In this article we will discuss about:- 1. Definition of Multiline Breeding 2. Features of Multiline Breeding 3. Types 4. Procedure 5. Merits and Demerits.
Definition of Multiline Breeding:
The deliberate seed mixtures of isolines, closely related lines or unrelated lines are referred to as multi-lines, and a variety which is developed for commercial cultivation from any of these mixtures is known as multiline variety. The terms multi-lines and blends are used as synonyms.
However, some persons prefer to designate mixtures of isoline as multi-lines and mixtures of lines differing for several characters as blends. Isogenic lines or isolines refer to those lines that are genetically identical except for the allele at one locus.
In other words, isogenic lines have only one gene difference. The use of multiline cultivars was first suggested in oats by Jensen in 1952. Borlaug and Gibler in 1953 outlined the method for developing multi-lines in wheat. Later on several workers used this method in various self-pollinated species.
Features of Multiline Breeding:
Main features of multiline varieties are briefly presented below:
1. Application:
The multiline approach is applicable to self-pollinated species only. Multiline cultivars are commercially used in self/pollinated crops like oat, wheat, soybean, groundnut and many other crops.
2. Genetic Constitution:
Multiline cultivars are mixtures of several pure-lines. The pure-lines may be isogenic lines, closely related lines or unrelated lines. Thus, multi-lines are homozygous but heterogeneous populations or genetically diverse populations. The genotypes which are mixed together to constitute a multiline have phenotypic similarities for several characters like height, maturity, grain colour and size etc.
3. Adaptation:
Multi-lines are more adaptable to environmental variations than pure-lines by virtue of their genetic diversity. In other words, multi-lines have more buffering capacity to environmental changes than pure lines. The pure lines are adapted to specific environment but have poor adaptability. Multi-lines have broad genetic base which provides them greater adaptability.
4. Disease Control:
The use of multiline cultivars is an effective way to minimize the yield losses due to the attack of multiracial disease. In a multiline cultivar, each component genotype has a resistant gene for a different race of a disease. All races of a disease will never appear at a time and all the genotypes of a heterogeneous mixture are never attacked at a time.
Multi-lines cause delay in the spread of the pathogen in the field, because the resistant plants act as barriers in the spread of a disease. In this way multiline cultivars provide better protection from the attack of new race of a disease.
5. Quality of Produce:
The produce of multiline cultivars is generally less uniform and less attractive than that of a pure-line, because it is a mixture of several pure lines.
6. Yield:
The yield of a multiline would be lesser than that of the most productive cultivar of a pure-line under normal conditions. But under adverse conditions, the yield of a multiline would be much higher than that of most productive pure-line cultivar. Because highly productive pure-line cultivars are more prone to biotic and abiotic stresses due to narrow genetic base.
Types of Multiline Breeding:
There are three types of multi-lines: viz.:
(1) Mixtures of isolines,
(2) Mixtures of closely related lines, and
(3) Mixtures of unrelated or distinctly different genotypes.
These are briefly described below:
1. Mixtures of Isolines:
Isolines are genotypes having one gene difference only. Isolines are developed by backcross method. The resistant genes for different races of a disease are transferred into one popular variety by separate backcross programmes. Six to ten isolines are developed and their seeds are mixed in equal quantity to constitute a multiline. Such varieties have been developed in oat, soybean and wheat in USA.
2. Mixtures of Closely Related Lines:
Sometimes, multiline cultivars are constituted by mixing the seed of closely related lines. Closely related lines are developed from crosses having one parent in common. This approach of multi-lines development is being followed now-a-days at important breeding centres like CIMMYT.
A mixture of closely related lines of wheat KSML 3 was released from Ludhiana to provide better resistance to rust disease. The six components of multiline were derived from crosses with popular cultivar Kalyan Sona as the common parent. Different types of crosses were made to develop each of the components including single crosses and limited backcrossing. In this case the component lines are not isolines. They differ in several characters from each other.
3. Mixtures of Unrelated Lines or Cultivars:
Multi-lines are also constituted from seed mixtures of distinctly different cultivars. Such multi-lines are developed when phenotyipc uniformity is not essential. First pure-lines are developed by pedigree, bulk or single seed descent methods. The performance of each pure-line is evaluated and then superior pure-lines each with different gene for resistance are mixed together to constitute multiline. This is almost similar to varietal blends.
Procedure of Developing Multiline Breeding:
The development of a multiline consists of four important steps:
(1) Selection of recurrent parent,
(2) Selection of donor parents,
(3) Transfer of resistant genes into recurrent parent, and
(4) Mixing of seed of the isogenic lines.
These are briefly discussed below:
(1) Selection of Recurrent Parent:
The recurrent parent should be a high yielding popular variety. The recurrent parent should be the best cultivar of a region.
(2) Selection of Donor Parents:
Parents with resistance to various races of a disease should be chosen as donor parents. The resistance should be thoroughly examined under artificial epiphytotic conditions before use of the donor parents in the crossing programmes. The donor parents should be adapted varieties as far as possible.
Because in un-adapted parents disease resistance is sometimes linked with several un-desirable characters and transfer of resistant genes from such parents to the recurrent parent becomes difficult task. Several donor parents are selected to incorporate different resistant genes against various races.
(3) Transfer of Resistance:
The resistant genes are transferred from donor parents to the recurrent parent through a series of several separate backcross programmes. Generally 4-5 backcrosses are sufficient to retain the genotype of recurrent parent with added resistance in the backcross derivatives. The backcross derivatives are evaluated for disease resistance during backcrossing and also at the end of backcrossing. The desirable lines from each backcross are mixed to form an isoline.
(4) Mixing of Isolines:
The various isolines developed by various backcrosses are mixed together to constitute a multiline cultivar. Generally 6-10 isolines are mixed to constitute a multiline cultivar.
Merits and Demerits of Multiline Breeding:
Merits:
1. Multi-lines are more adaptable to environmental changes than pure-line cultivars due to genetic diversity.
2. They provide better protection form the infection of new race of a disease. \
Multiline cultivars have been developed for commercial cultivation in oats, wheat, soybean and peanut in USA. In India three multiline varieties, viz. KSML 3, MLKS 11 and KML 7404 have been released in wheat from Punjab. The first two varieties involve 8 closely related lines and the third one involves 9 closely related lines.
Demerits:
1. The produce of multiline varieties is less attractive and less uniform due to mixture of several pure-lines.
2. Development of multiline cultivars involves several backcrosses and hence is costlier than conventional breeding methods.
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