Method of Plant Breeding in Cross Pollinated Plants – Recurrent Selection
Recurrent Selection:
The initial idea of recurrent selection was independently given by Hayes and Garber in 1919 and East and Jones in 1920. But the term recurrent selection was first coined by Hull in 1945.
Definition:
Recurrent selection is defined as reselection generation after generation, with intermating of selected plant to produce the population for the next cycle of selection. The idea of this method was to ensure the isolation of superior inbreds from the population subjected to recurrent selection. The isolation of an outstanding inbred line depends on two factors:
1) The proportion of superior genotypes present in the base population from which lines are isolated and
2) The effectiveness of selection during the inbreeding of desirable genes.
Types of Recurrent Selection:
These are four types of recurrent selection.
i) Simple recurrent selection (SRS)
ii) Recurrent selection for general combining ability (RSGCA)
iii) Recurrent selection for specific combining ability (RSSCA)
iv) Reciprocal recurrent selection (RRS)
i) Simple Recurrent Selection (SRS) :
A type of recurrent selection that does not include tester is referred as simple recurrent selection. It is also known as phenotypic recurrent selection.
Procedure of Simple Recurrent Selection:
1) A number of plants with desirable phenotype are selected and self-pollinated in the 1st year.
2) In the second year separate progeny rows are grown from selfed seed of the selected plants.
3) The progenies are inter crossed in all possible combination by hand.
4) Equal amount of seed from each cross is composited to produce the next generation. This complete the original selection cycle.
5) In the third year, bulked seeds are grown and superior plants are selected and selfed, like first year.
6) In the fourth year, progeny of selected plants are grown from selfed seed and intermating is done like first year.
7) The crossed seed is composited in equal quantity for use in the next cycle of selection. This complete first cycle of simple recurrent selection. Thus selection cycles may be repeated till the desired improvement is achieved.
Recurrent Selection for General Combining Ability (GCA):
A form of recurrent selection used to important the general combining ability of a population for a character and the heterozygous tester is referred as RSGCA. It is also known as half sub recur selection
Procedure of Recurrent Selection for General Combining Ability (RSGCA):
1) First Year:
Superior plants for the character under improvement are from the base population. The selected plants are selfed and also crosses heterozygous tester having broad genetic base. The selfed seed is kept storage.
2) Second Year:
The crossed seed is sown and the combining ability selected plants is evaluated and plants with good GCA are identified.
3) Third Year:
The progeny of selected plants with good GCA are grown from their selfed seed kept in cold storage. These progenies are inter mated in all possible combinations and their crossed seed is composited to form a new source of population for further selection. This complete original selection cycle. In the same way another cycle can be complete in these year called as first recurrent selection cycle. Many such cycles may be made to obtain desired results. Main features of this method to use for genetic improvement of quantitative characters in which selection is made or the basis of test crosses performance. This method also used for providing good of population and required three season or year for completion of each cycle.
Recurrent Selection for SCA:
It was originally proposed by Hull in 1945, a form of recurrent selection that is used to improve the SCA of a population for a character by using homozygous tester is referred as (RSSCA) recurrent selection for specific combining ability. It is also known as half site recurrent selection with homozygous tester.
Procedure:
The selection procedure of this method is same as for RSGCA, except that the tester is an inbred line which has narrow genetic base i.e tester used must be an outstanding inbred. The differences in the performance of test cross are due to difference in their specific combining ability.
1) First Year:
Several plants are selected and selfed. The selected plant ( male) are also crossed to an tester( female).
2) Second Year:
R.Y.T is conducted using test cross progeny. Outstanding progenies are identified.
3) Third Year:
Selfed seed from the progeny are planted in separate progeny row in a crossing block. All possible inter cross are made by hand among progeny. Equal amount of seed is composited, this complete the original selection cycle.
4) Fourth Year:
The combination seed is planted and operation of 1st year repeated.
5) Fifth Year:
Second year operation repeated.
6) Sixth Year:
Third year operation repeated.
This completes the first recurrent selection cycle.
Reciprocal Recurrent Selection (RRS):
A form of recurrent selection used to improve both GCA and SCA of a population for a character using two heterozygous testers is known as RRS. It is also known as recurrent reciprocal half sib selection. Comstock et al in 1949, proposed this method.
Main Features of these Methods:
1) It is used for improvement of polygenic characters.
2) Selection is made on the basis of test cross performance.
3) Two heterozygous tester are used as a source of population.
4) It is used for improving population for GCA and SCA for specific characters.
5) It is equally effective with incomplete, complete and over dominance.
6) It is used for improvement of those characters, which are governed by both additive and non-additive gene action.
7) This method also requires three seasons for completion of each cycle of selection.
Procedure:
1) 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.
2) Second Year:
The progeny of test crosses made with pollen parents of A and B population are evaluated in separate replicated triple. The superior progeny are identified.
3) 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 ‘blocks are composited in equal quantity to raise A1 generation. Similarly, crossed seeds of ‘B’ block are bulked to raise B1 generation. This (B1) completes original cycles of selection.
4) Fourth Year:
The A1 and B1 population are grown from the composite crossed seeds of respective population obtained in third year. The operations of first year are repeated.
5) Fifth Year:
Operation of second year are repeated.
6) Sixth Year:
Operation of third year are repeated.
This completed first cycle of reciprocal recurrent selection. Such cycles may be continued till the desired improvement is achieved.
Merits:
1) Recurrent selection is an efficient breeding method for increasing the frequency of superior genes for various economic characters.
2) It helps in breaking repulsion phase of linkage.
3) It helps in maintaining high genetic variability due to repeated intermating of heterozygous population.
Demerits:
1) It is not directly used for the development of new varieties.
2) This method involves lot of selection crossing and selfing work.
3) It permits selfing, which leads to loss of genetic variability.