Agriculture Reference
In-Depth Information
description of the different types of cultivar is presented
below.
are identified and improved by increasing the frequency
of desirable phenotypes within them.
Pure-line cultivars
Hybrid cultivars
Pure-line cultivars are homozygous, or near-homozygous,
lines. Pure-line cultivars can be produced most readily
in naturally self-pollinating species (e.g. wheat, barley,
pea, soybeans). But they can also be produced from
species that we tend to consider as cross-pollinating ones
(e.g. pure-line maize, gynoecious cucumber and onion).
There is no universally agreed definition of what con-
stitutes a pure-line cultivar, but it is generally accepted
that it is normally one in which the line is homozygous
for the vast majority of its loci (usually 90% or more).
The most common method used to develop pure-
line cultivars from inbreeding species is to artificially
hybridize two chosen (usually) homozygous parental
lines, self the heterozygous first filial generation (F
1
)
to obtain F
2
seed, and continue selfing future genera-
tions, upto a point where the line is considered to be
'commercially true breeding' maybe the F
6
or F
7
. At the
same time it has been common to carry out recurrent
phenotypic selection on the segregating population over
each generation.
Hybrid cultivars (single cross, three-way cross, and
double cross hybrids) are very homogeneous and
highly heterozygous. A true F
1
hybrid cannot be repro-
duced from seed of the hybrid generation because the
progeny would segregate and result in a very non-
uniform crop (although sometimes F
2
hybrid cultivars
are sown).
Hybrid breeding is perhaps the most complex of
the breeding methods. The process of cultivar devel-
opment involves at least two stages. The first stage is
to select desirable inbred lines from chosen out pol-
linated populations. These inbred selections are then
used in test crosses to allow their comparison and assess-
ment in relation to their general or specific combining
ability. Superior parents are selected and these are then
hybridized to produce seed of the hybrid cultivar. The
parent lines are then maintained and used to continually
reproduce the F
1
hybrids.
Clonal cultivars
Open-pollinated cultivars
Clonal cultivars are genetically uniform but tend to
be highly heterozygous. Uniformity of plant types is
maintained through vegetative rather than sexual repro-
duction. Cultivars are vegetatively propagated by asex-
ual reproduction (clones) including cuttings, tubers,
bulbs, rhizomes and grafts (e.g. potato, peaches, apples,
chrysanthemums). A cultivar can also be classified as a
clone if it is propagated through obligate apomixis (e.g.
buffelgrass).
Clonal varietal development begins by either sex-
ual hybridization of two parents (often clones) or
the selfing of one of them to generate genetic vari-
ability through the normal process of sexual repro-
duction. Most of the parental lines will be highly
heterozygous and segregation will begin at the F
1
stage.
Desirable recombinants are selected from amongst
the clonal propagules. Breeding lines are maintained
and multiplied through vegetative reproduction and
hence the genetic constitution of each selection remains
'
fixed
'.
Open-pollinated cultivars are heterogeneous popu-
lations comprised of different plants, which are
genetically non-identical. The component plants tend
to have a high degree of heterozygosity. Open-pollinated
cultivars are almost exclusively from cross-pollinating
species. Plants within these populations have been
selected to a standard that allows for variation in many
traits but which shows 'sufficient' stability of expres-
sion in the characters of interest. Stability of these
traits can be used to pass the DUS requirements neces-
sary for cultivar release. Examples of open pollinated
cultivars would include onions, rye and non-hybrid
sweet corn.
In developing outbreeding cultivars the initial
hybridization (the point at which the genetic diver-
sity and variation is exposed) is usually between two
outbreeding populations. In this case segregation is
apparent at the F
1
generation. Desirable populations
