Plant Breeding


Mankind has selectively bred plants for thousands of years
with the aim of improving their quality. Experimenting with
cereal crops, breeders have aimed to enhance their yield of
grain, the quality of their flour, and their resistance to
disease and drought. With other plants, breeders have tried
to improve the perfume and color of the flowers.
Improvement of plants, particularly food crops, is
obviously important and genetics has contributed to a
better understanding of the benefits and disadvantages of
particular breeding programs. Many cereal crops such as
corn are now planted largely as hybrid seed, produced by
outbreeding between different inbred varieties. The vigour
of the hybrid plant is probably a major contribution to the
increased corn output in the United States. This increased
output represents a major achievement for applied genetics.
In 1929 practically no hybrid corn was grown among the 100
million acres of corn in the United States.
But by 1970 the vast majority of 67 million acres was
planted with the hybrid variety, yielding twice as much
Plant breeders have a definite advantage over animal
breeders, because they can often produce fertile varieties
- indeed, new species - by crossbreeding between species.
This is because hybrids are often polypoid. Polyploidy, as
we know, can occur naturally in the wild. Some species of
cotton that we grow are polypodies that probably arose
originally by accidental crosses between different species
of cotton.
But breeders do not have to rely on accidents. They can
attempt to produce fertile polypodies by crossbreeding
between different species. One early attempt to produce
another hybrid species was made in 1927 by the Russian
geneticist G. D. Karpechenko, who crossbred two quite
distantly related species, a radish and a cabbage. Each
species has eighteen chromosomes (nine pairs); the hybrids
had the same number (nine radish chromosomes and nine
cabbage chromosomes) and were sterile. However, some
polyploids arose by chance. These had thirty six
chromosomes (nine pairs of radish, and nine pairs of
cabbage), and were fertile. Unfortunately, the hybrid was
not commercially successful because as luck would have it,
the plant had the leaves of a radish and the roots of a
Breeders can artificially encourage polyploidy by treating
the hybrids that result from crossbreeding between species
with a chemical called colchicine obtained from autumn
crocuses. This chemical allows the chromosomes to
reproduce, but prevents the formation of two separate
cells. The number of chromosomes in the nucleus is
therefore doubled. Several of these new polyploid varieties
promise to be very useful. For example, a new hybrid
cereal, called triticale, produced by crossbreeding rye
with species of wheat, adds rye's resistance to cold
winters to the usual properties of wheat.
Recent breeding programs have led to highly inbred wheats.
Much of the genetic variability, that accumulated over nine
thousand years of wheat cultivation, is missing from
present day varieties. If a new disease should arise, or if
the climate were to change suddenly, much of the wheat
might be damaged and lost. So it is a good idea to
introduce other genes into wheat by outbreeding. One way of
doing this is to crossbreed the inbred varieties with their
wild relatives, which may be resistant to viruses, insects,
or drought. For this reason, some wheat breeders believe it
is essential to conserve some stocks of primitive wheat in
seed banks, from which they will be able to take a
transfusion of genes, if and when the need arises.
Varieties of the same species of the cabbage family have
been selectively cultivated for their differing features of
taste and appearance. Some varieties became hard headed,
like a modern cabbage, some made masses of flower buds, as
in cauliflower and broccoli, and some made clusters of leaf
buds, as in Brussels sprouts. 


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