Principles of Plant Genetics and Breeding

Advantages and disadvantages The major advantages
and disadvantages of half-sib selection include the
following.
Advantages
1 The procedure is rapid to conduct.
2 Progeny testing increases the success of selection,
especially if quantitative gene action occurs or heritability
is low.
Disadvantages
1 The trait of interest should have high heritability for
success.
2 It is not readily applicable to species that cannot produce
enough seed per plant to conduct a yield trial.
3 Lack of pollen control reduces heritability by half.
Modifications The modifications or alternative procedures
for recurrent half-sib selection differ by: (i) the
testers used, (ii) the selection of one or both parents;
and (iii) the seed used for intercrossing. Some procedures
use a population as a tester and half-sib seed for
intercrossing. Selfed seed or clones may be used for
intercrossing.
Where genetic male sterility is incorporated in the
scheme, male-sterile plants may be tagged in the field at
the time of flowering. After open-pollination, each head
is harvested and threshed separately. Seed from one
head (family) is evaluated as one entry in a yield trial,
remnant seed is saved, superior entries are identified,
and the remnant seed is bulked and planted for recombination
to occur. Male-sterile plants are tagged again
and harvested individually to form the next cycle of
evaluation.
Half-sib selection with a testcross
Another way of evaluating genotypes to be composited
is by conducting a testcross.
Key features This variation of half-sib selection allows
the breeder to more precisely evaluate the genotype
of the selected plant by choosing the most suitable
testcross parent. The half-sib lines to be composited are
selected based on a testcross evaluation not progeny
performance. The tester may be inbred, in which case all
the progeny lines will have a common parental gamete.
Applications Like half-sib selection with a progeny
test, this procedure is applicable to cross-pollinated
species in which sufficient seed can be produced by
BREEDING CROSS-POLLINATED SPECIES 321
crossing to grow a replicated testcross progeny trial.
However, in procedures in which self-pollination is
required, the method cannot be applied to species with
self-incompatibility.
Procedure In season 1, the breeder selects 50–100
plants from the source population. A tester parent is pollinated
with pollen from each of the selected plants
(Figure 17.6). The crossed seed from the tester as well
as the open-pollinated selected plants are harvested
separately. In season 2, the testcross progenies are
grown in replicated plots. In season 3, an equal amount
of open-pollinated seed from 5–10 superior plants is
composited and grown in isolation for open-pollination
to occur.
Advantages and disadvantages The major advantages
and disadvantages of half-sib selection with a testcross
include the following.
Advantages
1 Control over the testcross parents permits a more
precise evaluation of the genotype of the selected
plant than would be obtained by open-pollination, as
in half-sib selection with progeny test.
2 It is rapid to conduct.
Disadvantages
1 This method of breeding is applicable to species that
can produce sufficient seed by crossing for replicated
testcross progeny trials.
2 When self-pollination is used, the method is applicable
to species without self-incompatibility issues.
Modifications Pollen from each selected plant may
be used to pollinate a tester plant and self-pollinate
the selected plant. Also, in season 3, equal quantities
of selfed seed may be composited and planted in
isolation.
Interpopulation improvement methods
The purpose of this group of recurrent selection
schemes is to improve the performance of a cross
between two populations. To achieve this, interpopulation
heterosis is exploited. The procedures are appropriate
when the breeder’s goal is hybrid production (i.e.,
the final product or cultivar is a hybrid). Developed
by P. E. Comstock and his colleagues, the procedures
allow the breeder to improve two genetically different
populations for GCA and SCA, thereby improving their
crossbred mean.