Agriculture Reference
In-Depth Information
Southern Hemisphere are frequently used to increase
either seed quantity or reduce heterozygosity in many
breeding schemes. The use of off-season sites is often
restricted to annual spring crops and there are only a
few good examples where they have helped accelerate
homozygosity in winter annuals and virtually none in
breeding biennials.
If off-season sites are to be incorporated into the
breeding scheme care must be taken to ensure that
'
selectional adaptability
' of the off-season site does not
have adverse effects on the segregating plant popula-
tions. For example the spring barley breeding scheme
at the Scottish Plant Breeding Station used to increase
F
4
breeding selections to F
5
by growing these lines over
winter in New Zealand. Although New Zealand has a
climate that is very similar to that found in Scotland
there is a completely different spectrum of races of
powdery mildew. As a result, mildew resistant selec-
tions made in New Zealand were of no relevance when
grown in Scotland and so meant that all New Zealand
trials needed careful spraying to avoid powdery mildew
being confounded with other performance characters.
Crossing
F
1
Bulk population
(mass selection)
F
5
Single
plants
F
6
F
7
Head rows
Early
yield trials
F
8-9
Advanced
yield trials
F
9 -10
Figure 4.1
Outline of a bulk breeding scheme used for
breeding inbreeding crop species.
Breeding schemes for pure-line cultivars
and hence predominate in future generations. Simi-
larly, these bulk populations are usually grown under the
stress and disease pressures common to the cultivated
crop, and it is assumed that the frequency of adapted
genotypes in the population increases. It is therefore
very important that the bulks are grown in a suitable and
representative environment. After a number of rounds
of bulk increase, individual plants showing desirable
characteristics are selected at the F
6
stage. From each
selected plant, a plant (or head) row is grown and the
produce from the best lines/rows are bulk harvested, for
initial yield trials. More advanced yield trials are grown
from bulk harvest of desirable populations.
The major advantage of the bulk method is that con-
scious selection is not attempted until plants have been
selfed for a number of generations and hence plants are
nearly homozygous. This avoids the difficulty of selec-
tion among segregating populations where phenotypic
expression will be greatly affected by levels of domi-
nance in the heterozygotes. This method is also one of
the least expensive methods of producing populations of
inbred lines. The disadvantage of this scheme is the time
from initial crossing until yield trials are grown. In addi-
tion, it has often been found that the natural selection,
There are probably as many different breeding schemes
used by breeders of self-pollinating crops as there are
breeders of inbreeding species. There are, however, three
basic schemes:
bulk methods, pedigree methods
and
bulk/pedigree methods
. It should be noted that all the
breeding schemes described would involve more than
a single cross at the
crossing stage
. A number of these
crosses will be two-parent crosses (Female parent
×
Male parent, say P
1
×
P
2
)
, although many breed-
ers use three- and four-way parent cross combinations
([P
1
×
P
2
]×
P
3
, and [P
1
×
P
2
]×[
P
3
×
P
4
]
, respectively).
Bulk method
The outline of a bulk scheme is illustrated in Figure 4.1.
In this scheme, genetic variation is created by artificial
hybridization between chosen parents.
The F
1
and several subsequent generations, in the
illustration upto and including the F
5
generations, are
grown as bulk populations. No conscious selection is
imposed on these generations and it is assumed that the
genotypes most suited to the environment in which the
bulk populations are grown will leave more offspring
