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.
Definition of Synthetic Variety:
A Variety which is produced by crossing in all combination a number of inbred lines that combine well with each other. Once synthesized, a synthetic is maintained by open-pollination in isolation is referred as synthetic variety.
Hayas and Garber suggested the commercial utilization of synthetic varieties in Maize in 1919. Synthetic varieties have been of great value in the breeding for those cross – pollinated crop, where pollination control is difficult. E .g Alfalfa, cloves, forage crop species etc. Even in maize improvement synthetic varieties are becoming increasingly important.
A synthetic varieties can 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 varieties. Generally 5-8 good general combining inbreds are used to constitute a synthetic variety. Synthetic variety consists of several heterozygous initially. Since subsequently the variety is maintained by open pollination, some degree of selfing occurs resulting in fixation of some genes. A result in later generation synthetic variety consists of several heterozygotes. Thus a synthetic variety has a heterogeneous population.
Steps in Development of Synthetic Variety:
Development of synthetic variety consists of three major steps. i.e.
1) Evaluation of lines for GCA.
2) Production of synthetic variety,
3) Multiplication of synthetic variety.
1) Evaluation of Lines for GCA:
The lines that make up a synthetic variety may be inbred lines, clones, open pollinated variety of short term inbred lines. Inbred lines are evaluated for general combining ability because synthetic variety exploit that portion of heterosis which is produced by GCA. There are three different methods of evaluating, which is produced by GCA. There are three cross, Polycross and single cross.
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 Polycross, selected inbreds are allowed to intermated by open pollination in isolation and in single cross all possible single crosses are made among selected inbreds. These crosses are evaluated for GCA of yield in replicated that using local variety as a check. Thus, inbred lines with good GCA are identified and finally selected for development of synthetic variety.
2) Production of Synthetic Variety:
A synthetic variety may be produced in one of the following two ways:
a) Equal amount of seed from the parental lines (syno) are mixed and planted in isolation. Open- pollination is allowed and produce crosses in all combinations. The seed from this population is harvested in bulk; the population raised from this seed is the Syn1 generation.
b) All possible crosses among the selected lines are made in isolation. Equal amount of seed from each cross is composite to produce the synthetic variety. The population derived from this composited seed is known as synthetic one generation.
c) Multiplication of Synthetic Variety: After a synthetic variety has synthesized. It is multiplied in isolation for one or more generation, before its distribution for cultivation. This is done to produce commercial quantities and is a common practical in most of the crops. E. g Grasses, Clover, Maize.
The open – pollinated progeny from the syn1 generation is termed as syn1that, syn2 as syn3 etc. the performance of syn2 is expected to be lower than that of syn1 due to the production of new genotype and decrease. In heterozygosity as a consequences of random mating. However, there would net be a noticeable decline in the subsequent generations produced by open – pollination (syn3, syn2, syn5, etc). Since the zygotic equilibrium for any gene is reached after one generation of random mating. The synthetic varieties are maintained by open pollinated, seed, and may be further improved through population improvement, particularly recurrent selection.