In the production of commercial hybrid varieties of sexually propagated species, inbreds are highly desirable in comparison to open-pollinated varieties or other populations with a broad genetic base. This is because of the following reasons:
- Inbreds can be maintained indefinitely without a change in their genotype, while the genetic make-up of open pollinated varieties is likely to be modified by the evolutionary forces;
- The hybrids derived from inbreds are homogeneous, or nearly so, year after year, while those produced from open-pollinated varieties are likely to be variable, and hence their performance cannot be accurately predicted;
- The uniformity of the inbred-derived hybrids is also desirable from the viewpoint of uniform quality of the produce. For these reasons, hybrid varieties in case of maize and other crops are produced almost exclusively from inbred lines. The operations involved in the production of hybrid varieties in such a case are:
- Development of inbred lines,
- Evaluation of inbreds, and
- Production of hybrid seed.
- Development Of Inbreds
Inbred lines are developed from a genetically variable population by continued inbreeding. The population from which inbreds are isolated is the source population. The source population is generally an open-pollinated variety, but it may as well be a synthetic, a single cross or a double cross.
- Isolation of inbreds by inbreeding
Inbreds are developed by a suitable system of close inbreeding. But self-pollination is desirable, wherever possible, as it leads to homozygosity very rapidly. The procedure for the isolation of inbreds through self-pollination is described below.
First year
A number of plants with desirable phenotypes are selected from a source population and are self-pollinated. The selected plants should be vigorous and free from diseases. They may be selected on the basis of their GCA estimates obtained by testing the performance of their testcross progeny. Experimental evidence clearly reveals that open pollinated plants differ in their GCA and that GCA can be successfully selected for.
Second year
About 30-40 plants are space planted from the selfed seed from each of the selected plants. Best plants are selected from the best progeny rows and are self-pollinated.
Third to Sixth years
The process of the second year is repeated. But as the number of generations of self-pollination increases, individual plant progenies would become more and more homogeneous. Consequently, in the later phases of inbreeding selection is primarily among the progenies rather than within the progenies. Most of the material would be discarded due to deficiencies and weaknesses, but a few outstanding lines would be maintained. These lines would be the inbreds that might be useful in a hybrid programme.
Seventh year
At this stage, individual plant progenies would be almost homogeneous, as they would be expected to be nearly homozygous. Selfing may be discontinued and the inbreds are generally maintained by sib-pollination.
- Development of Inbreds from Haploid Plants
This method presents a short -cut to the lengthy inbreeding programme for isolating inbreds. Haploid plants may also be obtained by parthenogenetic development of the male gamete, but frequency of such haploids is very low. A substantial number of inbreds developed by the haploid technique has been evaluated for combining ability. They are comparable to a random sample of the conventional inbreds developed by selfing.
- Selection during Inbreeding
Generally, a strict selection is practised during inbreeding. Close inbreeding leads to a rapid and random fixation of genes and does not allow the accumulation of desirable genes through recombination. It is generally accepted that selection accompanied with close inbreeding is more or less ineffective in increasing the frequency of desirable genes. But selection is highly effective for characters with high heritability, and in eliminating weak and undesirable lines. Thus selection during inbreeding is ineffective in improving the combining ability of inbreds, but is effective in improving the performance of inbreds themselves, which is an important factor in hybrid seed production.
- Evaluation of inbreds
The most important operation in a hybrid programme is the identification of inbreds that would produce an outstanding hybrid suitable for commercial use. And undoubtedly it is the most expensive operation in the development of hybrid varieties. The modern practice of inbred evaluation may be divided into the following four steps:
- Phenotypic evaluation,
- Top-cross test for GCA,
- Single cross test for SCA, and
- Prediction of double cross performance from the data on the performance of single crosses. These steps, briefly outlined below, are followed in the given order.
- Phenotypic evaluation
- Top-cross test
- Single cross evaluation
- Prediction on double cross performance
It is based on the phenotypic performance of inbreds themselves. It is highly effective for characters with high heritabilitiy, i.e., high GCA. To some extent, it is effective in improving the yielding ability of hybrids as the shows a small (usually 0.2) but positive correlation with the performance of their hybrids. Thus inbreds with very poor performance can be safely rejected. The performance of inbreds is tested in a replicated yield trial, and the inbreds showing poor performance are discarded.
A simple way of producing topcross seed in maize is to plant alternate rows of the tester and the inbreds to be tested. The inbreds are detasselled; the seed from the inbreds is harvested and it represents the topcross seed. The performance of the topcross progeny is evaluated in replicated yield trials, preferably over locations and years. Based on the topcross test, about 50% of the inbreds are eliminated. This reduces the number of inbreds to a manageable size for the next step.
The performance of single crosses is evaluated in a replicated yield trail, preferably over years and locations. Outstanding single crosses are identified and may be released as hybrid varieties where production of single crosses is commercially feasible. More commonly e.g., in the case of maize the performance of single crosses is used to predict the performance of double crosses.
The predicted performance of any double cross is the average performance of the four nonparental single crosses involving the four parental inbreds.
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