Primary steps in the development of a synthetic

There are three primary steps in the development of a synthetic:

Assembly of parents.

Assessment of GCA.

Random mating to produce synthetic cultivars.

The parents used in synthetics may be clones or inbred lines. Whereas forages can be increased indefinitely by clonal propagation, inbred lines are needed to perpetuate the genotypes used in hybrid production. The parental materials are reproducible and may be substituted with new genotypes as they become available, for some improvement in the synthetic cultivar. The parents are selected after progeny testing or general combining ability analysis using a test cross or topcross, but most frequently a polycross, for evaluation.

1 Test for GCA

Testers

_ Polycross. A polycross test is generally preferred because it is simple and convenient to conduct and also, by nature, provides an efficient estimate of GCA, a desired attribute in synthetic production. Furthermore, it allows an adequate amount of seed to be obtained for more comprehensive testing using commercial standards. It provides a greater insurance to cultivars against genetic shifts that could arise during seed increase. However, any significant amount of selfing or non-random cross-pollination could result in bias. The component clones may vary in self-fertility and other biological characteristics that impact fertilization. To minimize such deviations from a perfect polycross, the Latin square design may be used to establish the polycross nursery. In theory, the polycross allows each clone in the nursery to be pollinated by about the same pollen sources as a result of random pollination from all the entries in the same plot.

_ Topcross. Selected clones are grown in alternative rows with an open-pollinated cultivar as tester. The test cross seed includes both selfs and intercrosses among the clones being evaluated.

_ Diallel cross. A diallel cross entails achieving all possible single crosses involving all the parents. This is laborious to conduct. It requires that each parent

be grown in isolation. It provides information on both GCA and SCA.

Procedure

A procedure for crops in which selections are clonally propagable is as follows:

_ Year 1: The source nursery. The source population consists of clones. The source nursery is established by planting several thousands of plants assembled from many sources to provide a broad genetic base of the clonal lines for selection.

The germplasm in the nursery is screened and evaluated to identify superior individuals according to the breeding objectives.

_ Year 2: Clonal lines. The breeder first selects 100 to 200 superior plants on phenotypic basis to multiply clonally to produce clonal lines. A clonal line nursery is established, each line consisting of about 20–25 plants derived from the same parental line.The breeder may impose various biotic and abiotic selective pressure

to aid in identifying about 25–50 most desirable clones.

_ Year 3: Polycross nursery. The selected clonal lines are planted in a polycross nursery to generate seed for progeny testing. Ideally, the layout of the polycross in the field should allow each clone to be pollinated by a random sample of pollen from all the other entries. A method of layout to achieve this objective is a square plot in which every clone occurs once in every row. Covering with a fine mesh tent or separating the plots by adequate distance isolates each square plot. The mesh is removed once the pollination period is over. A large number of replications of the single randomized clones is suggested for achieving a highly mixed pollination. Seed from each clone is harvested separately. The polycross test is valid if the layout ensures random interpollination. Alternative methods of evaluating clones for quantitatively inherited traits are available. Self-fertilization may

be used but it often yields a little amount of seed. A diallel cross is cumbersome to conduct, especially for large entries. A topcross evaluates SCA. The polycross is used because it evaluates GCA.

_ Year 4: Polycross progeny test. Seed is harvested from the replicated clones and bulked for planting progeny rows for performance evaluation. The progeny test evaluates yield and other traits, according to the breeding objective. The top performing 5–10 clones are selected for inclusion in the synthetic cultivar.

_ Year 5: Syn-0 generation. The selected clones are vegetatively propagated and randomly transplanted into an isolated field for cross-fertilization to produce syn-0 seed. Leguminous species may be isolated in an insect-proof cage and cross fertilized by using insects.

_ Year 6: Syn-1 generation. The syn-0 seed is increased by planting in isolation. Equal amount of seed is obtained from each parent and mixed to ensure random mating in the field. Bulk seed is harvested from seed increased in syn-1 generation that may be released as a commercial cultivar provided sufficient seed is produced.

_ Year 7: Subsequent syn generations. Frequently, the syn-1 seed is not sufficient to release to farmers. Consequently, a more practical synthetic breeding

scheme is to produce syn-2 generation by openpollinated increase of seed from syn-1. The syn-2 seed may be likened to a breeder seed. It is further increased to produce syn-3 and syn-4. Commercial seed classes are discussed in detail in Chapter 27. The pattern of loss in vigor, progressively with advancement of generations from syn-1, syn-2, and syn-n, is similar to what occurs when hybrids are progressively selfed from F1, F2 . . . Fn generations. It is important to maintain the original clones so that the synthetic can be reconstituted as needed. The steps described are only generalized and can be adapted and modified according to the species and the objectives of the breeder.