Although not all breeders would consider this a separate stage in a breeding program, the
steps listed in Table 3.1 technically begin after the last generation in which a family can be
traced to the progeny from a single plant (thefounding generation). If that were the F 5
generation, then we would consider a resulting variety to be F 5 -derived. Since homozygos-
ity continues to increase with each generation of selfing, breeders often use the notation
FX:Y, whereXdenotes the founding generation andYdesignates generations of selfing
and bulking that follow the founding generation. Varieties developed from early founding
generations (i.e., a relatively heterozygous founding plant) can show a considerable amount
of heterogeneity, especially in molecular traits that do not undergo selection.Rogueing(the
culling of undesirable plants or off-types) is done in generations following the founding
generation, and this reduces the amount of heterogeneity. However, it is common for phe-
notypic variation to show up within a variety, sometimes by surprise, when a new environ-
ment is encountered. For this reason, varieties may need to be described in terms of their
range in characteristics, and descriptions based on molecular traits are increasingly favored.
Although they are not technically part of a breeding program, many jurisdictions have
testing and recommendation procedures that enable comparison of plant varieties from
different breeding programs with one another under different conditions (years, locations,
and management practices) culminating in reports and recommendations that allow agricul-
tural producers to select varieties that are most adapted to their conditions. The agricultural
producer has a job that is parallel to that of the plant breeder: making the final selection
among numerous plant varieties that are available for production on the farm.
3.4.3. Outcrossing Species
Since pure lines cannot easily be maintained in a naturally outcrossing species, the devel-
opment and release of varieties in an outcrossing species is quite different from that in a
self-pollinated species. Rather than identifying the “perfect genotype,” the objective is to
identify the perfect set of genes that work happily together in a random mating population.
Some outcrossing plant species, such as rye, can tolerate a high degree of inbreeding, and
can be effectively bred and grown as if they were self-pollinated species. The primary
difference is the increased need for isolation (to prevent uncontrolled outcrossing) during
variety development and seed production. However, other species do not tolerate inbreed-
ing, or they have specific mechanisms to prevent it. Matings between different plants often
produce offspring that are more fit than the parents, a concept calledhybrid vigororhetero-
sis. Thus, maintaining a heterogeneous population in a random mating state is beneficial.
However, there must still be some opportunity to select the breeding population such
that it produces relatively uniform progeny that have desired and predictable characteristics.
3.4.3.1. Mass Selection.Historically, varieties of outcrossing species were improved
in the same way that landraces of self-pollinated species were improved: by saving seed
in bulk, or by saving seed from selected plants. Either way, selection would have taken
place. For example, we know that selection for seeds that do not fall off the plant before
manual harvest (called “shattering”) was one of the earliest traits selected in the process
of crop domestication. This most likely happened simply because genotypes with seeds
that shattered were rapidly eliminated as soon as early agronomists started planting crops
intentionally from seed that was harvested for food. Other traits, such as lack of seed dor-
mancy, seed size, early flowering, and height, were probably selected in similar ways,
whereas traits such as fruit flavor may have required a more deliberate effort to propagate
favored genotypes. All traditional selection such as this is termed mass selection
3.4. METHODS OF PLANT BREEDING 69