Agriculture Reference
In-Depth Information
cultivar development might never have been developed,
or acceptance would have been delayed by at least
20 years, until cytoplasmic male sterile systems were
available.
Hybrid cultivars have been developed, however, in
sorghum, onions and other vegetables using a cytoplas-
mic male sterile (CMS) seed production system; in sugar
beet and some
Brassica
crops (mainly Brussels sprouts,
kale and rapeseed) using CMS and self-incompatibility
to produce hybrid seed; and in tomato and potato using
hand emasculation and pollination.
If hybrid cultivars are to be developed from a crop,
then the species must:
•
That technical problems (usually associated with seed
production) are simply challenges to be faced
•
That the biological arguments are irrelevant
Skeptics (of which there seem to be fewer, or who are
less vociferous) argue on the basis of experimental data
available to date that:
•
There is no good evidence for over-dominance, and
so it is definitely possible to develop pure-line, inbred
cultivars which are as productive as hybrid lines
The economic attractions of seed companies should
be weighed against high seed costs for the farmers;
that technical skills could be put to better (more
productive) use
•
Show a high degree of heterosis
•
The biological reality is all-important
•
Be capable of being manipulated so as to produce
inexpensive hybrid seed
•
These latter skeptics usually, however, accept that in
the case of out breeding species hybrids give a faster
means of getting yield increases, while in the longer
term inbred lines would match them, but in inbreeding
crops this differential in speed is not present.
•
Not easily be produced uniformly, and have a high
premium for crop uniformity
There are many differing opinions regarding the exact
contribution of hybrids in agriculture. In hybrid maize
there would seem little doubt that there have been
tremendous advances made. However, this has been
the result of much research time and also large finan-
cial investments. In addition, it should be noted that
inbred parents in hybrid breeding programs have been
improved just as dramatically as their hybrid prod-
ucts. Most other hybrid crops (with the exception of
sorghum) are also outbreeders. Almost all outbreed-
ing crops show degrees of inbreeding depression and,
therefore, its counterpart heterosis. In such cases there
are strong arguments, certainly in practical terms, for
exploiting heterozygosity to produce productive culti-
vars. This implies that hybrid cultivars can offer an
attractive alternative over open-pollinated cultivars or
even synthetic lines, although seed production costs will
always be a major consideration. In inbreeding crops,
hybrid cultivar production is much more difficult to
justify on '
biological grounds
'.
Committed '
hybridists
', of whom there are many
(especially within commercial seed companies) would
argue that:
Heterosis
The performance of a hybrid is a function of the genes
it receives from both its parents but can be judged by
its phenotypic performance in terms of the amount of
heterosis it expresses. Many breeders (and geneticists)
believe that the magnitude of heterosis is directly related
to the degree of genetic diversity between the two par-
ents. In other words, it is assumed that the more the
parents are genetically different the greater the hetero-
sis will be. To this end, it is common in most hybrid
breeding programmes to maintain two, or more, dis-
tinct germplasm sources (
heterotic groups
). Breeding
and development is carried out within each source and
the different genetic sources are only combined in the
actual production of new hybrid cultivars. For exam-
ple, maize breeders in the United States found that
they observed significant heterosis by crossing Iowa Stiff
Stalk breeding lines with Lancaster germplasm. Since
this discovery, these two different heterotic groups have
not been inter-crossed to develop new parental lines but,
rather, have been kept genetically separated for parental
development.
It has proved difficult to clearly and convincingly
define
•
Yield heterosis is there for the exploitation
•
Hybrid
cultivars
are
economically
attractive
to
breeding organizations and seed companies
the
underlying
causes
of
heterosis
in
crop
