The use of synthetic varieties for commercial cultivation was first suggested in maize (Hayes and Garber, 1919). After release, synthetic varieties are maintained by open pollination.
Definition of Synthetic Variety:
A Synthetic Variety may be define as varieties that is developed by intermating in all possible combinations many inbred lines with good general combining ability and mixing the seed of F1 crosses in equal quantity is called a synthetic variety.
Synthetic Variety:
In practical plant breeding, heterosis can be fully exploited in the form of hybrids 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.
The main features of synthetic varieties are given below:
(i) Relevance:
Synthetic varieties are relevant to cross-pollinated crops. Such varieties are developed in crops like maize, pearl millet, alfalfa, and many other cross-pollinated species.
(ii) Base material:
A synthetic variety can be developed from inbreeds, 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 inbreeds are used to constitute a synthetic variety.
(iii) Genetic concept:
The basic concept in the development of synthetic varieties is an exploitation of heterosis or hybrid vigor. Such varieties are constituted from good general combining inbreeds. 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 additive gene action, whereas hybrids exploit more of non-additive (overdominance and epistatic) gene action.
(iv) 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 the 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.
(v) 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 yields in fluctuating environments.
(vi) Disease resistance:
Synthetic varieties have better resistance to plant diseases due to their heterogeneous nature and broad genetic base.
(vii) Reconstitution:
A synthetic variety can safely be grown for 4-5 years without a reduction in the yield potential (Lonnquist and McGill, 1956). The yielding ability can be maintained in advanced generations by mass selection. Thus farmers can use their seed for five years. After five years, it would be desirable to reconstitute the synthetic variety. The reconstitution should be based on new developments and new requirements.
(viii) Yield level:
The yield of synthetic varieties is always higher than open-pollinated parental varieties 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.
(ix) Maintenance:
After release, a synthetic variety is maintained by open pollination.
(x) Designation:
The F1, F2, F3, F4, and F5 generations of a synthetic variety are designated ts Syn1, Syn2, Syn3, Syn4, and Syn5 respectively.
Steps in development of Synthetic Variety:
Development of synthetic varieties consists of four major steps: viz (i) isolation of inbred lines, (ii) evaluation of inbred lines for general combining ability, (iii) intermating of good general combining inbreeds in all possible combinations, and (iv) mixing the seed of al F₁ crosses in equal quantity.
(i) Isolation of inbreeds:
Various materials, viz., inbred lines, clones, open-pollinated varieties, and material developed by recurrent selection are used for the development of synthetics Jenkins (1940) suggested that inbred lines with one generation selfing can be used for the development of a synthetic variety. The synthetic variety developed from inbred lines can be reconstituted exactly when the parental material is inbred or clones. The exact reconstitution is not possible when the parental lines are open-pollinated populations or short-term inbreeds because short-term inbreeds are also heterozygous for many gene loci.
(ii) Evaluation of inbreeds 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 the top cross method, poly-cross method, and single crosses. In the top cross, the inbreeds are crossed with a common tester, and the progenies are evaluated in replicated trials in general combining the ability of yield and yield contributing characters. In poly-cross, selected reds can intermate by open pollination in isolation. The top cross progeny are evaluated for GCA of yield using local check-in replicated trials.
In the third method, all possible single crosses are made among selected inbreeds. These crosses are evaluated for CA of yield in replicated trials using a local variety as a check. Thus inbred lines with good general combining ability are identified and finally selected for the development of synthetic variety. The first two methods are in common use for the evaluation of inbreeds in terms of GCA. because a large number of inbreeds can be evaluated by these methods. Single cross method (diallel crosses) can evaluate only limited inbreeds at a time.
(iii) Intermating of good general combining inbreeds:
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.
(iv) Mixing of F1 seeds:
The seed of all possible F1 crosses made between the selected inbred lines is mixed in equal quantity or equal number to constitute a synthetic variety. The variety thus developed is called 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 (Syn2).
Procedure of developing synthetic variety from already available inbred line:
| Year | Main breeding Activity |
| 1 | Selection of inbred lines, crossing with a common tester, harvesting top-crossed seed separately. |
| 2 | Evaluation of top crosses in the replicated trial using standard hybrid or open-pollinated variety as check. Identification and selection of good general combining inbred lines based on top cross performance. |
| 3 | Making all possible single crosses among inbred lines selected for GCA and harvesting crossed seeds of single crosses separately. |
| 4-5 | Mixing seeds of single crosses in equal quantity and seed multiplication by open pollination in isolation for one or two generations. |
| 6 | Release as a new variety and distribution of seed to the farmers for commercial cultivation. |
Factor Affecting The performance of Synthetic Variety:
Three main factors affect the performance of synthetic varieties in advanced generations. These factors are (i) the number of parental lines included, (ii) the mean performance of these parental lines, and (iii) the mean performance of all possible crosses among the n lines.
1. Number of inbreed lines:
The performance of synthetic varieties depends on the number of inbred lines which constitute the variety. In maize, it was observed that the yield of synthetics increased by increasing the number of inbred lines up to 3, and thereafter the yield decreased (Kinman and Sprague, 1945). The decrease in yield by the inclusion of the beyond 5 resulted due to a decrease in prepotency. The prepotency is compensated ap inbred lines due to an increase in variability and thereby in heterozygosity. The highest yielding synthetic variety was obtained when only 5 or 6 best combining inbreeds were included.
2. Mean performance of inbreeds:
The mean performance of parental inbreeds also affects the yield potential of synthetic varieties. A positive association is found between the yield of a synthetic variety and its component lines. High-yielding and vigorous mem 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 inbreeds can be identified based on top cross performance. The yields of synthetic varieties are the highest when the yields of parental lines are high. Moreover, a smaller number of parental lines is required for the development of yielding synthesis when the yields of parental lines are high.
3. Mean performance of F1 crosses:
The third factor that influences the yield potential of synthetic varieties is the mean performance of F, crosses among selected inbred lines High performing F, crosses is expected to give rise to high-yielding synthetic varieties. The improvement in the mean yield of F1 will enhance the yield level of the synthetic variety
Merits of Synthetic Variety:
(1) Use of synthetic varieties permits appreciable exploitation of heteroses in the cross-pollinated species where hybrid seed production is very difficult.
(ii) The seed of synthetic varieties is much cheaper than single or double cross hybrid Moreover, the seed can be afforded even by small farmers.
(mm) Synthetic varieties are more adaptable to environmental changes than hybrids due to greater variability and broad genetic bases.
(iv) Synthetic varieties have vast genetic variability which provides them better prosecut from the infestation of new races of a disease.
(v) There is no need to purchase fresh seed every year. Farmers can use their own saved seed for 4-5 years
Demerits of Synthetic Variety:
1) The production of synthetic varieties is generally less uniform and less attractive hybrids due to greater variability and heterogeneity.
(ii) The yield of synthetic varieties is generally poorer than single or double hybrids, because of partial exploitation of heterosis. They exploit GCA only
() Synthetic varieties are utilized in cross-pollinated species only, whereas hybrid can be developed in both cross and self-pollinated species.
Achievements:
Synthetic varieties have been developed in cross-pollinated crops like maize, pearl millet, sunflower, sugarbeet, alfalfa, and several other crops in the USA. In India, synthetic varieties have been developed in pearl millet at ICRISAT and in sugar beet at Pant Nagar University. ding in sugarbeet, Pant Synthetic 3 is worth mentioning. In cauliflower, synthetic 3 has been developed.
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