The method allows the breeder to advance the
maximum number of F2 plants through the F5 generation. This is achieved by advancing
one randomly selected seed per plant through the early segregating stages. The
focus on the early stages of the procedure is on attaining homozygosity as
rapidly as possible, without selection. Discriminating among plants starts
after attainment of homozygosity.
Growing plants in the greenhouse under artificial conditions
tends to reduce flower size and increase cleistogamy. Consequently, single seed
descent is best for self-pollinated species. It is effective for breeding small
grains as well as legumes, especially those that can tolerate close planting
and still produce at least one seed per plant. Species that can be forced to mature
rapidly are suitable for breeding by this method. It is widely used in soybean
breeding to advance the early generation.
A large F1 population is generated to ensure
adequate recombination among parental chromosomes. A single seed per plant is
advanced in each subsequent generation until the desired level of inbreeding is
attained. Selection is usually not practiced until F5 or F6. Then, each plant is
used to establish a family to help breeders in selection and to increase seed
for subsequent yield trails.
Each individual in the final population is
descendent from a different F2 plant. Each of these plants undergoes a decrease
in heterozygosity at a rapid rate, each generation. Barring the inability of a
seed to germinate or a plant to set seed, the effect of natural selection is
practically non-existent in the single seed descent procedure. Only one seed
per plant is advanced, regardless of the number produced. That is, a plant
producing one seed is equally represented in the next generation as one
producing 1000 seeds. Selection is conducted on homozygous plants rather than
segregating material. An efficient early generation testing is needed to avoid
genetic drift of desirablealleles. Single seed descent is similar to bulk selection
in that the F6/F7 comprises a large number of homozygous lines, prior to
selection among progenies. A wide genetic diversity is carried on to relatively
advanced generations.
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