Principles of Plant Genetics and Breeding. George Acquaah

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      Repeated selfing has no genetic consequence in self‐pollinated species (no inbreeding depression or 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.

5.6 Genotype conversion programs

      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.

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
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