George Acquaah - Principles of Plant Genetics and Breeding

After several backcrossing generations, homozygous BC nS 1resistant plants of these crosses were selected ( Figure B5.3). Since we have facilities for genome‐wide analysis, we genotyped all selected plants with AFLP markers to compare their genetic background with the recurrent parent MM. BC nS 1resistant plants that were genetically most similar to MM were maintained as NILs.

These NILs harboring dominant Ol genes are valuable advanced breeding lines and have been used by seed companies for breeding tomato cultivars with resistance to tomato powdery mildew, which are now available on the market. The NILs for the Ol‐qtls are still in development via MAS.

1 Egashira, H., Ishihara, H., Takshina, T., and Imanishi, S. (2000). Genetic diversity of the ‘peruvianum‐complex’ (Lycopersicon peruvianum (L.) Mill. and L. chilense Dun.) revealed by RAPD analysis. Euphytica. 116: 23–31.

2 Huang, Y., Komoto, J., Konishi, K. et al. (2000). Mechanisms for auto‐inhibition and forced product release in glycine N‐methyltransferase: crystal structures of wild‐type, mutant R175K and S‐adenosylhomocysteine‐bound R175K enzymes. J Mol Biol 298 (1): 149–162.

3 Kiss, L., Cook, R.T.A., Saenz, G.S. et al. (2001). Identification of two powdery mildew fungi, Oidium neolycopersici sp. nov. and O. lycopersici, infecting tomato in different parts of the world. Mycological Research 105 (2001): 684–697.

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Repeated selfing has no genetic consequence in self‐pollinated species (no inbreeding depressionor loss of vigor following selfing). Similarly, self‐incompatibility does not occur. Because a self‐pollinated cultivar is generally one single genotype reproducing itself, breeding self‐pollinated species usually entails identifying one superior genotype (or a few) and multiplying it. Specific breeding methods commonly used for self‐pollinated species are pure line selection, and also pedigree breeding, bulk populations, and backcross breeding.

To facilitate breeding of certain major crops, projects have been undertaken by certain breeders to create breeding stock of male sterile lines that plant breeders can readily obtain. In barley, over 100 spring and winter wheat cultivars have been converted to male sterile lines by USDA researchers. In the case of CMS, transferring chromosomes into foreign cytoplasm is a method of creating CMS lines. This approach has been used to create male sterility in wheat and sorghum. In sorghum, kafir chromosomes were transferred into milo cytoplasm by pollinating milo with kafir, and backcrossing the product to kafir to recover all the kafir chromosomes as previously indicated.

As previously indicated, crossing is a major procedure employed in the transfer of genes from one parent to another in the breeding of sexual species. A critical aspect of crossing is pollination control to ensure that only the desired pollen is involved in the cross. In hybrid seed production, success depends on the presence of an efficient, reliable, practical, and economic pollination control system for large‐scale pollination. Pollination control may be accomplished in three general ways:

1 Mechanical controlThis approach entails manually removing anthers from bisexual flowers to prevent pollination, a technique called emasculation, removing one sexual part (e.g. detasselling in corn), or excluding unwanted pollen by covering the female part. These methods are time consuming, expensive, and tedious, limiting the number of plants that can be crossed. It should be mentioned that in crops such as corn, mechanical detasselling is widely used in the industry to produce hybrid seed.

2 Chemical controlA variety of chemicals called chemical hybridizing agents, or by other names (e.g. male gametocides, male sterilants, pollenocides, androcides) are used to temporally induce male sterility in some species. Examples of such chemicals include Dalapon®, Estrone®, Ethephon®, Hybrex®, and Generis®. The application of these agents induces male sterility in plants, thereby enforcing cross‐pollination. The effectiveness is variable among products.

3 Genetical controlCertain genes are known to impose constraints on sexual biology by incapacitating the sexual organ (as in male sterility) or inhibiting the union of normal gametes (as in self‐incompatibility). These genetic mechanisms will be discussed further.

Allogamyoccurs when fertilization of the flower of a plant is effected by pollen donated by a different plant within the same species. This is synonymous with ( cross‐pollinationor) cross‐fertilizationor out breeding, involving the actual fusion of gametes (sperm and ovum). An incomplete list of allogamous species is presented in Table 5.3.

Table 5.3Examples of predominantly cross‐pollinated species.

Though predominantly pollinated, some of these species may have another reproductive mechanism in breeding and crop cultural systems. For example, banana is vegetatively propagated (and not grown from seed) and so are cassava and sweet potato; cabbage and maize are produced as hybrids.

Allogamous species depend on agents of pollination, especially wind and insects, and hence tend to produce large amounts of pollen, and have large, bright‐colored fragrant flowers to attract insects. They commonly have taller stamens than carpels or use other mechanisms to better ensure the dispersal of pollen to other plant flowers. Other provisions that promote cross‐fertilization are mechanisms that control the timing of the receptiveness of the stigma and shedding of pollen and thereby prevent autogamy within the same flower. In protandry, the anthers release their pollen before the stigma of the same flower is receptive (protandrous flower). In protogyny, the stigma is receptive before the pollen is shed from the anthers of the same flower (protogynous flower). Several mechanisms occur in nature by which cross‐pollination is ensured, the most effective being dioecy, monoecy, dichogamy, and self‐incompatibility. Some mechanisms are stringent in enforcing cross‐pollination (e.g. dioecy), while others are less so (e.g. monoecy). These mechanisms are exploited by plant breeders during controlled pollination phase of their breeding programs, so that only desired pollen sources participate in siring the next plant generation.