WHY DEVELOP SYNTHETIC VARIETIES

After reviewing the known advantages and disadvantages of synthetic varieties, the value of the best possible synthetic varieties from a given population has been compared to that of the best single-cross hybrids (or the best clones) and to that of the best lines. ft has been demonstrated that the best synthetic varieties are always inferior to the best single-cross hybrids and hardly superior to the original population (from 0 to 10%), and that this inferiority is more accentuated the higher the inbreeding depression. In fact, selection between varieties or its equivalent hardly compensates for the inbreeding depression if the latter is sufficiently high. If, however, it is weak, then the development of line varieties is mora justified. The threshold depends on the genetic variances among lines and among hybrids. Under conditions in which synthetic varieties are most justified, the original population could be taken as a variety without significant loss of potential gain. To develop synthetic varieties, ft is therefore necessary to concentrate on recurrent selection rather than on selection between narrow-base varieties in order to improve the overall value of the population.