In this article we will discuss about:- 1. Definition of Synthetic Varieties 2. Features of Synthetic Varieties 3. Steps 4. Factors 5. Merits and Demerits.
Definition of Synthetic Varieties:
In practical plant breeding, heterosis can be fully exploited in the form of hybrid in cross pollinated species and also in some self-pollinated crops. In cross pollinated species, heterosis can also be exploited partially in the form of synthetic and composite varieties.
A variety which is developed by intermating in all possible combinations a number of inbred lines with good general combining ability and mixing the seed of F1 crosses in equal quantity is referred to as synthetic variety. The use of synthetic varieties for commercial cultivation was first suggested in maize. After release, synthetic varieties are maintained by open pollination.
Features of Synthetic Varieties:
Main features of synthetic varieties are given below:
1. Relevance:
Synthetic varieties are relevant to cross pollinated crops. Such varieties are developed in crops like maize, pearl-millet, alfalfa, and many others cross pollinated species.
2. Base Material:
A synthetic variety can be developed from inbreds, clones and open pollinated varieties. The end products of recurrent selection which are already tested for GCA are generally used to constitute synthetic variety. Generally, 5-8 good general combining inbreds are used to constitute a synthetic variety.
3. Genetic Concept:
The basic concept in the development of synthetic varieties is exploitation of heterosis or hybrid vigour. Such varieties are constituted from good general combining inbreds. However, heterosis is partially utilized by synthetic varieties because some level of inbreeding takes place due to open pollination in later generations. Synthetics exploit more of additive gene action, whereas hybrids exploit more of non-additive (over-dominance and epistatic) gene action.
4. Genetic Constitution:
A synthetic variety consists of several heterozygotes initially. Since subsequently the variety is maintained by open pollination, some degree of selfing occurs resulting in fixation of some genes. As a result, in later generations a synthetic variety consists of several heterozygotes and homozygotes. Thus synthetic variety has a heterogeneous population.
5. Adaptation:
Synthetic variety constitutes a polymorphic and stable population. Hence synthetic varieties are highly adaptable to environmental variations. In other words, synthetic varieties provide stable yield in the fluctuating environments.
6. Disease Resistance:
Synthetic varieties have better resistance to plant diseases due to their heterogeneous nature and broad genetic base.
7. Reconstitution:
A synthetic variety can safely be grown for a period of 4-5 years without reduction in the yield potential. The yielding ability can be maintained in advanced generations by mass selection. Thus farmers can use their own seed for five years. After a period of five years, it would be desirable to reconstitute the synthetic variety. The reconstitution should be based on new developments and new requirements.
8. Yield Level:
The yield of synthetic varieties is always higher than open pollinated parental variety but lower than the yield of single and double cross hybrids. The main advantage of synthetic varieties is that their seed is much cheaper than those of hybrids.
9. Designation:
The F1, F2, F3, F4 and F5 generations of a synthetic variety are designated as Syn1, Syn2, Syn3 Syn4 and Syn5 respectively.
Synthetic varieties differ from mass selected varieties and lines breeding in the way of selection of component genotypes. The constituent genotypes are selected on the basis of general combining ability for a synthetic variety, whereas combining ability is not tested in mass selected varieties and line breeding.
In mass selected varieties the component genotypes are selected on the basis of phenotypic performance and in the line breeding on the basis of progeny performance. Moreover, the synthetic variety is developed by crossing selected genotypes in all possible combinations and mixing the seeds of all F1 crosses in equal quantity. Such crosses are not made to develop mass selected or line breeding varieties.
Steps in Development of Synthetic Variety:
Development of synthetic varieties consists of four major steps, viz.:
(1) Isolation of inbred lines,
(2) Evaluation of inbred lines for general combining ability,
(3) Intermating of good general combining inbreds in all possible combinations, and
(4) Mixing the seed of all F1 crosses in equal quantity.
(1) Isolation of Inbreds:
Various materials, viz. inbred lines, clones, open pollinated varieties and material developed by recurrent selection are used for development of synthetics. Jenkins (1940) suggested that inbred lines with one generation selfing can be used for development of a synthetic variety.
The synthetic variety developed from inbred lines can be reconstituted exactly when the parental material is inbreds or clones. The exact reconstitution is not possible when the parental lines are open pollinated population or short term inbreds because short term inbreds are also heterozygous for many gene loci.
(2) Evaluation of Inbreds for GCA:
Inbred lines are evaluated for general combining ability. There are three methods of evaluating inbred lines for general combining ability (gca).
These are: top cross method, poly cross method, and single crosses. In top cross, the inbreds are crossed with a common tester and the progeny are evaluated in replicated trials for general combining ability of yield and yield contributing characters. In poly cross, selected inbreds are allowed to intermate by open pollination in isolation.
The top cross progeny are evaluated for gca of yield using local check in replicated trial. In the third method, all possible single crosses are made among selected inbreds. These crosses are evaluated for gca of yield in replicated trial using local variety as a check. Thus inbred lines with good general combining ability are identified and finally selected for development of synthetic variety.
The first two methods are in common use for evaluation of inbreds in terms of gca, because large number of inbreds can be evaluated by these methods. Single cross method (diallel crosses) can evaluate only limited inbreds at a time.
(3) Intermating of Good General Combining Inbreds:
Inbred lines selected for superior gca are crossed in all possible combinations. The all possible single crosses would be n (n – 1)/2, where n is the number of inbred lines. With 6 inbred lines, there would be 6(6 – 1)/2 = 15 single crosses. The seed of each cross is obtained in adequate quantity to produce a synthetic variety.
(4) Mixing of F1 Seeds:
The seed of all possible F1 crosses made between the selected inbred lines is mixed together in equal quantity or equal number to constitute synthetic variety. The variety thus developed is called as Syn1.The seed of such variety is generally multiplied by open pollination in isolation for one or two generations (Syn1 and Syn2) and then distributed to the farmers for commercial cultivation (Syn3). Procedure of developing synthetic cultivar from already available inbred lines is briefly presented in Table 23.1.
Factors Affecting Performance of Synthetic Varieties:
There are three main factors which affect the performance of synthetic varieties in advanced generations.
These factors are:
(1) The number of parental lines included,
(2) The mean performance of these parental lines, and
(3) The mean performance of all possible crosses among the n lines.
(1) Number of Inbred Lines:
The performance of synthetic varieties depends on the number of inbred lines which constitute such variety. In maize, it was observed that the yield of synthetics increased by increasing the number of inbred lines upto 5 and thereafter the yield decreased.
The decrease in yield by inclusion of inbreds beyond 5 resulted due to decrease in prepotency. The prepotency is compensated upto 5 inbred lines due to increase in variability and thereby in heterzygosity. The highest yielding synthetic variety was obtained when only 5 or 6 best combining inbreds were included.
(2) Mean Performance of Inbreds:
The mean performance of parental inbreds also affects the yield potential of synthetic varieties. A positive association is found between the yield of a synthetic variety and the yield of its component lines. High yeiding and vigorous inbreds give rise to high yielding synthetics. On the other hand, low yielding synthetic varieties are obtained when low yielding and less vigorous lines are used as constituent lines.
The performance of inbred lines can be evaluated while testing them for combining ability. The high combining inbreds can be identified on the basis of top cross performance. The yields of synthetic varieties are the highest when the yields of parental lines are high. Moreover, smaller numbers of parental lines are required for the development of high yielding synthetics when the yields of parental lines are high.
(3) Mean Performance of F1 Crosses:
The third factor which influences the yield potential of synthetic varieties is the mean performance of F1 crosses among selected inbred lines. High performing F1 crosses are expected to give rise to high yielding synthetic varieties. Thus improvement in the mean yield of F1 will enhance the yield level of synthetic variety.
Merits and Demerits of Synthetic Varieties:
Merits:
1. Use of synthetic varieties permits appreciable exploitation of heterosis in those cross pollinated species where hybrid seed production is very difficult.
2. The seed of synthetic varieties is much cheaper than single or double cross hybrids. Moreover, the seed can be afforded even by small farmers.
3. Synthetic varieties are more adaptable to environmental changes than hybrids due to greater variability and broad genetic base.
4. Synthetic varieties have vast genetic variability which provides then better protection from the infestation of new races of a disease.
5. There is no need to purchase fresh seed every year. Farmers can use their own saved seed for 4-5 years.
Synthetic varieties have been developed in cross pollinated crops like maize, pearl-millet, sunflower, sugar-beet, alfalfa, lucerne and several other crops in USA. In India, Synthetic varieties have been developed in pearl millet at ICRISAT and in sugar beet at Pantnagar University. In sugar-beet, Pant synthetic 3 is worth mentioning. In cauliflower synthetic 3 has been developed.
Demerits:
1. The produce of synthetic varieties is generally less uniform and less attractive than hybrids due to greater variability and heterogeneity.
2. The yield of synthetic varieties is generally poorer than single or double cross hybrids, because of partial exploitation of heterosis. They exploit GCA only.
3. Synthetic varieties are utilized in cross pollinated species only, whereas hybrids can be developed in both cross and self-pollinated species.
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