Agriculture Reference
In-Depth Information
which occurs through bulk population growth, is not
always that which is favourable for growth in agricul-
tural practice. In addition, natural selection can, of
course, only be effective in environments where the
character is expressed. This often prevents the use of
bulk methods at off-season sites.
Other methods have been used to produce homozy-
gosity in bulk breeding schemes. These include single
seed descent and doubled haploidy. Breeding schemes
that use these techniques, have increased the popular-
ity of bulk breeding scheme in recent years, as the time
from crossing to evaluation can be minimized. How-
ever, the basic philosophy is similar, being to produce
near-homozygous lines, and thereafter select amongst
those inbred lines. Where rapid acceleration to homozy-
gosity techniques are used, it is essential to ensure that
no negative selection occurs. For example, research
has shown that creating homozygous breeding lines of
rapeseed (
B. napus
) through embryogenesis produces a
higher than random, frequency of plants with low eru-
cic acid in the seed oil. If low erucic acid content is
desired, this poses a selection advantage. If, however, an
industrial oilseed cultivar were desired (one with high
erucic acid content) then using embryogenesis would
be detrimental.
Crossing
F
1
Single
plants
F
2
F
3
Head rows
F
4
Head rows
F
5
Head rows
Early
yield trials
F
6-7
Advanced
yield trials
F
8-9
Figure 4.2
Outline of a pedigree breeding scheme used
for breeding inbreeding crop species.
environmental conditions from year to year, make
selection difficult. Other disadvantages of the pedigree
method are that it requires more land and labor than
other methods; experienced staff with a '
good breeders'
eye
' are necessary to make plant selections; selection is
carried out on single plants where errors of observation
are very large while actual yield testing is not possible
in the early generations.
If selection was effective on a single plant basis, pedi-
gree breeding schemes would allow inferior genotypes
to be discarded early in the breeding scheme, without
the need of tested in more extensive, and costly, yield
trials. Unfortunately, pedigree breeding schemes offer
little opportunity to select for quantitatively inherited
characters and even single gene traits can cause prob-
lems when selecting on a single plant basis in highly
heterozygous populations.
Pedigree method
The outline of a pedigree breeding scheme is shown in
Figure 4.2. In a pedigree breeding scheme, single plant
selection is carried out at the F
2
through to the F
6
gen-
erations. The scheme begins by hybridization between
chosen homozygous parental lines, and segregating F
2
populations are obtained by selfing the heterozygous
F
1
s. Single plants are selected from amongst the segre-
gating F
2
population. The produce from these selected
plants is grown in plant/head rows at the F
3
gener-
ation. A number of the 'most desirable' single plants
(in Figure 4.2 four plants) are selected from the 'better'
plant rows and these are grown in plant rows again at the
F
4
stage. This process of single plant/head selection is
repeated until plants are 'near' homozygous (i.e. F
6
)
.At
this stage the most productive rows are bulk harvested
and used as seed source for initial yield trials at F
7
.
In addition to being laborious (as a considerable
amount of record keeping is required) and relatively
expensive, annual discarding may lead to the loss of
valuable genotypes, particularly under the changing
Bulk/pedigree method
The outline of a bulk/pedigree breeding scheme is illus-
trated in Figure 4.3. This type of breeding scheme
uses a combination of bulk population and single
