Agriculture Reference
In-Depth Information
4
Breeding Schemes
The most effective breeding schemes will utilize
the positive attributes of a crop species while mini-
mizing difficulties that might arise through the selec-
tion process. In the following section the breeding
schemes for self-pollinating, out-pollinating, hybrid
and clonal cultivars will be explained, along with men-
tion on the schemes used for developing multilines and
synthetics.
INTRODUCTION
All successful breeding programmes have been designed
around a breeding scheme. The breeding scheme deter-
mines the passage of breeding lines through the selection
process, and through planting material increase for
cultivar release. The process of selection will be car-
ried out over a number of years, and under differing
environmental conditions. The early selection stages
of breeding programmes will involve screening many
thousands of different genotypes. The early screening
is therefore relatively crude, and in many instances
involves only selection for single gene traits. After each
round of selection, the '
better
', more adapted, or more
disease resistant genotypes will be retained for further
evaluation while the least adapted lines will be dis-
carded. This process will be repeated over a number of
years, at each stage the number of individual genotypes
or populations is reduced and evaluation is conducted
with greater precision in estimating the worth of each
entry.
The breeding scheme used will be highly dependant
on the crop species and the type of cultivar (inbred,
hybrid, clone, synthetic, etc.) that is being developed.
The general philosophy for developing a clonal cultivar
like potato is therefore different from an inbred cereal
cultivar, say barley. In the former, breeding selections
are genetically fixed through vegetative propagation, but
there will be a low rate of multiplication of planting
materials. In the latter, there will be more rapid increase
of planting material, although the segregating nature of
the early generation breeding lines will complicate the
selection process.
DEVELOPMENT OF
SELF-POLLINATING CULTIVARS
Some crops that are generally produced as inbred culti-
vars are: barley, chickpea, flax, lentil, millet, peas, rice,
soybean, tobacco, tomato and wheat.
One and a half centuries ago most inbred crop species
were grown in agriculture as 'landraces'. Landraces were
locally grown populations which were, in fact, a collec-
tion of many different genotypes grown in mixture and
which were, of course, both genetically and phenotyp-
ically variable. Pure-line cultivars were developed first
from these landraces by farmers who selected specific
(presumably more productive) lines from the mixed
populations and maintained these in isolation, selfed
selections and eventually developed homozygous, or
near-homozygous, lines. It is reasonable to assume that
these homozygous lines were indeed more productive
than the original landraces because by the end of the
19th century, landraces had almost completely dis-
appeared in countries with an advanced agricultural
system.
These early 'pure-line breeders' used the naturally
existing genetic variation within the landraces they
