Line Varieties

Self-pollinating crops are usually bred as inbred lines following three different breeding methods. The aim is to breed a pure line after several generations of selfing or by DH production. All methods start with crosses and generation of F1 populations. During bulk breeding, plants are selfed for two or three more generations to produce lines with a high level of homozygosity. Then selection starts in the F4 or F5 generations. Alternatively, only a single seed is harvested from each plant during selfing generations. Plants are grown under conditions where ripening is accelerated to shorten the generation time (single seed descent).

Using the pedigree method, selection starts as early as the F2 generation, followed by successive selections until the F5 or F6 generation. Single plants are selected and their offspring tested in the field. Thus the genotype of the selected plant is determined by testing its offspring generation.

In all cases the result is an inbred line with >95% homozygosity. In the past decade the doubled haploid (DH) method became popular because plants with 100% homozygosity can be obtained from F1 plants using different methods like anther or microspore culture or pollination with inductor plants. Clearly, the transformation of DHs or the combination of DH production and transformation is desirable because homozygous plants are obtained in one step.

A limited number of lines are tested in the field under different environments and the line with the best performance is selected as a new variety. When there is heterosis and a male sterility system is available, hybrid breeding is an alternative. In general, however, heterosis is low for self-pollinating crops. There are only rare examples of transgenic lineal cultivars, e.g. herbicide-tolerant soybean. One reason is that a line can be easily propagated by the farmer so that he does not have to buy seeds every year, thus reducing the profitability of a transgenic variety.

6.4.2 Open-Pollinated Varieties

In the past, open-pollinated crops were bred as population varieties. Panmictic populations were grown and mass selection was performed. Seeds from selected plants were grown as an improved variety. Mass selection was repeated to further improve the populations.

Today open-pollinated crops are mostly bred as synthetic varieties (synthetics) or hybrids. Synthetics are open-pollinated varieties when a limited number of selected parents are used for seed production. The parents can be inbred lines or clones. This method is used when a male sterility system is lacking, however cross-pollination between parental components must be guaranteed (e.g. by self incompatibility).

6.4.3 Hybrid Varieties

Hybrid breeding is the mostly preferred breeding method for open-pollinated crops today. Thus, many genetically modified varieties are hybrids. Hybrid breeding

Gene pool A

Gene pool B

Gene pool A

Inbred lines

Tester

Gene pool B

Topcross test ^ 1

Single cross test

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