Principles of Plant Genetics and Breeding

316 CHAPTER 17
Key features
The selection units are individual plants. Selection is
solely on phenotypic performance. Seed from selected
plants (pollinated by the population at large) are bulked
to start the next generation. No crosses are made, but a
progeny test is conducted. The process is repeated until
a desirable level of improvement is observed.
Procedure
Year 1 Plant the source population (local variety, synthetic
variety, bulk population, etc.). Rogue out
undesirable plants before flowering, and then
select several hundreds of plants based on the
phenotype. Harvest and bulk.
Year 2 Repeat year 1. Grow selected bulk in a preliminary
yield trial, including a check. The check
is the unselected population (original), if the
goal of the mass selection is to improve the
population.
Year 3 Repeat year 2 for as long as progress is made.
Year 4 Conduct advanced yield trials.
The mass selection may be longer, depending on the
progress being made.
Genetic issues
The effectiveness of the method depends on the heritability
of the trait since selection is solely on the phenotype.
It is also most effective where additive gene action
operates. Effectiveness of mass selection also depends on
the number of gene involved in the control of the trait
of interest. The more additive genes are involved, the
greater the efficiency of mass selection. The expected
genetic advance through mass selection is given by the
following (for one sex – female):
∆G m = [( 1 / 2)iσ 2 A ]σ p
= [( 1 / 2)iσ 2 A ]/[σ2 A +σ2 D +σ2 AE +σ2 DE
+σ 2 e +σ2 me ]
where σp = phenotypic standard deviation in the population,
σ2 A = additive variance, σ2 D = dominance variance,
and the other factors are interaction variances. ∆Gm doubles with both sexes. This large denominator makes
mass selection inefficient for low heritability traits.
Selection is limited to only the female parents since there
is no control over pollination.
Advantages and disadvantages
The major advantages and disadvantages of individual
plant selection methods include the following.
Advantages See Chapter 16.
Disadvantages
1 Using phenotypic selection makes the selection of
superior plants often difficult.
2 Lack of pollen control means both desirable and
undesirable pollen will be involved in pollination of
the selected plants.
3 If selection intensity is high (small population size
advanced) the possibility of inbreeding depression is
increased.
Modifications
1 Stratified or grid system. Proposed by C. O.
Gardener, the field is divided into small grids (or subplots)
with little environmental variance. An equal
number of superior plants is selected from each grid
for harvesting and bulking.
2 Honeycomb design. Proposed by A. Fasoulas, the
planting pattern is triangular rather than the conventional
rectangular pattern. Each single plant is at the
center of a regular hexagon, with six equidistant plants,
and is compared to the other six equidistant plants.
There are other modifications that are sometimes complex
to apply and have variable effects on selection response.
Family selection methods
Family selection methods are characterized by three
general steps:
1 Creation of a family structure.
2 Evaluation of families and selection of superior ones
by progeny testing.
3 Recombination of selected families or plants within
families to create a new base population for the next
cycle of selection.
Generally, the duration of each step is one generation,
but variations exist.
Half-sib family selection methods
The basic feature of this group of methods is that half-sib
families are created for evaluation and recombination,