Breeding Method in Asexually Propagated Crop – Hybridization
Clonal crops are generally improved by crossing two or more desirable clones, followed by selecting in the F1 progeny and in the subsequent clonal generations. Once the F1 has been produced, the breeding procedure is essentially the same as clonal selection. The improvements through hybridization involved three steps: 1) Selection of parents, 2) Production of F1 progeny, and 3) Selection of superior clones.
Selection of Parents:
The selection of parents to be used in hybridization is very important since the value of F1 progeny would depend upon the parents used. Parents are generally selected on the basis of their known performance both as varieties and as parents in hybridization programmes. The performance of a clone hybridization programme depends on its propotency and general combining ability. It would be highly desirable to know the relative values of GCA and SCA in the crops to be improved. If GCA is more important, a small number of parents with good GCA should be used in hybridization programmes. On the other hand , when SCA is more important ,a large number of parents should be used to produce a large number of F1 families.
Production of F1 Progeny:
Generally clonal crops are cross pollinated and they may show self- incompatibility. The selected parents may be used to produce single crosses involving two parents or an equivalent of a Polycross involving more than two parents.
Selection among F1 Families:
When the breeding value of parents is not known and the relative contribution of GCA and SCA is not available, a large number of crosses have to be made in order to ensure that at least some of the crosses would produce outstanding progeny in F1. This is particularly true in a species, where crop improvement has not been done or has been done at a small scale. The general worth of individual F1 families or populations is estimated visually. The presence of outstanding individuals in these families is also noted. Inferior families are eliminated. Promising families with outstanding individuals are then grown at a much larger scale for selection. The procedure is designed to save time, space and labour by planting only small populations of large number of crosses at the preliminary stage.
Selection within F1 Families:
The selection procedure within F1 families is essentially the same as that in the case of clonal selection.
First Year:
Clones to be used as parents are grown and crosses are made to produce F1 progeny.
Second Year:
Sexual progeny from the cross i.e seedlings obtained from seeds are grown. Undesirable plants are eliminated. Few hundreds to few thousand desirable plants are selected.
Third Year:
Clones from individual plants are grown separately. Poor are clones are eliminated. Upto 200 superior clones may be selected for preliminary yield trial.
Fourth Year:
a replicated preliminary yield trial is conducted. A suitable check is included for comparison. Few outstanding clones are selected for trails at several locations.
Fifth to Seventh Year:
Replicated yield trials are conducted at several locations. A suitable check is included for comparison. One or few outstanding clones are identified and released as new varieties.
Eighth Year:
The clones released as varieties are multiplied and distributed among farmers.