Agriculture Reference
In-Depth Information
basic assumption of any backcrossing system is that the
technique is unlikely to result in an increase of perfor-
mance of the recurrent parent, other than for the single
character being introduced. Even when a gene has been
introduced it is difficult to ensure its distribution over
the whole population.
Base
population
Select 'best'
individuals
Seed production
Improved
population
In most cases, seed production of open-pollinated cul-
tivars simply involves taking a sample of seed from the
population but, to avoid the problems noted before,
under increasingly stringent conditions, to avoid con-
tamination or cross-pollination from other populations
or cultivars.
Figure 4.5
Basic recurrent phenotypic selection scheme.
the traits of interest in the selection process. Recurrent
phenotypic selection has been shown to be effective but
mainly in cases where there is high heritability of the
characters being selected for (e.g. some disease and pest
resistances). The techniques are not nearly as effective
where traits have a lower heritability such as yield or
quality traits.
It is common practice (and a good idea) to retain
a sample of the base population so that the genetic
changes due to selection can be evaluated in a later
season.
DEVELOPING SYNTHETIC
CULTIVARS
A synthetic cultivar basically gives rises to the same end
result as an open-pollinated cultivar (i.e. population
improvement), although a synthetic cultivar cannot
be propagated by open-pollination without changing
the genetic make-up of the population. This has per-
haps been a primary reason for the rapid change over
from open-pollinated cultivars to synthetic ones, since
it means that farmers need to return to the seed com-
panies for new seed each year. It has been commercial
seed companies that have been responsible for breed-
ing almost all synthetic varieties. For example, before
1950 there were only two alfalfa breeders working
in private seed companies while 23 were breeding
in the public sector. By 1980 there were 17 public
sector alfalfa breeders but now there are more than
three times (52) private breeders developing synthetic
lines.
A synthetic cultivar must be reconstructed from
parental lines or clones. Within the United States maize
is almost exclusively grown as hybrid cultivars although
in many countries maize crops are grown as synthetics.
Synthetics have also been used almost exclusively in the
development of alfalfa, forage grass and forage legume
cultivars, and have also been used to develop varieties
from other crop species (e.g. rapeseed).
The breeding method used for the development
of synthetic cultivars is dependent on the ability to
develop homozygous lines from a species or to propagate
Progeny testing
There are actually a variety of possibilities within this
main heading, for example half- or full-sib selection
with test crossing; and selection from S
1
progeny
testing. All the schemes basically involve selecting indi-
viduals from within the population, crossing or selfing
these to produce seed. Part of the seed is sown for
assessment and part is retained. Once the results of
the assessments are available the remnant seed from the
progenies that have been shown to be superior are then
sown as a composite population for plants again to be
selected and so on. At any stage seeds can be taken out
for commercial exploitation.
Backcrossing on open pollinated-cultivar
development
Backcrossing is not as commonly used with outbreed-
ing crops as with self-pollinating ones, nevertheless is it
used. When used the biggest difference is that the recur-
rent and non-recurrent parents in the backcrosses are
plant populations rather than homozygous lines. The
