Genetic

Contamination from outcrossing may produce heterozygotes in the population. Unfortunately, where the dominance effect is involved in the expression of the trait, heterozygotes are indistinguishable from homozygous dominant individuals. Including heterozygotes in a naturally selfing population will provide material for future segregations to produce new off-types. Mass selection is most effective if the

expression of the trait of interest is conditioned by additive gene action.

Mass selection may be conducted in self-pollinated populations as well as cross-pollinated populations, but with different genetic consequences. In self pollinated populations, the persistence of inbreeding will alter population gene frequencies by reducing heterozygosity from one generation to the next. However, in cross-pollinated populations, gene frequencies are expected to remain unchanged unless the selection of plants was biased enough to change the frequency of alleles that control the trait of interest.

Mass selection is based on plant phenotype. Consequently, it is most effective if the trait of interest has high heritability. Also, cultivars developed by mass selection tend to be phenotypically uniform for qualitative traits that are readily selectable in a breeding program. This uniformity notwithstanding, the cultivar could retain significant variability for quantitative traits. It is helpful if the selection environment is uniform. This will ensure that genetically superior plants are distinguishable from mediocre plants. When selecting for disease resistance, the method is more effective if the pathogen is uniformly present throughout the field without “hot spots”. Some studies have shown correlated response to selection in secondary traits as a result of mass selection. Such a response may be attributed to linkage or pleiotropy.

Advantages and disadvantages

There are both major advantages and disadvantages of mass selection for improving self-pollinated species.

Advantages

_ It is rapid, simple, and straightforward. Large populations can be handled and one generation per cycle can be used.

_ It is inexpensive to conduct.

_ The cultivar is phenotypically fairly uniform even though it is a mixture of pure lines.

Disadvantages

_ To be most effective, the traits of interest should have high heritability.

_ Because selection is based on phenotypic values, optimal selection is achieved if it is conducted in a uniform environment.

_ Phenotypic uniformity is less than in cultivars produced by pure line selection.

_ With dominance, heterozygotes are indistinguishable from homozygous dominant genotypes. Without progeny testing, the selected heterozygotes will

segregate in the next generation.

Modification

Mass selection may be direct or indirect. Indirect selection will have high success if two traits result from pleiotropy or if the selected trait is a component of the trait targeted for improvement. For example, researchers improved the seed protein or oil by selecting on the basis of density separation of the seed.

Pure-line selection

The theory of the pure line was developed in 1903 by the Danish botanist Johannsen. Studying seed weight of beans, he demonstrated that a mixed population of self-pollinated species could be sorted out into genetically pure lines. However, these lines were subsequently non-responsive to selection within each of

them. Selection is a passive process, since it eliminates variation but does not create it.The pure-line theory may be summarized as follows:

_ Lines that are genetically different may be successfully isolated from within a population of mixed genetic types.

_ Any variation that occurs within a pure line is not heritable but due to environmental factors only. Consequently, as Johansen’s bean study showed, further selection within the line is not effective.

Lines are important to many breeding efforts. They are used as cultivars or as parents in hybrid production. Also, lines are used in the development of genetic stock

and synthetic and multiline cultivars.