Principles of Plant Genetics and Breeding

Procedure: cycle 1 This is the same as for cycle C 0 .
Genetic issues The genetic gain per cycle is given by:
∆G FS = iσ A 2 /2σFS
Season 1
Season 2
Season 3
where σ FS = phenotypic standard deviation of the fullsib
families.
Selfed (S1 or S2 ) family selection
An S1 is a selfed plant from the base population. The key
features are the generation of S1 or S2 families, evaluating
them in replicated multienvironment trials, followed
by recombination of remnant seed from selected families
(Figure 17.4).
Applications The S 1 appears to be best suited for selfpollinated
species (e.g., wheat, soybean). It has been
used in maize breeding. One cycle is completed in three
seasons in S 1 and four seasons in S 2 . A genetic gain per
cycle of 3.3% has been recorded.
Procedure
Season 1 Self-pollinate about 300 selected S 0 plants.
Harvest the selfed seed and keep the remnant
seed of each S 1 .
Season 2 Evaluate S 1 progeny rows to identify superior
progenies.
Season 3 Random mate selected S 1 progenies to form a
C 1 cycle population.
Genetic issues The main reason for using this scheme
is to increase the magnitude of additive genetic variance.
In theory the genetic gain is given by:
S 0
S 1
Figure 17.4 Generalized steps in breeding based on S 1 /S 2 progeny performance.
BREEDING CROSS-POLLINATED SPECIES 319
Source
population
Select 50–100 plants
Self-pollinate (S 0);
keep remnant seed
Grow replicated S 1
progeny test; identify
superior progeny
Composite equal amounts of
remnant seed of superior
progenies and grow in
isolation for random mating
∆G S1 = iσ 2 A1 /σ PS1
where σ2 A1 = additive genetic variance among S1 , and
σPS1 = phenotypic standard deviation among S1 families.
The additive genetic variation among S2 is two times
that of S1 . The S1 and S2 , theoretically, have the highest
expected genetic gain per cycle for intrapopulation
improvement. However, various reports have indicated
that, in practice, full-sib and testcross selections have
produced greater genetic gain for both populations per
se and the population crosses.
Family selection based on a testcross
The key feature of this approach to selection is that it
is designed to improve both the population per se as
well as its combining ability. The choice of the tester
is most critical to the success of the schemes. Using a
tester to aid in selection increases the duration of a
cycle by 1 year (i.e., a 3-year cycle instead of a 2-year
one as in phenotypic selection). The choice of a tester is
critical to the success of a recurrent selection breeding
program. The commonly used testers may be classified
into two: (i) narrow genetic base testers (e.g., an inbred
line); and (ii) broad genetic base testers (e.g., openpollinated
cultivars, synthetic cultivars, double-cross
hybrid). Broad base testers are used for testing GCA in
the population under improvement, whereas narrow
genetic base testers are used to evaluate SCA and possibly
GCA.
Generally, plants are selected from the source population
and are selfed in year 1. Prior to intermating, the