Breeding and Seed Production of Ornamental Crops (2+1)

Introduction of Selection

Introduction to Selection

Selection is the oldest method of crop improvement and was developed as an art in the olden days. Selection consists of permitting the reproduction of some desirable genotypes from a given population and using it further for commercial cultivation.
Types of selection
1) Natural Selection
2) Artificial Selection
1) Natural Selection:

• Selection is a continuous process and in nature it is governed by natural environmental conditions e.g. temperature, soil, weather, prevalence of pest and disease etc. In this process, the genotypes showing adaptability to a given environment leaves behind more progeny. It can be used for further multiplication and termed as natural selection.
2) Artificial selection:
• The selection, which is carried out by man is called as artificial selection. It often permits only the selected plants to reproduce. The progeny from remaining plants is generally discarded. Artificial selection progressively reduces the variability. Artificial selection is essentially based on phenotype of the plants.
• The effectiveness of selection primarily depends on the degree to which the phenotype reflects the genotypes. Selection has two basic limitations
1. Selection is effective for heritable difference only. Its effectiveness is greatly affected by heritability of character under selection.
2. Selection does not create variation; it only utilizes the variation already present in population.

Therefore, the requirements of selection are:

1. Variation must be present in the population.
2. Variation must be heritable.

Progeny Test

• It is the evaluation of worth of plant on the bases of performance of their progeny The Progeny Test was developed by Louis de-Vilmorin. Therefore it is also known as Vilmorin Isolation Principle or Vilmorin Principal.
• The Progeny Test serves two valuable functions

1. To determine the breeding behavior of plant whether homozygous or heterozygous eg. AA or Aa, respectively.
2. To find out whether the character for which the plant is selected is heritable i.e. due to genotypes.

Pure line

• A pureline is a progeny of a single homozygous plant of a self pollinated spp. Therefore, all the plants in a pure line will have the same genotype. The phenotypic differences within a pure line are due to environment and have no genetic basis. Therefore variation in a pure line is not heritable and selection within a pureline is ineffective.
• The concept of pureline was proposed by Johannsen,1903 on the basis of his studies with beans (Phaseolus vulgaris) variety Princess.

Effects of self pollination on genotypes

• Self pollination increases homozygosity with a corresponding decrease in heterozygosity. Inbreeding also increases homozygosity and reduces heterozygosity. Inbreeding is the mating between
individuals related by descent that is, having a common parent or parents in their ancestry. Some examples of inbreeding are:
• Sib mating: mating between brother-sister
• Half sib mating: brother-step sister mating. Self pollination is the most intense form of inbreeding, since in this case the same individual functions as the male as well as female parent. Increase of
homozygosity by self pollination is shown in the figure:





After 10 generarations of selfing, all the plants in the population would be homozygous i.e. AA or aa, whereas the frequency of heterozygosity would be negligible.
Therefore, self pollination has two main effects on a crop:

1. All plants in population becomes completely homozygous.
2. The population is the mixture of several homozygous genotypes.

Origin of variation in Pure Lines
The variation in a pureline can only be observed due to:

(1) Mechanical mixture during harvesting: During threshing, cleaning or storage of seeds, other genotypes may get mixed with a pureline. Such contaminations are quite common and may be removed with careful handling.
(2) Natural Hybridization:- In most self pollinated species, a low amount of cross pollination does occur. Natural hybrids can be avoided by isolating a pure line from other genotypes with a couple of rows of the same pure line. In nature, the frequency of mutations affecting quantitative characters is not known. The frequency of spontaneous mutation is10^-6 .