Principles of Plant Genetics and Breeding

286 CHAPTER 16
Pedigree 2: MK2-57*3/SV-2.
Equivalent formula: MK2-57/3/MK2-57/2/MK2-
57/SV-2.
Interpretation: the genotype MK2-57 was backcrossed
three times to genotype SV-2.
Mass selection
Mass selection is an example of selection from a biologically
variable population in which differences are genetic
in origin. The Danish biologist, W. Johansen, is credited
with developing the basis for mass selection in 1903.
Mass selection is often described as the oldest method of
breeding self-pollinated plant species. However, this by
no means makes the procedure outdated. As an ancient
art, farmers saved seed from desirable plants for planting
the next season’s crop, a practice that is still common
in the agriculture of many developing countries. This
method of selection is applicable to both self- and crosspollinated
species.
Key features
The purpose of mass selection is population improvement
through increasing the gene frequencies of desirable
genes. Selection is based on plant phenotype and
one generation per cycle is needed. Mass selection is
imposed once or multiple times (recurrent mass selection).
The improvement is limited to the genetic variability
that existed in the original populations (i.e.,
new variability is not generated during the breeding
process). The goal in cultivar development by mass
selection is to improve the average performance of the
base population.
Applications
As a modern method of plant breeding, mass selection
has several applications:
1 It may be used to maintain the purity of an existing
cultivar that has become contaminated, or is segregating.
The off-types are simply rogued out of the
population, and the rest of the material bulked. Existing
cultivars become contaminated over the years by
natural processes (e.g., outcrossing, mutation) or by
human error (e.g., inadvertent seed mixture during
harvesting or processing stages of crop production).
2 It can also be used to develop a cultivar from a base
population created by hybridization, using the procedure
described next.
3 It may be used to preserve the identity of an established
cultivar or soon-to-be-released new cultivar.
The breeder selects several hundreds (200–300) of
plants (or heads) and plants them in individual rows
for comparison. Rows showing significant phenotypic
differences from the other rows are discarded,
while the remainder is bulked as breeder seed. Prior
to bulking, sample plants or heads are taken from
each row and kept for future use in reproducing the
original cultivar.
4 When a new crop is introduced into a new production
region, the breeder may adapt it to the new
region by selecting for key factors needed for successful
production (e.g., maturity). This, hence, becomes
a way of improving the new cultivar for the new production
region.
5 Mass selection can be used to breed horizontal
(durable) disease resistance into a cultivar. The
breeder applies low densities of disease inoculum
(to stimulate moderate disease development) so
that quantitative (minor gene effects) genetic effects
(instead of major gene effects) can be assessed. This
way, the cultivar is race-non-specific and moderately
tolerant of disease. Further, crop yield is stable and
the disease resistance is durable.
6 Some breeders use mass selection as part of their
breeding program to rogue out undesirable plants,
thereby reducing the materials advanced and saving
time and reducing costs of breeding.
Procedure
Overview
The general procedure in mass selection is to rogue out
off-types or plants with undesirable traits. This is called
by some researchers, negative mass selection. The
specific strategies for retaining representative individuals
for the population vary according to species, traits of
interest, or creativity of the breeder to find ways to
facilitate the breeding program. Whereas rouging out
and bulking appears to be the basic strategy of mass
selection, some breeders may rather select and advance
a large number of plants that are desirable and uniform
for the trait(s) of interest (positive mass selection).
Where applicable, single pods from each plant may be
picked and bulked for planting. For cereal species, the
heads may be picked and bulked.
Steps
The breeder plants the heterogeneous population in
the field, and looks for off-types to remove and discard