Advantages and disadvantages

A pure line suggests that a cultivar has identical alleles at all loci. Even though plant breeders may make this assumption, it is one that is not practical to achieve in a breeding program. What plant breeders call pure-line cultivars are best aptly called

“near” pure-line cultivars because, as researchers such as K.J. Frey observed, high mutation rates occur in such genotypes. Line cultivars have a very narrow genetic base and tend to be uniform in traits of interest.In case of proprietary dispute, lines are easy to unequivocally identify.

Application

Pure-line breeding is desirable for developing cultivars for certain uses:

_ Cultivars for mechanized production that must meet a certain specification for uniform operation by farm machines.

_ Cultivars developed for a discriminating market that puts a premium on eye-appeal.

_ Cultivars for the processing market.

_ Advancing “sports” that appear in a population.

_ Improving newly domesticated crops that have some variability.

_ The pure-line selection method is also an integral part of other breeding method,s such as the pedigree selection and bulk population selection.

Procedure

Overview

The pure-line selection in breeding entails repeated cycles of selfing following the initial selection from a mixture of homozygous lines. Natural populations of self-pollinated species consist of mixtures of homozygous lines with transient heterozygosity originating from mutations and outcrossing.

Steps

_ Year 1. The first step is to obtain a variable base population  and space plant it in the first year, select, and harvest desirable individuals

_ Year 2. Grow progeny rows of selected plants.Rogue out any variants. Harvest selected progenies individually. These are experimental strains.

_ Year 3–6. Conduct preliminary yield trials of the experimental strains including appropriate check cultivars.

_ Year 7–10. Conduct advanced yield trials at multilocations. Release highest yielding line as new cultivar.

Genetic issues

Pure-line breeding produces cultivars with a narrow genetic base and, hence, that are less likely to produce stable yields over a wider range of environments. Such cultivars are more prone to being wiped out by pathogenic outbreaks. Because outcrossing occurs to some extent within most self-pollinated cultivars, coupled with the possibility of spontaneous mutation variants may arise in commercial cultivars over time. It is tempting to select from established cultivars to develop new lines, an action that some view as unacceptable and unprofessional practice. As previously

discussed, pure-line cultivars depend primarily on phenotypic plasticity for production response and stability across environments.

Advantages and disadvantages

There are both major advantages and disadvantages of the application of the pure-line method for improving self-pollinated species.

Advantages

_ It is a rapid breeding method.

_ The method is inexpensive to conduct. The base population can be a landrace. The population size selected is variable and can be small or large, depending on the objective.

_ The cultivar developed by this method has great “eye appeal”.

_ It is applicable to improving traits of low heritability, because selection is based on progeny performance.

_ Mass selection may include some inferior pure lines. In pure line selection, only the best pure line is selected for maximum genetic advance.

Disadvantages

_ The purity of the cultivar may be altered through admixture, natural crossing with other cultivars, and mutations. Such off-type plants should be rogued out to maintain cultivar purity.

_ The cultivar has a narrow genetic base and, hence, is susceptible to devastation from adverse environmental factors because of uniform response.

_ A new genotype is not created. Rather, improvement is limited to the isolation of the most desirable or best genotype from a mixed population.

_ The method promotes genetic erosion because most superior pure lines are identified and multiplied to the exclusion of other genetic variants.

_ Progeny rows takes up more resources.