Purpose and expected outcomes

As previously discussed, self-pollinated species have a genetic structure that has implication in the choice of methods for their improvement. They are naturally inbred and hence inbreeding to fix genes is one of the goals of a breeding program for self-pollinated species in which variability is generated by crossing. However, crossing does not precede some breeding methods for self-pollinated species. The purpose of this chapter is to discuss specific methods of selection for improving self-pollinated species. After studying this chapter, the student should be able to discuss the characteristics, application, genetics, advantages, and disadvantages of the following methods of selection:

1 Mass selection.

2 Pure line selection.

3 Pedigree selection.

4 Bulk population.

5 Single seed descent.

The student should also be able to discuss:

6 The technique/method of backcrossing.

7 The method of multiline breeding.

8 The method of breeding composites.

9 The method of recurrent selection.

Types of cultivars

There are six basic types of cultivars that plant breeders develop. These cultivars derive from four basic populations used in plant breeding – inbred pure lines, open-pollinated populations, hybrids, and clones. Plant breeders use a variety of methods and techniques to develop these cultivars.

Pure-line cultivars

Pure-line cultivars are developed for species that are highly self-pollinated. These cultivars are homogeneous and homozygous in genetic structure, a condition attained through a series of self-pollination. These cultivars are often used as parents in the production of other kinds of cultivars. Pure-line cultivars

have a narrow genetic base. They are desired in regions where uniformity of a product has a high premium. It should be pointed out, though, that genetic uniformity occurs in other types of cultivars besides pure lines, for example hybrids and vegetatively propagated cultivars.

Open-pollinated cultivars

Contrary to pure-lines, open-pollinated cultivars are developed for species that are naturally cross-pollinated. The cultivars are genetically heterogeneous and heterozygous. Two basic types of open-pollinated cultivars are developed. One type is developed by improving the general population by recurrent  selection or bulking and increasing material from selected superior inbred lines. The other type, called a synthetic cultivar, is derived from planned matings involving selected genotypes. Open pollinated cultivars have a broad genetic base. Another important type of cultivar developed for openpollinated species is the hybrid cultivar.

Hybrid cultivars

Hybrid cultivars are produced by crossing inbred lines that have been evaluated for their ability to produce hybrids with superior vigor over and above those of the parents used in the cross. Hybrid production exploits the phenomenon of hybrid vigor to produce superior yields. Heterosis is usually less in crosses involving self-pollinated species than those involving cross-pollinated species. Hybrid cultivars are homogeneous but highly heterozygous. Pollination is highly controlled and restricted in hybrid breeding to only the designated pollen source. In the past, physical human intervention was required to enforce this strict pollination requirement, making hybrid seed expensive. However, with time, various techniques have been developed to capitalize on natural reproductive control systems to facilitate hybrid production. Hybrid production is more widespread in cross-pollinated species, because the natural reproductive mechanisms are more readily economically exploitable than in self-pollinated species.