exactly one random line in each following generation by taking a single seed from each F 2
family in every segregating generation. The result is that maximum genetic diversity is pre-
served until late generations when selection will commence, and no recordkeeping is
required. The SSD method can also be used to derive populations of nearly inbred lines
(NILs). These populations are useful in genetic experiments because segregation can be
considered random such that “good” lines can be contrasted with “bad” lines to identify
genetic determinants. However, it is this same feature that leads to the primary criticism
of SSD as a breeding method: poor material that might easily be removed in early gener-
ations continues to occupy space and resources in the breeding program.
3.4.2.3. Bulk Breeding Methods. Abulk breeding methodis any method whereby
generations are advanced by bulking and planting seed from the previous generation.
However, if all seed from a given generation is harvested, then there will likely be too
much seed to plant in the following generation, so some seed must be discarded or held
in reserve. The SSD method is actually a special type of bulk breeding whereby each gen-
eration is advanced by saving only one seed per plant from the previous generation. In other
methods of bulk breeding, the reduction of seed is achieved randomly, or through a selec-
tion process that is applied uniformly but indiscriminately (e.g., harvesting the earliest-
ripening plants, or sieving to keep the largest seed). Some breeders favor bulk breeding
as a method of generation advance because it is extremely simple. However, several
issues must be considered. If the intention is to preserve a random nonselected population,
Figure 3.9.The pedigree breeding method is used in self-pollinated species to derive pure-line
varieties when it is desirable to practice selection in early generations.
3.4. METHODS OF PLANT BREEDING 65