In this article we will discuss about:- 1. Definition of Clonal Selection 2. Features of Clonal Selection 3. Breeding Procedure 4. Achievements 5. Merits and Demerits.
Definition of Clonal Selection:
Progeny of a single plant obtained by asexual reproduction is known as clone. A procedure of selecting superior clones from the mixed population of asexually propagating crops is referred to as clonal selection. Crops which are propagated asexually or by vegetative means are known as asexually propagated or vegetatively propagated or clonal crops.
There are some agricultural (sugarcane, potato, sweet potato, etc.) and horticultural (banana, mango, citrus apple, pears, peaches, loquat, litchi, etc.) crops that propagate by asexual means.
The main reasons of asexual reproduction are:
(1) Non flowering in many cases,
(2) Reduced flowering and seed set,
(3) To avoid inbreeding depression in certain crops, and
(4) Apomixis in some species.
(1) Non Flowering Species:
This group includes garlic, ginger, betel and several yams.
(2) Low Seed Setting Species:
Sugarcane, potato, and sweet potato, etc.
(3) Normal Flowering and Seed Setting Species:
Citrus, mango, pear, peach, apple, litchi, loquat, and many ornamental plants. These are highly heterozygous and vegetative propagation is essential to maintain the heterozygous balance.
(4) Apomictic Species:
Seeds develop asexually in such species. Many fruit crops exhibit apomixis.
Features of Clonal Selection:
Clones have several important features such as:
(1) Homogeneous constitution,
(2) Heterozygosity,
(3) Vigorous growth,
(4) Wider adaptation,
(5) Source of Variation, and
(6) Segregation in F1.
These are briefly discussed below:
(1) Homogeneous Constitution:
The progeny of a clone is genetically identical. In other words, all the plants of a clone have similar genetic constitution. Thus, clones are homogeneous. There is no genetic variation within a clone. The variation is only environmental hence selection is not effective within a clone.
(2) Heterozygosity:
The asexually propagated crops are heterozygous and hence clone is also heterozygous. Progeny of a clone looks similar phenotypically but is heterozygous. If a clone is subjected to inbreeding, it will produce various types of segregants and exhibit high inbreeding depression.
(3) Vigorous Growth:
Clones have hybrid vigour which is conserved due to asexual reproduction. Most of the varieties of sugarcane and potato are hybrids. In other words, clonal selection is useful in conserving the heterosis for a long period, because clones are stable and are not prone to segregation.
(4) Wider Adaptation:
Generally, clones are more adaptable to environmental variation due to high level of heterozygosity than pure lines. A deliberate mixture of genetically different but phenotypically similar clones give better yield in variable environments than a single clone and also provides better protection form the infestation of diseases.
(5) Source of Variation:
There are three sources of variation in a clone, viz. bud mutations, mechanical mixtures, and occasional sexual reproduction. The frequency of bud mutation is very low. But once bud mutation occurs it will lead to deterioration of a clone by adding new variants in the population. Viral and bacterial diseases also lead to deterioration of a clonal variety.
(6) Segregation in F1:
When hybridization is done between different clones, segregation occurs in F1 generation. Each F1 plant is potentially a new variety; therefore, selection is practised in F1. A brief comparison of a pure line variety and a clonal variety is presented in Table 17.1.
Breeding Procedure of Clonal Selection:
Improvement of asexually propagated crops by selecting superior clones is known as clonal selection.
Superior clones can be isolated from three types of material, viz., (i) local variety, (ii) introduced variety, and (iii) intercrossed populations. In other words, clonal selection can be practised in above three types of material.
The general procedure of clonal selection is outlined as follows:
1. In the first year, superior plants are selected from a mixed population of vegetatively propagated crop. Superior plants are selected on the basis of yield, maturity, disease resistance, etc.
2. In the second year, the progeny of each selected plant is asexually propagated and grown separately for seed increase and evaluation. Clones superior to check variety are selected and evaluated in replicated preliminary trials in the third year.
3. Several clones are evaluated in multi-location trial for 3-4 years using standard variety as a check. Clones which are superior to check variety in yield, disease resistance and quality are identified for release.
4. The best clone is released as a variety and its seed is multiplied for distribution. Thus release of new variety by this method takes 9-10 years.
Besides clonal selection, interspecific hybridization and mutation breeding are also used for the improvement of asexually propagated crops. These methods have been successfully used in sugarcane and potato. Interspecific hybridization has been extensively used in the breeding of sugarcane.
Many modern cultivars have been derived from crosses of Saccharam officinarum with S. spontaneum or S. barberi. These crosses are useful in combining high sugar content of the first species with disease resistance, cold tolerance and vigour of last two species. Backcross method is used for transfer of desirable gene from wild species to the cultivated one in sugarcane and potato.
Achievements of Clonal Selection:
In India, clonal selection has been successfully used for developing new varieties in potato, sugarcane, banana, citrus and grapes.
Varieties Kufri Red and Kufri safed in potato; Ko 11, Ko 22 and Neelam in mango; and Bombay green, Pride monthan and High gate in banana have been developed by clonal selection. Several varieties of sugarcane (Co 541, CoS 510, Co 1148, Co 1158 etc.) and potato (Kufri Sinduri, Kufri Kuber, Kufri Kundan, Kufri Chamatkar, etc.) have been developed by interspecific hybridization followed by clonal selection.
Merits and Demerits of Clonal Selection:
Merits:
1. This method is useful in conserving heterosis for several generations. The variety evolved by this method retains all the characteristics of the parental clone for several years.
2. Clonally propagated varieties are highly uniform like pure-lines. Moreover, they are highly stable because there is no risk of deterioration due to segregation and recombination.
3. This is an effective method for genetic improvement of asexually propagated crop plants. In other words, it is useful in isolating the best genotype from a mixed population of asexually propagated crops.
Demerits:
1. Varieties developed by clonal selection are highly prone to new races of a disease.
2. Clonal selection cannot create new variability and, therefore genetic makeup cannot be improved by this method without hybridization.
No comments yet.