In this article we will discuss about:- 1. Introduction to Recurrent Selection 2. Features of Recurrent Selection 3. Types 4. Merits and Demerits.
Introduction to Recurrent Selection:
The initial idea about recurrent selection was independently given by Hayes and Garber in 1919, and East and Jones in 1920. The procedure of recurrent selection was described by Jenkins in 1940 and the term recurrent selection was coined by Hull in 1945. Hull defined recurrent selection as reselection generation after generation with interbreeding of selects to provide for genetic recombination.
Thus it is a cyclic selection that is used to improve the frequency of desirable alleles for a character in a breeding population. In other words, it is an important method of population improvement.
Recurrent selection is a modified form of progeny selection. However, it differs from progeny selection in two main aspects. Firstly, the selected plants are self-pollinated in recurrent selection, whereas they are open pollinated in progeny selection. Secondly, the progeny of selected plants are intermated in all possible combinations in this method, whereas they are open pollinated in progeny selection.
Features of Recurrent Selection:
Main features of recurrent selection are given below:
1. Application:
Recurrent selection was originally developed as a method of breeding cross pollinated species. Now it is also used in self-pollinated species. However, it is more commonly used in cross pollinated species than in self-pollinated species.
2. Base Population:
A heterozygous base population is required to start recurrent selection.
In cross pollinated species, a base population may be any of the five populations, viz.:
(a) An open pollinated variety,
(b) A synthetic variety,
(c) Progeny of intercrosses among selected inbreds,
(d) A double cross, and
(e) A single cross.
3. Important Steps:
A simple recurrent selection scheme consists of five main steps:
(a) Selection of superior plants from base population,
(b) Selfing of selected plants,
(c) Growing progeny of selected plants in the next season from selfed seed,
(c) Intermating among progeny, and
(e) Bulking of crossed seed in equal quantity.
This completes original cycle of recurrent selection. The bulk seed is used for next cycle of selection which also involves above five steps.
4. Use of End Product:
The population developed by recurrent selection can be used in three main ways :
(a) In producing homozygous inbreds by selfing,
(b) In the production of hybrid varieties, and
(c) In the production of synthetic varieties.
5. Basic Assumptions:
Recurrent selection is based on three basic assumptions, viz., absence of epistasis, absence of multiple alleles and absence of linkage disequilibrium. However, none of these assumptions is considered valid.
6. Impact:
Recurrent selection is used to improve the frequency of desirable alleles for a character in a population. In this method the heterozygosity that is lost due to selfing is recovered by intermating of selected progeny.
Types of Recurrent Selection:
There are four types of recurrent selection, namely:
(1) Simple recurrent selection,
(2) Recurrent selection for general combining ability,
(3) Recurrent selection for specific combining ability, and
(4) Reciprocal recurrent selection.
Each type is used under specific conditions as discussed below:
1. Simple Recurrent Selection:
A type of recurrent selection that does not include tester is referred to as simple recurrent selection. It is also known as phenotypic recurrent selection. This method is an extension of mass selection.
Main features of simple recurrent selection are given below:
i. The tester is not used in this scheme.
ii. It does not measure the combining ability.
iii. The selection is based on phenotype or simple test.
iv. This method is useful only for those characters which have high heritability.
v. This method requires only two seasons for the completion of one selection cycle as given below:
In the first year, the superior plants for the character under improvement are selected from the heterozygous base population. These plants are grown from the selfed seed and intermating is done among the progeny. The crossed seed is bulked in equal quantity. This completes original cycle of selection.
In the third year, bulked seed is grown and superior plants are selected and selfed like first year. In the fourth year, progeny of selected plants are grown from selfed seed and intermating is done like first year. The crossed seed is composited in equal quantity for use in the next cycle of selection. This completes first cycle of simple recurrent selection. (Fig. 21.1). Thus selection cycles may be repeated till the desired improvement is achieved.
2. Recurrent Selection for GCA:
A form of recurrent selection that is used to improve the general combining ability of a population for a character and includes heterozygous tester is referred to as recurrent selection for general combining ability (RSGCA). It is also known as half sib recurrent selection with heterozygous tester. This is an extension of the Jenkin’s (1940) scheme used for development of short term synthetics.
Main features of this scheme are given below:
i. This method is used for genetic improvement of quantitative characters.
ii. The selection is made on the basis of test cross performance.
iii. A heterozygous tester with broad genetic base is used for testing general combining ability. Generally, an open pollinated variety is used as a tester.
iv. This method is used for improving general combining ability of a population for a character.
v. This method is more effective with incomplete dominance and less effective with over dominance.
vi. This method is used for the improvement of those characters which are governed by additive gene action.
vii. This method requires three seasons or years for completion of each cycle of selection as given below:
First Year:
Superior plants for the character under improvement are selected from the base population. The selected plants are selfed and also crossed to a heterozygous tester having broad genetic base. The selfed seed is kept in cold storage.
Second Year:
The crossed seed is sown and the combining ability of the selected plants is evaluated and plants with good gca are identified.
Third Year:
The progeny of selected plants with good gca are grown from their selfed seed kept in cold store. These progeny are intermated in all possible combinations and their crossed seed is composited to form a new source population for further selection.
This completes original selection cycle. In the same way another cycle can be completed in three years (4th to 6th year). This is called first recurrent selection cycle. Many such cycles may be made to obtain desired results.
3. Recurrent Selection for SCA:
A form of recurrent selection that is used to improve the sea of a population for a character by using homozygous tester is referred to as recurrent selection for specific combining ability (RSSCA). It is also known as half sib recurrent selection with homozygous tester. This method was originally proposed by Hull in 1945.
Main features of this scheme are briefly presented below:
i. This method is used for the genetic improvement of polygenic character.
ii. Selection is made on the basis of test cross performance.
iii. A homozygous tester with narrow genetic base is used for testing specific combining ability. In other words, an inbred is used as a tester.
iv. This method is used for improving specific combining ability of the population for a character.
v. This method is more effective with over dominance and less effective with incomplete dominance.
vi. This scheme is used when a character is governed by non-additive (dominance and epistasis) gene action.
vii. This method requires three seasons or years for completion of each cycle of selection.
The selection procedure of this method is same as for RSGCA except that the tester is an inbred line in this case which has narrow genetic base. The differences in the performance of test crosses are due to difference in their specific combining ability. A comparison of recurrent selection for gca and recurrent selection for sca is presented in (Table 21.1.).
4. Reciprocal Recurrent Selection:
A form of recurrent selection that is used to improve both gca and sca of a population for a character using two heterozygous testers is known as reciprocal recurrent selection (RRS). It is also termed as recurrent reciprocal half sib selection. This scheme was proposed by Comstock, et al. in 1949.
Main features of this method are given below:
i. This scheme is also used for the improvement of polygenic characters.
ii. Selection is made on the basis of test cross performance.
iii. Two heterozygous populations each of which is the tester for other are used in this method. The two populations may be designated as A and B.
iv. This method is used for improving a population both for gca and sca for specific character.
v. This method is equally effective with incomplete, complete and over dominance.
vi. This method is used for the improvement of those characters which are governed by both additive and non-additive gene action.
vii. This method requires three seasons or years for completion of each cycle of selection as given below:
First Year:
Several phenotypically superior plants are selected from population A and B. The pollen of some selected plants of A population is used to cross large number of randomly selected plants of population B. Similarly, pollen of some selected plants of B population is used to cross large number of plants of population A. All the plants of population A and B used as pollen parents in the crosses are selfed.
Second Year:
The progeny of test crosses made with pollen parents of A and B populations are evaluated in separate replicated trials. The superior progeny are identified.
Third Year:
The selfed seeds of those A and B plants whose progeny were found superior in replicated trials are grown in separate block. All possible crosses are made among the progeny of A plants and also among the progeny of B plants. The crossed seeds of A block are composited in equal quantity to raise A generation. Similarly, crossed seeds of B block are bulked to raise B1 generation. This completes original cycle of selection (Fig 21.2).
Fourth Year:
The A1 and B1 populations are grown from the composite crossed seeds of respective population obtained in third year. Then operations of first year are repeated.
Fifth Year:
The operations of second year are repeated.
Sixth Year:
The operations of third year are repeated.
The last three years constitute first cycle of reciprocal recurrent selection. Such selection cycles may be continued till the desired improvement is achieved.
Merits and Demerits of Recurrent Selection:
Merits:
1. Recurrent selection is an efficient breeding method for increasing the frequency of superior genes in a population for various economic characters. Thus it is an important method of population improvement.
2. Repeated intermating of heterozygous progeny provides greater opportunities for recombination to occur. Thus this method helps in breaking repulsion phase linkages.
3. This method also helps in maintaining high genetic variability in a population due to repeated intermating of heterozygous plants/populations.
4. The selection is made on the basis of test cross performance (except in simple recurrent selection) and only selected plants are allowed for intermating.
Demerits:
1. This method is not used directly for the development of new varieties. The new varieties are developed by the use of end product in hybridization. This is only a method of population improvement.
2. This method involves lot of selection, crossing and selfing work.
3. This method permits selfing which leads to loss of genetic variability.
Recurrent selection has been successfully used for the improvement of oil content in maize, fibre strength in cotton and sugar content in sugar-beet and sugarcane.
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