Principles of Plant Genetics and Breeding. George Acquaah

may be temporarily overcome by techniques or strategies such as the removal of the stigma surface (Figure 5.7) (or application of electric shock), early pollination (before inhibitory proteins form), or lowering the temperature (to slow down the development of the inhibitory substance). Self‐incompatibility promotes heterozygosity. Consequently, selfing self‐incompatible plants can create significant variability from which a breeder can select superior recombinants. Self‐incompatibility may be used in plant breeding (for F₁ hybrids, synthetics, triploids), but first homozygous lines must be developed.

Figure 5.7 Cut‐style and embryo rescue in lily. Overcoming reproductive barriers. (a) Pollination barriers; (b) post‐fertilization reproductive barriers. Courtesy of JM van Tuyl/P Ariens Wageningen UR ‐ Plant Breeding.

Self‐incompatibility systems for hybrid seed production have been established for certain crops (e.g. cabbage, kale) that exhibit sporophytic incompatibility (Figure 5.8). Inbred lines (compatible inbreds) are used as parents. These systems generally are used to manage pollinations for commercial production of hybrid seed. Gametophytic incompatibility occurs in vegetatively propagated species. The clones to be hybridized are planted in adjacent rows.

Figure 5.8 Application of self‐incompatibility in practical plant breeding. Sporophytic incompatibility is widely used in breeding of cabbage and other Brassica species. The single cross hybrids are more uniforms and easier to produce. The top cross is commonly used. A single self‐incompatible parent is used as female, and is open‐pollinated by a desirable cultivar as pollen source.

       Male sterility

Male sterility is a condition in plants whereby the anthers or pollen are nonfunctional. The condition may manifest most commonly as absence of/or extreme scarcity of pollen, severe malformation or absence of flowers or stamens, or failure of pollen to dehisce. Just like self‐incompatibility, male sterility enforces cross‐pollination. Similarly, it can be exploited as a tool to eliminate the need for emasculation for producing hybrid seed. There are three basic kinds of male sterility based on the origin of the abnormality:

      1 True male sterility – This is due to unisexual flowers that lack male sex organs (dioecy and monoecy), or bisexual flowers with abnormal or nonfunctional microspores (leading to pollen abortion).

      2 Functional male sterility – The anthers fail to release their contents even though the pollen is fertile.

      3 Induced male sterility – Plant breeders may use chemicals to induce 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.