Genetic Resources in Agroecosystems - Agroecology: The Ecology of Sustainable Food Systems - page 190

male flowers). An alternative technique, used extensively
in self-pollinated plants such as sorghum, is to introduce
genetically controlled male sterility, called cytosterility ,
into one of the inbred parental lines. This line is then used
as the seed-producing parental line, because it can be
pollinated only by pollen from the other, nonsterile paren-
tal line.
The hybrid offspring of two selectively inbred
parents are usually quite different from either parent.
They are often larger and produce larger seeds or fruits,
or have some other desirable characteristic not possessed
by either parent. This response, known as hybrid vigor ,
or heterosis , is one of the great advantages of a hybrid
variety. Another desirable characteristic (from the stand-
point of conventional agriculture) is genetic uniformity:
all the hybrid seeds of a particular cross will have the
same genotype.
Hybrid varieties, however, have an inherent disadvan-
tage. Seeds produced by hybrid plants — through either
self- or cross-pollination — are usually undesirable for
planting since sexual recombination will produce a variety
of new gene combinations, most of which will not exhibit
the hybrid vigor of the parents. Therefore, farmers must
purchase hybrid seed each year from seed producers.
In crop types with tubers or other means of asexual
reproduction, such as potatoes and asparagus, once a
hybrid is produced with a suite of desirable traits, it is
then propagated asexually as a clone . With advances in
techniques of tissue culture, this method of propagating
hybrids without seed has been applied more widely. Small
amounts of tissue from different parts of important hybrid
cultivars can be used to rapidly reproduce clones under
strictly controlled conditions.

T RANSGENIC M ODIFICATION

Plant breeding using the techniques described above is
tedious, time-consuming, and dependent to some extent
on luck. Genes occur in the company of many other thou-
sands or millions of genes on chromosomes, and the plant
breeder cannot determine how a few genes of interest are
distributed and recombined in each generation. Moreover,
these techniques are restricted to breeding parents that are
closely related — usually within the same species.
No such limitations exist for genetic engineers. Using
various techniques developed in recent decades, they can
transfer single genes from one organism — a bacterium,
for example — to another completely unrelated organism,
such as a higher plant. Genetic engineering enables crop
geneticists to introduce specific traits such as resistance
to freezing or herbivory, into a crop species, and to create
customized organisms, each with its own unique suite of
traits.
As noted in Chapter 1, the end results of genetic
engineering are called transgenic, genetically modified,
or genetically engineered (GE) organisms. Transgenic
crops being planted today on a commercial basis include
strains of corn, soybeans, wheat, rice, cotton, canola
seed, sugar beets, tomatoes, lettuce, peanuts, and pota-
toes. The area planted to these and other GE crops has
increased regularly every year since the mid-1990s; in
2004, they covered an estimated 200 million acres (81
million ha). Throughout this period of rapid growth in
the planting of biotech crops, one company — Monsanto
Corporation — has maintained an 80 to 90% share of
the market.
GE crops are created with a variety of goals in mind.
Some are intended to be resistant to attack by a particular
pest, some to create food with better nutritive value, some
to resist the application of herbicides. Because of these
and other characteristics, genetic engineering has been
touted as the technological answer to many of the
challenges faced by agriculture: producing more food,
producing better food, reducing the need for pesticides
and herbicides, and growing crops on marginal land.
Transgenic modification of crop organisms has been
controversial ever since it began to be practiced on a
commercial basis in the 1990s, and for good reason.
Growing GE crops poses a variety of potential problems
and serious risks (Table 14.1). As just one example,
several researchers have recently cautioned against the
use of transgenic rice on the grounds of its potential
effects on food safety, cross pollination with “wild rice”
varieties and other types of environmental impact on
nontarget organisms (Bottrell, 1996; Lu and Snow, 2005;
Saito and Miyata, 2005).
Some of the potential drawbacks of growing trans-
genic crops are listed below. They are not all hypothetical;
most have already been documented to occur.

Induced Polyploidy

Many of today's important crop types, such as wheat,
corn, coffee, and cotton, arose long ago as natural poly-
ploids. Since polyploid plants are often more robust and
have larger fruits or seeds than their normal diploid par-
ents, people found them desirable when they occurred
in early cropping systems, and they were selected,
even though farmers were not aware of what made them
different.
When it was discovered by modern cytologists that
many favorable traits in crop plants were the result of
polyploidy, methods were developed to artificially induce
it. Through the use of colchicine or other chemical
stimulators during the first steps of meiosis, artificial
multiplication of the number of chromosomes has become
possible. Induced polyploidy has produced some of the
most useful lines of wheat, for example, such as the hexap-
loid Triticum aestivum . Once produced, polyploids
themselves can then be used to perpetuate pure lines or
develop new hybrids.