Principles of Plant Genetics and Breeding

298 CHAPTER 16
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. One
other major application of single-seed descent is in conjunction
with other methods.
Procedure
Overview
A large F 1 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 F 5 or F 6 . Then,
each plant is used to establish a family to help breeders
in selection and to increase seed for subsequent yield
trials.
Steps
Year 1 Crossing is used to create the base
population. Cross selected parents to
generate an adequate number of F 1
for the production of a large F 2
population.
Year 2 About 50–100 F 1 plants are grown
in a greenhouse in the ground, on a
bench, or in pots. They may also be
grown in the field. Harvest identical
F 1 crosses and bulk.
Year 3 About 2,000–3,000 F 2 plants are
grown. At maturity, a single seed per
plant is harvested and bulked for
planting F 3 . Subsequently, the F 2
plants are spaced enough to allow
each plant to produce only a few
seeds.
Years 4–6 Single pods per plant are harvested to
plant the F 4 . The F 5 is space planted
in the field, harvesting seed from only
superior plants to grow progeny rows
in the F 6 generation.
Year 7 Superior rows are harvested to grow
preliminary yield trials in the F 7 .
Year 8 and later Yield trials are conducted in the
F 8 –F 10 generations. The most superior
line is increased in the F 11 and F 12
as a new cultivar.
Comments
1 If the sample is too small, superior genetic combinations
may be lost because only one seed from each
plant is used.
2 It may be advantageous to use progeny rows prior to
yield testing to produce sufficient seed as well as to
help in selecting superior families.
3 The breeder may choose to impose some artificial
selection pressure by excluding undesirable plants
from contributing to the subsequent generations (in
the early generations). This is effective for qualitative
traits.
4 Record keeping is minimal and so are other activities
such as harvesting, especially in the early generations.
Genetic issues
Each individual in the final population is a descendent
from a different F 2 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 as equally represented in the next generation as
one producing 1,000 seeds. Selection is conducted on
homozygous plants rather than segregating material.
An efficient early generation testing is needed to avoid
genetic drift of desirable alleles. Single-seed descent is
similar to bulk selection in that the F 6 /F 7 comprises a
large number of homozygous lines, prior to selection
among progenies. A wide genetic diversity is carried on
to relatively advanced generations (F 6 /F 7 ).
Advantages and disadvantages
Single-seed descent has certain advantages and disadvantages,
the major ones including the following.
Advantages
1 It is an easy and rapid way to attain homozygosity
(2–3 generations per year).
2 Small spaces are required in early generations (e.g.,
can be conducted in a greenhouse) to grow the
selections.
3 Natural selection has no effect (hence it can not
impose an adverse impact).
4 The duration of the breeding program can be
reduced by several years by using single-seed descent.