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[SANET-MG] hybrid seed
August 12, 2005
Prof. Joe Cummins
“Hybrid Seed”
Hybrid seed began with maize in the 1920’s and that development led to
production of seed hybrids in vegetables and flowers. Crops such as rice
and some forage crops have recently begun producing hybrid seed. Hybrid
seeds are produced from out-breeding crops( in contrast to self
pollinating inbreeding crops). Inbred lines are produced by repeatedly
self pollinating the normally out-breeding crop then the established
inbred lines are crossed to produce hybrids. 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 produced by out
crossing. The precise basis of heterosis is still unclear but most
widely related to effects called epistasis (interactions between a
number of genes, such as the genes involved in a metabolic pathway) or
to over-dominance (heterozygotes superior to dominant homozygotes).
Hybrid seed is planted to produce a crop which is harvested , however,
saving seed from the crop and planting it is undesirable because the
genes in the hybrid segregate in the off-springs producing a variable
progeny that is harvested with difficulty and prone to disease. The
hybrid is maintained by perpetually crossing inbred lines. For practical
reasons only the seed companies produce hybrid seeds and those seeds
must be repurchased each year.
Hybrid maize arose through the advocacy of a few influential Americans.
Foremost among those advocated was Henry A. Wallace who became
vice-president of the United States Wallace graduated from University in
agriculture and upon graduating studied statistics on his own.. He later
taught that 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). He later entered politics and subsequently was
made Secretary of Agriculture then elected vice-president of the United
States. Wallace was noted for his concern for the common man and
envisioned hybrid corn a means of providing bountiful food at low prices
for average people. Fuller histories of the development of hybrid maize
are available (2,3).
The first corn hybrids were produced by detasseling the maternal inbred
line , that operation was usually done by young girls employed for the
summer in seed production. Later male sterile lines were developed. The
male sterile maternal lines were fertilized with paternal lines
containing a restorer gene that allowed the final hybrid seed to produce
male and female flowers. The male sterile lines are most frequently
altered in the mitochondrial genome leading to the inhibition of male
inflorescence development (4). A number of such lines are now available
, however , the early development of male sterile lines led to disaster.
The primary line used in the 1960s contained the T (Texas) cytoplasm
male sterility gene , by 1970 over 85% of the commercial maize planted
contained that gene , the gene harbored a pleiotropic sensitivity to an
invading 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 genes 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 favor of risky but profitable genetic adventures.
Rice hybrids have been produced using cytoplasmic male sterility. Over
dominant epistatic loci were implicated as the basis for heterosis (and
inbreeding depression, a phenomenon in which inbred lines suffer
decreased yield)(6). Epistasis was earlier implicated as a genetic basis
for heterosis in an elite rice hybrid (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 (production of 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 in China , India or
Thailand (10). About two thirds of commercial onions are hybrids. These
are produced using male sterility genes (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.
A number of genetically engineer male sterile crops have been developed
and tested in the field. In Canada a nuclear male sterile gene was
intruded into canola and that construction was approved and has been in
commercial; production. The nuclear gene included a barnase ribonuclease
gene controlled by a tapetum promoter. Barnase attacks the pollen cells
ablating them thus rendering the plant male sterile. In the hybrid male
fertility is restored using the barstar inhibitor of barnase (15).
Barnase is well known to be toxic to animal cells and there is every
likelihood that the toxin is carried over into the canola press cake
used both for both food and feed(16).
In looking at the development of hybrid seed where production is limited
to seed companies for the practical reason that it is the most
economical way to maintain appropriate inbred lines and seed production
in isolation from the food production areas of open pollinating crops.
However , biotechnology went a step further and demanded that seed
production be restricted to companies even when there was no rational
basis for the restriction , other than the greed of corporations. Based
on the hybrid crop experience the question of genetic use restriction
was addressed by Goeshl and Swanson. They argued that developed
countries could benefit from the additional production hoped to be
gained by use restriction with its high end technology. However,
developing countries will suffer from their inability to afford the
costly technology. They predict net deterioration in the developing
countries flowing from the gap in productivity (17). On top of that 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.
References
1.Higgins,A. . The Life of Henry A. Wallace: 1888-1965 2005
http://www.winrock.org/wallacecenter/wallace/bio.html
2.Duvick,D. Biotechnology in the 1930s:the development of hybrid maize
Nature Reviews Genetics 2001, 2, 60-6
3.Troyer,A. Review and Interpretations Background of US Hybrid Corn Crop
Sci. 1999,39,601-26
4.Wen,L. and Chase,C. Pleiotroppic effects of a nuclear restorer of
fertility locus on mitochondrial transcripts in male fertile and S male
sterile maize Curr Genet 1999,35, 521-26
5. Crow,J. 90 years ago : the beginning of hybrid maize Genetics
1998,148,923-8
6. Luo LJ, Li ZK, Mei HW, Shu QY, Tabien R, Zhong DB, Ying CS, Stansel
JW, Khush GS and Paterson AH. Overdominant epistatic loci are the
primary genetic basis of inbreeding depression and heterosis in rice.
II. Grain yield components. Genetics. 2001 Aug;158(4):1755-71
7. Yu SB, Li JX, Xu CG, Tan YF, Gao YJ, Li XH, Zhang Q and Maroof MA.
Importance of epistasis as the genetic basis of heterosis in an elite
rice hybrid. Proc Natl Acad Sci U S A. 1997 Aug 19;94(17):9226-31
8. Riday,H. and Brummer,C. Heterosis of agronomic traits in alfalfa Crop
Sci. 2002,42,1081-87
9.Robinson,R. Rationale and methods for producing hybrid cucurbit seeds
Journal of New Seeds 1999, 1,1-47
10.Burke,T. Hybrid seed production in capsicum Journal of New Seeds
1999, 1,49-67
11.Pathak,C. Hybrid sed production in onion Journal of New Seeds 1999,
1, 89-108
12. Zhiyuan,F,Wang,X, Dongyu,Q. and Guanshu,L. Hybrid seed production in
cabbage Journal of New Seeds Journal of New Seeds 1999,1, 108-29
13.Khan,M. Engineered male sterility Nature 2005,436, 783-4
14. Ruiz ON and Daniell H. Engineering Cytoplasmic Male Sterility via
the Chloroplast Genome by Expression of {beta}-Ketothiolase. Plant
Physiol. 2005 Jul;138(3):1232-46
15. Decision document DD95-04: Determination of environmental safety of
environmental safety of plant genetic systems Inc. novel hybridization
system for canola Canadian Food Inspection Agency 1995
http://www.inspection.gc.ca/english/plaveg/bio/dd/dd9504e.shtml
16. Ho,M. and Cummins,J. Chronical of an ecological disaster foretold
ISIS report 2003 http://i-sis.org.uk/
17.Goesschl,T. and Swanson,T. The development impact of genetic use
restriction technologies : a forecast based on the hybrid crop
experience Environment and Development Economics 2003,8,149-65
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