Recurrent selection

Recurrent selection

Recurrent selection

It should also be recognized that the effectiveness of the selection during production of inbreeds largely depended on phenotype, and genetic potential could not be tested. Therefore, top crosses (crossing individual inbreeding plants with open pollinated variety to find out the genetic potential of inbreeding plants, by comparing top cross progenies) were made is several generations to find out if selection in early generations can be made on the basis of these tests. Jenkins (1935) showed that inbreeds become stable early during inbreeding and therefore can be selected early during the process of inbreeding. This phenomenon was described as ‘early testing’.

On the basis of failure of mild inbreeding and the success of ‘early testing’, it was recommended that a successful breeding system should ensure that variability is maintained to allow effective selection; this was possible only when regular intercrossing is done among selected plants which will also increase the frequency of desirable genes in every cycle of selection followed by intercrossing. Such a breeding scheme was given the name ‘recurrent selection’ by Hull in 1945. It was, therefore, involved the following steps-

  1. plants from heterozygous, source are self-pollinated to give S1 lines and these individual plants were also simultaneously evaluated for some desirable characters;
  2. plants with inferior performance are discarded and superior plants are propagated from selfed seed (the superiority of plants is evaluated by different methods in different recurrent selection schemes see later);
  3. all possible intercrosses are made among the S1 progenies of selected superior plants; if all possible intercrosses cannot be made, open pollination is allowed;
  4. the resulting intercross populations serves as a source material for an additional cycle of selection.

In the above scheme, it is apparent, as pointed out by Hull, that “recurrent selection was meant to include reselection generation after generation, with interbreeding of selects to provide for genetic recombination. Thus selection among isolates, inbred lines or clones is not a recurrent selection, until selects are interbred and a new cycle of selection is initiated”.

Classification of Recurrent Selection

Recurrent selection is sometimes classified into two types-

  1. Phenotypic recurrent selection, where phenotype of S0 plants is the basis of selection (S0 plants are those which have not been selfed so far; S1 plants mean population derived after first cycle of selfing).
  2. Genotypic recurrent selection, where the selection is based on some kind of a test, either selfed progeny test or a top cross, etc.

According to another classification, four types of recurrent selection schemes have been proposed-

  1. simple recurrent selection,
  2. recurrent selection for general combining ability,
  3. recurrent selection for specific combining ability,
  4. reciprocal recurrent selection.

These four schemes will be separately described and it will be seen that only the first scheme (simple recurrent selection) can be (sometimes, but not always) classified as a phenotypic recurrent selection scheme, the other three are genotypic recurrent selection schemes, since they involve some kind of a test for selecting S0 plants.

(I) Simple recurrent selection

In this, some plants are selected on the basis of phenotype or on the basis of comparing selfed progenies. The selected plants are propagated through S1 progenies and intercrossed. The intercrossed seed becomes the source material for the next cycle of recurrent selection. Therefore, one cycle in this scheme takes two years as outlined below for two selection cycles in four years.

  1. Original selection cycle-
  • First year- Self-pollinate a number of plants or ears (about 100) and select superior ones (about 10 best plants).
  • Second year- Raise S1 generation from progenies of selected plants and make all possible intercrosses by hand; allow open pollination, if all intercrosses by hand are not possible.
  1. First recurrent selection cycle-
  • Third year- Sow seed from intercrosses made ; self all hybrid plants (about 100) and select superior plants (about 10) on the basis of phenotype.
  • Fourth year- Sow selfed seed from plants (about 10 as above) selected in the third year and make all possible intercrosses. The recurrent cycles may be repeated till the desired level of improvement is achieved.

From the above scheme, it is obvious that the simple recurrent selection will be effective only where visual selection based on phenotype is very effective- e.g. discase resistance, etc. It will not be effective for improving combining ability for characters like yield. Following this scheme in maize, an improvement in oil content was achieved by Sprague and Brimhall (1950) and improvement is resistance against Helminthosporium turcicum (leaf blight) was achieved by Jenkins, Robert and Finley (1954).

(II) Recurrent selection for general combining ability (GCA)-

In this scheme, a number of superior plants are selected from the source population (in any recurrent selection programme, source can be an open pollinated variety, synthetic variety, single cross of double cross hybrids) and these S0 plants are selfed and crossed to heterozygous tester stock (with broad genetic base). Individuals with good general combining ability are propagated by selfed seed and intercrossed in all possible combinations. A composite of intercrossed seed is then used for further selection cycle.

A selection cycle in this scheme of recurrent selection for general combining ability takes three years (unlike simple recurrent selection, where one cycle takes two years), as shown in the following steps shown year-wise (note that an additional year in needed in testing top cross progenies).

  1. Original selection cycle-
  • First year- A number of selected (S0) plants (which appeal to the breeder) are self-pollinated and also crossed to a tester (an open pollinated variety with broad genetic base). The selfed seed is harvested separately and saved for planting in the third year. The test cross seed from each plant crossed, is harvested separately and used for a replicated yield trial in the second year.
  • Second year- A replicated yield trial is conducted using the crossed seed harvested in the first year. The superior progenies are identified.
  • Third year- Selfed seed (S1) harvested in the first year is sown in progeny rows, but only from those plants which are identified to be superior on the basis of trial conducted in the second year. These progenies are intercrossed in all possible combinations. Equal amount of seed from each intercross is composited to raise a crop for next cycle of selection.
  1. First cycle of recurrent selection-
  • Fourth year- The composite intercrossed seed harvested in the third year, is sown to raise source population for the first cycle of recurrent selection. Several selected plants are selfed and crossed to the tester used in the first year.
  • Fifth year- Yield trial is conducted as done in the second year and superior lines are selected.
  • Sixth year- Selfed seed (only form plants selected on the basis of fifth year) harvested in fourth year is sown in progency rows and all possible intercrosses are made. This completes the first cycle of recurrent selection.

Recurrent selection cycle may be repeated till the desired level of improvement for general combining ability is achieved. It is obvious that this scheme is primarily based on early testing proposed by Jenkins in 1935. An improvement in the general combining ability was actually achieved in some lines following this scheme in the ‘Krug’ variety of maize used by Lindquist in 1905.

(III) Recurrent selection for specific combining ability (SCA)-

The recurrent selection for specific combining ability was proposed by Hull in 1945 and its main objective was to improve and isolate such lines which will combine well with a given outstanding inbred line to give superior hybrids. Consequently, it differs from recurrent selection for general combining ability only in the choice of tester, which is a homozygous and outstanding inbred line that will be used as a parent in future programme for hybrid varieties (note that in the previous case of recurrent selection for general combining ability, heterozygous tester in the form of open pollinated variety with a broad genetic base was used). As you can see, in this method care will have to be observed in selecting the tester which will be used as one of the parents in future hybrid variety programme.

Since in this programme lines are developed specifically for a tester, much labour goes waste if an inbred line other than the tester needs to be used or if tester needs to be improved. There does not seem to be any evidence available to suggest effectiveness of this method suggested by Hull in 1945.

(IV) Reciprocal recurrent selection-

Reciprocal recurrent selection was proposed by Comstock, Robinson and Harvey in 1949. Its main objective was to improve two populations to identify and isolate lines that will combine well not only with each other, but also with other lines. In other words, the procedure is meant to bring above simultaneous improvement in general as well as specific combining abilities.

Two heterozygous source populations A and B are taken, such that they must be diverse (genetically unrelated). A number of plants from A are selfed and crossed with B and simultaneously a number of plants from Bare-selfed and crossed with A. The superior plants are identified on the basis of field trials conducted in the second year and the selfed seeds of these identified plants from A and B are separately sown in progeny rows in third year to make all possible intercrosses. The intercrossed seed from A as well as B is used to initiate a cycle of recurrent selection in both these populations. The selection cycles are repeated till the desired level of improvement is achieved. In the initial cycles improvement in general combining ability is achieved, but later as homozygosity increases, improvement in specific combining ability becomes effective.

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