Cambridge International A Level Biology Revision Guide

Chapter 19: Genetic technology
The most likely detrimental effects on the environment
of growing a herbicide-resistant crop are that:
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the genetically modified plant will become an
agricultural weed
pollen will transfer the gene to wild relatives,
producing hybrid offspring that are invasive weeds
herbicide-resistant weeds will evolve because so much
of the same herbicide is used.
In 1993, an investigation to compare invasiveness of
normal and genetically modified oil seed rape plants was
carried out. Three genetic lines were compared: nonengineered
oilseed rape and two different genetically
engineered versions of the same cultivar. The rates of
population increase were compared in plants grown in
a total of 12 different environments. The environments
differed in, for example, the presence and absence
of cultivated and uncultivated background vegetation, and
presence and absence of various herbivores and pathogens.
There was no evidence that genetic engineering increased
the invasiveness of oil seed rape plants. Where differences
between normal and genetically modified plants existed,
the genetically engineered plants were slightly less invasive
than the unmodified plants.
The risk of pollen transfer, by wind or by insects, is
real. Oil seed rape interbreeds easily with two related
species: wild radish and wild turnip. Its flowers are
adapted for insect pollination, but are also pollinated by
wind. Although ‘safe’ planting distances are specified for
trials of genetically modified plants (for example, 200 m
for oil seed rape), pollen from various plants has been
found between 1000 and 1500 m away from those plants.
Bees visiting some flowers have been found to forage at
distances of more than 4000 m. Safe planting distances
should be increased to allow the organic farming industry
to maintain its ‘GM-free’ certification.
Experimental crosses between glufosinate-resistant
oil seed rape and both wild radish and wild turnip have
shown that resistance can be passed to the hybrid offspring
and that it persists through several further generations of
their offspring. However, there is as yet little evidence of
this occurring outside the laboratory.
Herbicide-resistant mutant plants of various species
have been found growing near fields where glyphosate
has been much used. However, the herbicide is not only
used on resistant crop species. Gene technology is not
directly responsible for this evolution of resistance,
which may arise in the absence of any genetically
modified crop.
Insect-resistant crops
Another important agricultural development is that of
genetically modified plants protected against attack by
insect pests. Maize is protected against the corn borer,
which eats the leaves of the plants and then burrows into
the stalk, eating its way upwards until the plant cannot
support the ear. Cotton is protected against pests such as
the boll weevil (Figure 19.21). In both plants, yield
is improved.
Insect-resistant tobacco also exists, and is protected
against the tobacco bud worm, but as yet it has not been
grown commercially.
The most likely detrimental effects on the environment
of growing an insect-resistant crop are:
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the evolution of resistance by the insect pests
a damaging effect on other species of insects
the transfer of the added gene to other species of plant.
However, less pesticide is used, reducing the risk of spray
carrying to and affecting non-target species of insects in
other areas. Remember also that only insects that actually
eat the crop are affected.
a
b
Figure 19.21 a Corn borer, b boll weevil.
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