The Rural Voice, 1989-05, Page 23 (2)a plant breeder enhances the desired
trait while retaining the elite
characteristics.
The implications are obvious. The
only source of the genetic diversity
necessary for the creation of elite
crops are the landraces which have
been informally adapted to a multitude
of conditions. Yet those landraces are
being wiped out at a rate which Pluck -
nett et al. say is "unprecedented."
They point out that the replacement
of landraces by elite crops is not the
only factor in this genetic erosion.
The worldwide shift to monoculture,
the clearing of land, the creation of
reservoirs, overgrazing, the gathering
of fuel wood, and the extinction of
tribal people who know the traits and
uses of many varieties of plants, all
play a significant role. As well, the
needs of machines, consumer prefer-
ence, market forces, and the require-
ments of the food industry dictate the
need for large plantings of uniform
crops in the First and Third Worlds.
Genetic simplification and erosion
have several consequences. As human
populations continue to grow, the
yield per unit area of the major crops
must also rise to feed the new people.
This has to happen because the agri-
cultural land base cannot expand —
nearly all the arable land on earth is
already under cultivation. If breeders
cannot find genes that will improve
crop yields, then there will be food
shortages.
Another consequence of genetic
simplification is that the plants of
modem crops are all genetically simi-
lar and so all equally susceptible to
natural calamities. Compounding this
is the massive shift to monoculture:
genetically similar plants in field after
field sit waiting for a new disease to
run rampant through them.
The Irish potato famine, which
began in 1845, is an example of an
epidemic caused by genetic uniform-
ity. The potatoes in that crop were
descended from only two samples
brought back from the New World,
and they did not have resistance to late
blight. When the disease hit, it de-
stroyed the crop and caused the death
or emigration of nearly 4 million Irish
There are indications that large-scale climatic shifts
are underway worldwide, which will create a need
for a different spectrum of crop plants. How well
breeders can respond to these sorts of problems
depends on how much genetic diversity they
are able to work with.
peasants. Plucknett et al. point out
that at the time there were varieties
resistant to the disease growing in
South America. Had some of these
varieties been planted, the epidemic
may have been slowed or halted.
And there are always the
"unknowns" to consider. Garrison
Wilkes, and editor of Plant Genetic
Resources: a Conservation Imper-
ative, writes that since we cannot be
certain of our future needs in terms of
crop plants, we must keep our options
open. The genetic resources of land -
races are indispensable raw material.
Disease resistance, for example,
always needs to be updated as patho-
gens mutate or spread in unpredictable
ways. And there are indications that
large-scale climatic shifts are under-
way worldwide, which will create a
need for a different spectrum of crop
plants. How well breeders can
respond to these sorts of problems
depends on how much genetic
diversity they are able to work with.
Plant breeders have been aware of
the problems of genetic erosion, and
are vocal about halting it. But a
simple return to the old ways will not
work: the food shortages caused by a
return to low -yielding landraces would
result in massive starvation, mostly in
the Third World. Breeders say it is
immoral to coerce some farmers to
grow low -yielding landraces in the.
name of genetic diversity while others
use high -yielding varieties. Instead,
we must retain modern agricultural
production while conserving genetic
diversity.
One way of retaining this diversity
is to collect landrace seeds before they
disappear and store them, much the
same way that zoos collect and
preserve rare animals. If a plant
breeder needs the genetic material
contained within the seeds, some of
the seeds can be grown and used to
generate a living line of plants for
breeding purposes.
The pioneering work for these
gene banks was done by Alphonse de
Candolle in 1855 and 1902 as well as
N. I. Vavilov in 1940 and 1957.
Later, germplasm collections were
established in the USSR and the U.S.
with mandates to collect and store
seeds from all the world's crops. And
the International Board for Plant
Genetic Resources (IBPGR) was set
up in 1974 to develop a worldwide
network of plant genetic resource
centres, with special emphasis on the
Third World.
The types of germplasm collec-
tions vary. The working collections of
plant breeders, for example, are limit-
ed and changing. Dr. Brad Fraleigh,
genetic resource manager of Plant
Genetic Resources Canada (PGRC),
says breeder collections are different
in scope than gene bank collections
because the aim of a breeder is not to
preserve genetic diversity, but to
reduce it.
Gene banks, on the other hand,
cover a wide spectrum and want to
conserve diversity. They usually use
mid-term or long-term storage. In
mid-term storage, dry seeds are held at
0 to -5°C for 10 to 30 years. For long-
term storage, the seeds are dried and
vacuum-packed, then stored at -20°C
for up to 100 years.
The size of the seed sample needed
to obtain all the genetic diversity in a
population is disputed. Some research
indicates that a sample of only 100
seeds is all that is required, while
other data show that 2,500 seeds are
necessary. Most of the larger seed
banks store several thousand seeds in
each accession.
MAY 1989 21