Factors affecting performance of synthetic cultivars

Three factors are key in determining the performance of a synthetic cultivar.

Number of parental lines used. Synthetic cultivars are maintained by open pollination. Consequently, the F2 yield should be high to make it a successful cultivar. The Hardy– Weinberg equilibrium is reached in syn-2 for each individual locus and, hence, should remain unchanged in subsequent generations. It follows then that the F3 should produce as well as syn-2. Some researchers have even shown that F3 and F4 generations yielded as much or slightly better than F2 generations, provided the number of lines included in the synthetic cultivar is not small. With nผ2, the reduction in performance will be equal to 50% of the heterosis. Consequently, n has to be increased to an optimum number without sacrificing high GCA. When n is small, the yields of syn-1 are high, but so are the decline in syn-2 yields. On the other hand increasing n decreases syn-1 yields and syn-2 decline. A balance needs to be struck between the two effects. Some researchers estimate the

optimum number of lines to include in a synthetic to be five or six.

Mean performance of the parental lines. The lines used in synthetics should have high performance. A high value of parental lines reduces the reduction in performance of syn-2 over syn-1. Preferably, the parents should be non-inbreds or have minimum inbreeding.

Mean syn-1. In theory, the highest value of syn-1 is produced by a single cross. However, alone it will suffer from a higher reduction in performance. It is important for the mean F1 yield of all the component F1 crosses to be high enough such that the syn-2 yield would remain high in spite of some decline.

Advantages and limitations of development of synthetics

Advantages

_ The method is relatively easy to implement.

_ It can be used to produce variability for hybrid breeding programs.

_ Advanced genotypes of synthetics show little yield reduction from syn-1, making it possible for farmers to save and use seed from the current season to plant the next season.

Disadvantages

_ Because inadequate seed is often produced in syn-1, the method fails to exploit to the maximum the effects of heterosis, as is the case in conventional F1 hybrid breeding. The method of synthetics is thus a compromise to the conventional means of exploiting heterosis. 352 CHAPTER 17

_ Natural selection changes the genotypic composition of synthetics, which may be undesirable.

Backcross breeding

The key concern in the application of the backcross technique to cross-pollinated species is the issue of inbreeding. Selfing cross-pollinated species leads to inbreeding depression. The use of a recurrent parent in a backcross with crosspollinated species is tantamount to inbreeding. To minimize the loss of vigor, large populations should be used to enable the breeder sample and maintain the diversity of the cultivar and to insure against the harmful effects of inbreeding. Just like self-pollinated species, it is relatively straight forward to improve a qualitative trait conditioned by a single dominant gene. The breeder simply selects and advances individuals expressing the trait. Where a recessive gene is being transferred, each backcross should be followed by one round of intercrossing to identify the recessive phenotype. Improving inbred lines is equivalent to improving self-pollinated species. The key to success is for the breeder to maintain a broad gene base by using adequate number of backcrosses and a large segregating population.