Agriculture Reference
In-Depth Information
A second consideration with hybrid lines that has
been postulated is that the heterozygosity of the cul-
tivar makes it more stable over a range of different
environments. This may be true but there is no direct
evidence for such a basic biological effect and it does not
explain the extremely high genotype by environment
interactions found in hybrid maize.
Crossing
population 1
Crossing
population 2
F
1.pop.1
F
1.pop.2
Types of hybrid
There are a number of different types of hybrid, apart
from the single cross types concentrated on above. The
different types of hybrid differ in the number of parents
that are used in hybrid seed production. Consider four
inbred parents (A, B, C and D), types of hybrid would
include:
Inter-cross selections
Population 1
×
Population 2
Select for
combining ability
×
×
•
Single cross hybrids (e.g. A
B, B
D, etc.)
×
×
×
•
Three-way hybrids (e.g.
[(A
B)
C], [D
×
(C
A)], etc.)
New cultivar
Inbred
pop.1
×
Inbred
pop.2
×
×
×
•
Double cross hybrids (e.g. [A
B]
[C
D],
×
×
×
[C
A]
[B
D], etc.)
Figure 4.7
Outline of a hybrid breeding scheme.
Single cross hybrid are more uniform than three-way
or double cross hybrids and are generally more produc-
tive, but are most expensive to produce hybrid seed.
The procedure used to develop inbred parent lines
in hybrid cultivar development is similar to that used
to breed pure-line cultivars and the advantages and
disadvantages of various approaches are the same.
Breeders have used, bulk methods, pedigree methods,
bulk/pedigree methods, single seed descent and out of
season extra generations (off-station sites) to achieve
homozygosity. One of the most important objectives
is to maintain high plant vigour and to ensure that
the inbred lines are as productive as possible. This is
not always easy, particularly in species where there is
a high frequency of deleterious recessive alleles present
in the outbred populations. Breeders must decide the
level of homozygosity that is required. On one hand,
the more homozygous (the extreme, of course, being
100% homozygosity) the inbred lines are, then the more
uniform will be their resulting hybrid. More heterozy-
gous “inbred lines” may, however, be more productive
as parents and hence help reduce the cost of hybrid seed
production.
Combining ability (or more relevantly, general com-
bining ability,
Breeding system for hybrid cultivars
The
three
major
steps
in
producing
hybrids
are
therefore:
•
development of inbred lines to be used as parents;
•
test cross these lines to identify those that combine
well;
exploit the best single crosses as hybrid cultivars.
•
The system used to develop hybrid cultivars is illus-
trated in Figure 4.7. The scheme involves six stages:
•
Produce two, or more, segregating populations
•
Develop inbred lines (parents)
•
Evaluate performance of inbred lines phenotypically
•
Evaluate general combining ability of selected inbred
lines;
•
Evaluate hybrid cross combinations
•
Increase inbred parental lines
GCA) is evaluated with the aim of
