including corn, sorghum, sugar beet, carrot, and flax. This system has real advantages in breeding ornamental species because all the offspring is male sterile, hence allowing them to remain fruitless (Figure 5.10). By not fruiting, the plant remains fresh and in bloom for a longer time.
Cytoplasmic‐Genetic Male SterilityCMS may be modified by the presence of fertility‐restoring genes in the nucleus. CMS is rendered ineffective when the dominant allele for the fertility‐restoring gene (Rf) occurs, making the anthers able to produce normal pollen (Figure 5.11). As previously stated, CMS is transmitted only through the egg, but fertility can be restored by Rf genes in the nucleus. Three kinds of progeny are possible following a cross, depending on the genotype of the pollen source. The resulting progenies assume that the fertility gene will be responsible for fertility restoration.
Figure 5.9 Genetic male sterility as used in practical breeding.
Figure 5.10 Cytoplasmic male sterility as applied in plant breeding. N Normal cytoplasm, s sterile cytoplasm.
Figure 5.11 The three systems of cytoplasmic genetic male sterility. The three factors involved in CMS are the normal cytoplasm (N), the male sterile cytoplasm (S), and the fertility restorer (Rf, rf).
Exploiting male sterility in breeding
Male sterility is used primarily as a tool in plant breeding to eliminate emasculation in hybridization. Hybrid breeding of self‐pollinated species is tedious and time‐consuming. Plant breeders use male‐sterile cultivars as female parents in a cross without emasculation. Male‐sterile lines can be developed by backcrossing.
Using genetic male sterility in plant breeding is problematic because it is not possible to produce a pure population of male‐sterile plants using conventional methods. It is difficult to eliminate the female population, before either harvesting or sorting harvested seed. Consequently, this system of pollination control is not widely used for commercial hybrid seed production. On the contrary, CMS is used routinely in hybrid seed production in corn, sorghum, sunflower, and sugar beet. The application of male sterility in commercial plant hybridization is discussed in Chapter 19.
Dichogamy
Dichogamy is the maturing of pistils and stamens of a flower at different times. When this occurs in a self‐pollinated species, opportunities for self‐pollination are drastically reduced or eliminated altogether, thus making the plant practically cross‐pollinated. There are two forms of dichogamy: protogyny (stigma is receptive before the anther is mature to release the pollen) and protandry (pollen is released from the anther before the female is receptive).
5.5.3 Genetic and breeding implications of autogamy
Self‐pollination is considered the highest degree of inbreeding a plant can achieve. It promotes homozygosity of all gene loci and traits of the sporophyte. Consequently, should there be cross‐pollination the resulting heterozygosity is rapidly eliminated. To be classified as self‐pollinated, cross‐pollination should not exceed 4%. The genotypes of gametes of a single plant are all the same. Further, the progeny of a single plant is homogeneous. A population of self‐pollinated species in effect comprises a mixture of homozygous lines. Self‐pollination restricts the creation of new gene combinations (no introgression of new genes through hybridization). New genes may arise through mutation, but such a change is restricted to individual lines or the progenies of the mutated plant. The proportions of different genotypes, not the presence of newly introduced types, define the variability in a self‐pollinated species. Another genetic consequence of self‐pollination is that mutations (which are usually recessive) are readily exposed through homozygosity, for the breeder or nature to apply the appropriate selection pressure on (see Box 5.1).
