Agriculture Reference
In-Depth Information
parent lines clonally. In the case of maize, for exam-
ple, synthetic cultivars are developed using a three stage
process:
recurrent selection on the base population available or
by selections from those that have been produced by
designed cross pollinations. The second stage involves
clonal evaluation
and is conducted using replicated field
trials of asexually reproduced plant units. The aim of
the clonal testing is to identify which clonal populations
are phenotypically most suited to the environments
where they are grown. The clonal trials are often grown
in two or more locations to include an assessment of
environmental stability.
Using a test cross or polycross technique will then
genetically test the '
best
' clones identified from the
clonal screen. The aim of this 'genetic' test is to deter-
mine the general combining ability of each clonal line in
cross combinations with other genotypes in the selected
group of clones.
If a test cross (often called a '
top cross
') is used, all
the selected clones are hybridized to one (or more) test
parent. The test parent will have been chosen because
it is a desirable cultivar or it may be chosen because
of past experience of the individual breeder. The test
parent is a heterozygous clone that produces gametes of
diverse genotypes. This diversity of gametes produced
from the tester will help an assessment of the average
ability of each clone to produce superior progeny when
combined with alleles from many different individuals.
Test cross evaluations are most useful when the variation
that is observed within the different progeny is a result
of differences between clones under evaluation and not
due to only a small sample of genes coming from the
test parent.
A polycross does not use a common test parent but
rather a number of different parents. It therefore differs
from a test cross as the seed progenies to be evaluated
result from inter-crossing between the clones that are
under test (i.e. each clone under evaluation is used as
female and randomly mated to all, or a good range,
of other clonal selections). A polycross, like the test
cross, is used to determine the general combining ability
of the different clones. The seed so produced from a
polycross is then tested in randomized field trials. It is
essential that the trials are randomized and that the level
of replication is high enough to allow the possibility of
hybrid seed being from as many other clones as possible.
Seeds from test crossing and polycrossing are grown
in
progeny evaluation trials
to evaluate the genotype or
determine the general combining ability of each of the
clones. Progeny evaluation trials are very similar to any
•
Develop a number of inbred lines
•
Progeny test the inbred lines for general combining
ability
•
Identify the 'best' parents and intercross these to
produce the synthetic cultivar
This process is almost identical to the procedure used
for developing hybrid cultivars and only differs in the
last stage where many more parents will be included in
the synthetic than in a hybrid cultivar. To avoid rep-
etition this section on synthetics will only cover the
case of crops where inbred lines are not possible due to
inbreeding depression (e.g. clonal synthetics).
The process of developing a synthetic cultivar from
clonal lines is illustrated in Figure 4.6.
Clonal selections can be added to the nursery
ad infinitum
, on the basis of continued phenotypic
Develop clonal nursery
Select the phynotypically
'best' clones
Rep.1
Rep.2
Rep.3
Rep.4
Rep.5
Rep.6
Randomized mating
trial
Rep.7
Rep.8
Rep.9
Rep.10
Rep.11
Rep.12
Rep.13
Rep.14
Evaluate progeny to determine
genotypically 'best' clones
Select parents
in synthetic
Figure 4.6
Breeding procedure used to develop synthetic
cultivars from a clonally reproduced open-pollinated crop
species (i.e. alfalfa) and using polycross progeny testing.
