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ISIS Press Release 02/09/05
Hybrid Seed
Hybrid seed was the first step whereby agribusiness corporations wrested
control of seed away from farmers Prof.
Joe Cummins and Dr.
Mae-Wan Ho
A fully referenced
version of this article is posted on ISIS members’ website. Details here
A brief history
Hybrid seed began with maize in
the 1920s, and became extended to vegetables and flowers; and more recently,
rice and some forage crops. Hybrid seeds are produced from naturally
out-breeding crops, from which inbred lines are produced by repeated
self-pollination. The established inbred lines are crossed to produce first
generations (F1) hybrid seeds. The hybrid seeds are prized because they produce
uniform plants benefiting from the effect called heterosis (hybrid vigor).
Heterosis can result in a large increase in yield over the inbred lines or
comparable lines that are out-crossing. The precise basis of heterosis is still
unclear, but epistasis and over-dominance are thought to be involved. Epistasis
is the interaction between different genes, and over-dominance is a condition
where the heterozygotes (genes represented by two different versions) are
superior to either homozygotes (gene represented by the same versions). The F1
hybrid seed is heterozygous in many genes.
Hybrid
seed is planted to produce a crop that is harvested for use. Saving seed from
the crop and planting it is undesirable because the two different versions of
the genes in the F1 hybrid segregate out
in the offspring, producing an extremely variable progeny. In other words, the
superior qualities of the F1 hybrid will have all disappeared. The hybrid is
obtained by crossing the inbred lines, which therefore, have to be separately
maintained. Thus, only the seed companies produce hybrid seeds, and farmers
must buy those seeds from the company every year.
Hybrid maize arose
through the advocacy of a few influential Americans. Foremost among the
advocates was Henry A. Wallace, who became vice-president of the United States.
Wallace
graduated from University with an agriculture degree, and studied statistics
thereafter on his own. He later taught the subject at Iowa State University and used his knowledge to
develop the first commercial hybrid maize. In 1926, he founded the Hi-Bred Corn
company (now Pioneer Hi-Bred Seed Company, a subsidiary of Dupont Chemical
Company), and later entered politics. He was made Secretary of Agriculture
before being elected vice-president of the United States. Wallace was noted for his concern for the common man and envisioned hybrid corn as a
means of providing bountiful food
at low prices for the masses. The detailed history of hybrid corn and Wallace
makes fascinating reading [1-3].
The first corn
hybrids were made by detasseling the plants of the maternal inbred-line by
removing the male flowers so that the female flowers on the plants can only be
fertilized by pollen produced from plants of another, male line. The
detasseling operation used to be performed mainly by young girls employed
during the summer months. Later on, male-sterile lines were developed that did
not produce fertile male flowers or pollen. The male-sterile maternal lines
were fertilized with paternal lines that allowed the hybrid seed to produce
both male and female flowers. The male-sterile lines are most frequently altered in the
mitochondrial genome, leading to the inhibition of male flower development [4]. A number of
such lines are now available.
Disaster struck
The early development of
male-sterile lines led to disaster, however. The primary line used in the
1960s contained the T (Texas) cytoplasm male-sterility gene; and by 1970, over 85% of
the commercial maize planted contained that gene. The gene also caused a
pleiotropic (multiple effects due to a single gene) susceptibility to a fungus
disease. During a damp 1970 summer, the disease spread widely
particularly in the summer corn belt.
The impact on maize production was disastrous, leading to a return to
hand-detasseling for a number of years until alternate male-sterility lines
could be developed [5]. The lesson that should have been learned was that the
absence of diversity is bound to lead to disastrous epidemics; but that lesson
tends to get ignored in favour of risky but profitable genetic manipulations.
Hybrids galore
Rice hybrids have been produced
using cytoplasmic male-sterility. Over-dominance and epistatic genes were
implicated as the basis for heterosis (and inbreeding depression, a phenomenon
in which inbred lines suffer decreased yield) [6, 7]. Alfalfa interspecies
hybrids showed heterosis, interspecies hybrids are a little different from
those originating from inbred lines, but in
general they act similarly to inbred lines [8].
A large number of vegetable crops have
been hybridized. Hybrid cucumbers have been produced by hand pollination,
removal of male flowers, or gynoecy (property of producing only female
flowers). There does not seem to be an available male sterility gene (9). Hot
and sweet peppers have been hybridized. Both nuclear and cytoplasmic sterility are used in some cases. Most
hybrid-pepper seed production is carried out in China, India or Thailand [10]. About two-thirds of
commercial onions are hybrids. These are produced using male sterility lines
[11]. Hybrid cabbage shows strong heterosis, and
the use of such hybrids is expanding. The seed is produced using male sterile
lines [12].
Most of the male sterile lines used
commercially contain mitochondrial genes, but such genes are not readily
available in a number of crops. Genetic engineers have developed a system of
male-sterility based on transformation of the chloroplast with a gene for
beta-ketothiolase that interferes with fatty acid synthesis, leading to disrupted anther tissue and a
failure to produce pollen. The beta-ketothiolase
gene is controlled by a light sensitive promoter, so that male-fertility can be
restored in hybrids using several days of continual illumination [13, 14]. The
system was developed in tobacco but may be extended to food crops, barring
unforeseen complications.
Genetically modified male-sterility
A number of genetically modified (GM) male-sterile crops have been developed
and tested in the field. In Canada, a male-sterile transgene was introduced
into the nuclear gnome of canola, and that construction was approved for, and
has been in commercial production. The transgenic construct included a barnase
ribonuclease gene controlled by a tapetum promoter. Barnase kills pollen cells
thus rendering the plant male-sterile. In the hybrid male fertility is restored
using the barstar inhibitor of barnase [15], although barnase is well known
to be toxic to animal cells. Development continued, and the technology came
to be used to protect GM traits patented by agribusiness corporations such as
herbicide tolerance under the general rubric of genetic use restriction technology
(GURT). Such crops were extensively field tested in Europe; and we have warned
that the F1 hybrid grown in the field will actually spread the barnase transgene
as well as the herbicide tolerance gene in pollen with potentially harmful ecological
impacts (“Chronicle of an ecological disaster foretold”, SiS
18) [16]. Furthermore, the toxin may well be carried over into the canola
press cake used both for both food and feed.
The development of hybrid seed had left seed production to
seed companies for the practical reason that it is the most economical way to
maintain appropriate inbred lines, and seed production can be isolated from the
food production areas of open pollinating crops. But it had also prevented farmers from saving and
replanting seeds, making it necessary to purchase seeds every season.
Biotechnology has gone a step further and demanded that seed production be
restricted to companies even when there is no rational basis for the restriction,
other than corporate greed. Goeshl and Swanson addressed the question of GURT
based on the hybrid-crop experience. They argued that developed countries could
benefit from the additional production supposedly to be gained by the technology,
while developing countries will suffer from their inability to afford the high
extra cost. They predict net deterioration in the developing countries due to
the widening gap in productivity [17]. These predictions must be taken with
a very large grain of salt. There is at present no evidence that genetic use
restriction technologies, or indeed, any genetic modification technology have
led to increase in crop yield. Furthermore, both hybrids and GM crops lack the
diversity required for sustainability in the complex ecosystems of the developing
world. What is needed is seed production that takes into account the unique
requirements of developing countries, where the farmers’ rights to save, replant
and exchange seeds are integral to food sovereignty and food security (“SOS:
Save our seeds”, SiS27).
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