Development of synthetic cultivars

1. Synthetic cultivar versus germless composites

There are two basic types of open-pollinated populations of crops – those produced by population improvement, and synthetics. As previously discussed, population improvement methods can be categorized into two – those that depend on purely phenotypic selection and those that involve selection with progeny testing. A synthetic cultivar may be defined as an advanced generation of crossfertilized seed

mixture of parents that may be strains, clones, or hybrids. The parents are selected based on GCA. The primary distinction between these basic types of populations mentioned in this section is that population improvement cultivars can be propagated indefinitely as such. However, a synthetic cultivar is propagated for only a limited number of generations and then must be reconstituted from the parental stock. A synthetic population differs from a natural population by consisting of breeder-selected parental stocks. Germplasm composites is a broad term used to refer to the mixing together of breeding materials on the basis of some agronomic trait, followed by random mating. There are many ways to put a composite together. Germplasm composites are by nature genetically broad based and very complex. They can be

used as for commercial cultivation over a broad range of agroecological environments. However, they can also be used as reservoirs of useful genes for use in breeding programs.

2 Desirable features of a synthetic cultivar

K.J. Frey summarized three major desirable features of synthetic cultivars as:

Yield reduction in advanced generations is less than with a single or double cross. For example,in maize an estimated 15–30% reduction occurs between F1 and F2, as compared to only a reduction of 5–15% from syn-1 to syn-2. This slow rate of reduction in yield makes it unnecessary for producers to obtain new seed of the cultivar for planting in each season.

A synthetic cultivar may become better adapted to the local production environment over time, as it is produced in successive generations in the region.

A synthetic cultivar is genetically heterogeneous, a population structure that makes it perform stably over changing environmental conditions. Further, because of this heterogeneity, both natural and artificial selection can modify the genotypic structure of synthetic cultivars. That is, a breeder may achieve gain in performance by practicing selection in syn-2 and subsequent generations.

3 Application

The synthetic method of breeding is suitable for improving cross-fertilized crops. It is widely used to breed forage species. Successful synthetic cultivars have been bred for corn, sugar beets, and other species. The suitability of forage species for this method of breeding stems from several biological factors. Forages have perfect flowers, making it difficult to produce hybrid seed for commercial use. The use of male sterility may facilitate controlled cross-pollination, which is difficult to achieve in most forage species. To test individual plants for use in producing the commercial seed, it is essential to obtain sufficient seed from these plants. The amount of seed obtained from single plants of these species is often inadequate for a progeny test. Furthermore, forage species often exhibit self-incompatibility, a condition that inhibits the production of selfed seed. Synthetic cultivars are also used as gene pools in breeding progeny. Synthetic cultivars are advantageous in agricultural production systems where farmers routinely save seed for planting. One of the well-known and widely used synthetic is the Iowa stiff-stalk synthetic of maize.