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ISIS Report 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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