Agriculture Reference
In-Depth Information
Synthetic cultivars
species (e.g. barley or lima beans). After the initial
hybridizations have been carried out the composite-
cross population is multiplied in a chosen environment
such that the most adapted segregants will predomi-
nate and those less adapted to these conditions will
occur at lower frequencies. A composite-cross popula-
tion cultivar is therefore continually changing and can
be considered (in a very loose sense) similar to old land
races. Breeders' seed can never be maintained as the
cultivar was originally released.
Inter-crossing a set number of seed lines generates a
synthetic variety. In the simplest sense a 'first gener-
ation two-parent synthetic' is very similar to an F
1
hybrid. Synthetic lines can be derived from cross-
pollinated lines or self-pollinating lines, although the
latter instance is not common. Synthetic cultivars have
a series of categories (Syn.1, Syn.2,
, Syn.
n
) accord-
ing to the number of open pollinated generations that
have been grown since the synthetic line was generated.
For example the first generation synthetics are classi-
fied as Syn.1, if this population is then selfed or out
pollinated the next generation is classified as Syn.2 etc.
The use of synthetic cultivars has been most suc-
cessful in cases where crop species show partial self-
incompatibility (e.g. alfalfa). Examples of other crops
where synthetic varieties have been released include
rapeseed (
B. rapa
cultivar types),
...
ANNUALS AND PERENNIALS
Plant species are categorized into annuals and peren-
nials. World crop plants are fairly evenly distributed
between annuals (approximately 70 species) and peren-
nials (approximately 50 species). All major self-
pollinating crop species are annuals while the great-
est majority of cross-pollinating crops are perennials.
Perennials pose greater difficulty in breeding than most
annuals. Most perennials do not become reproductive
within the first years of growth from seed. Most peren-
nials are clonal cultivars and this can cause additional
difficulties in maintaining disease free parental lines and
breeding material. Winter annuals require vernaliza-
tion of chill treatment before moving from vegetative
to reproductive and can increase the time necessary for
developing cultivars.
rye,
pearl millet,
broom grass and orchard grass.
Multiline cultivars
Multiline cultivars are mixtures or blends of a number
of different cultivars or breeding lines. Each genotype
in the mixture will be represented by at least 5% of the
total seed lot. Many multilines are the result of devel-
oping near isogenic lines and using these to initiate the
mix. These cultivars are usually self-pollinating species.
A multiline is therefore not the same as a synthetic
where the aim is to maintain heterozygosity by inter-
crossing between the parent lines. Multilines became
popular with the aim of increasing disease resistance by
reducing the pressure for a pathogen to evolve/mutate
to overcome the biological resistance. For example, near
isogenic lines of barley, which differ in that each line has
a different qualitative disease resistance, could be mixed
to make a multiline. The main thought is to make the
epidemiology of the pathogen such that it would be less
likely to evolve virulence to all resistance genes in the
mixture.
REPRODUCTIVE STERILITY
Female and male sterility has been identified in many
crop species. Genetic and cytoplasmic male sterility
have been identified in several plant species. Plant
breeders in designing breeding programmes can uti-
lize sterility and breeding schemes can be designed
specifically to accommodate sterility, particularly in
developing hybrid cultivars. Sterility also can pose
problems and limits to the choice of parental cross
combinations that are possible.
Composite-cross cultivars
THINK QUESTIONS
Composite-cross populations are cultivars derived by
inter-crossing two or more cultivars or breeding lines.
These cultivar types have all tended to be inbreeding
(1) Complete the following table by assigning a
YES
or
NO
to each of the 16 cells.
