Recurrent selection

As previously stated, a composite cultivar, like a multiline, is a mixture of different genotypes. The difference between the two lies primarily in the genetic distance between the components of the mixture. Whereas a multiline is constituted of closely related lines, a composite may consist of inbred lines, all types of hybrids, populations, and other less similar genotypes. However, the components are selected to have common characters, such as similar growth period, degrees of resistance to lodging or to a pathogenic agent. This consideration is critical to having uniformity in the cultivar.

A composite cultivar should be distinguished from a composite cross that is used to generate multiple-parent crosses by successively crossing parents until the final parent contains all parents. Composites may serve as a continuous source of new entries for a breeding nursery. Any number of entries may be included in a composite, provided selection is judiciously made after evaluation. New entries may be added at any time. Technically, a composite may derive from a single diverse variety, a progeny from a single cross, or even several hundreds of entries. However, a good number of entries is between 10 and 20. The breeder’s objectives determine the kind of entries used for breeding a composite. Using elite and similar genotypes would make the composite more uniform, robust, but less genetically diverse. The reverse would be true if diverse entries are included. As a population improvement product, the yield of a composite can be improved by advancing it through several cycles of selection.

In species such as sorghum, which are predominantly self-pollinated, recessive male sterility gene that is stable across environments may be incorporated

into the composite  by crossing each entry to the source of the sterility gene prior to mixing. The F1 is first selfed and then backcrossed to the male sterile segregates.

The recurrent parents are then mixed to create the composite.

Recurrent selection is a cyclical improvement technique aimed at gradually concentrating desirable alleles in a population. It is one of the oldest techniques of plant breeding. The name was coined by F.H. Hull in 1945. It was first developed for improving cross-pollinated species and has been a major breeding method for this group of plants. Hence, detailed discussion of this method of breeding is deferred to Chapter 17. It is increasingly becoming a method of improving selfpollinated species. It has the advantage of providing additional opportunities for genetic recombination through repeated intermating after the first cross, something not available with pedigree selection. It is effective for improving quantitative traits.

_ Recurrent selection requires extensive crossing, which is a challenge in autogamous species. To overcome this problem, male sterility system may be incorporated into the breeding program. With male sterility, natural crossing by wind and/or insects will eliminate the need for hand pollination.

_ Adequate seed may be obtained by crossing under controlled environment where the crossing period can be extended.

There several advantages and disadvantages of the application of recurrent selection to breeding autogamous species.

Advantages

_ Opportunities to break linkage blocks exist because of repeated intercrossing exists.

_ It is applicable to both autogamous grasses and legumes.Disadvantages

_ Extensive crossing is required, something that is a challenge in autogamous species. Male sterility system may be used to facilitate this process.

_ Sufficient seed may not be available after intercrossing. This also may be resolved by including male sterility in the breeding program.

_ More intermatings may prolong the duration of the breeding program.

_ There is also the possibility of breaking desirable linkages.