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.
5.7 Artificial pollination control techniques
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.
5.8 What is allogamy?
Allogamy occurs 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‐pollination or) cross‐fertilization or out breeding, involving the actual fusion of gametes (sperm and ovum). An incomplete list of allogamous species is presented in Table 5.3.
Table 5.3 Examples of predominantly cross‐pollinated species.
| Common name | Scientific name |
| Alfalfa | Medicago sativa |
| Annual ryegrass | Lolium multiflorum |
| Banana | Musa spp. |
| Birdsfoot trefoil | Lotus corniculatus |
| Cabbage | Brassica oleracea |
| Carrot | Daucus carota |
| Cassava | Manihot esculentum |
| Cucumber | Cucumis sativa |
| Fescue | Festuca spp. |
| Kentucky bluegrass | Poa pratense |
| Maize | Zea mays |
| Muskmelon | Cucumis melo |
| Onion | Allium spp. |
| Potato | Solanum tuberosum |
| Radish | Raphanus sativus |
| Rye | Secale cereale |
| Sugarbeet | Beta vulgaris |
| Sunflower | Helianthus annuus |
| Sweet potato | Ipomoea batatus |
| Watermelon | Citrullus lanatas |
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.
5.8.1 Mechanisms that favor allogamy
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.
Monoecy
Some flowers are complete (possess all the four basic parts) while others are incomplete (are missing one or more of the four basic floral parts). Furthermore, in some species, the sexes are separate. When separate male and female flowers occur on the same plant, the condition is called monoecy. Sometimes, the male and female flowers occur in different kinds of inflorescence (different locations as in corn). Other examples of monoecious plants include most figs, birch, and pine trees. It is easier and more convenient to self‐pollinate plants when the sexes occur in the same inflorescence. In terms of seed production, monoecy and dioecy may appear to be inefficient because not all flowers produce seed. Some flowers produce only pollen.
