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OCR for page 5937
Proc. Natl. Acad. Sci. USA
Vol. 96, pp. 5937-5943, May 1999
Colloquium Paper
This paper was presented at the National Academy of Sciences colloquium "Plants and Population: Is There Time?"
held December 5-6, 1998, at the Arnold and Mabel Beckman Center in Irvine, CA.
Plant genetic resources: What can they contribute toward
increased crop productivity?
DAVID HOISINGTON*, MIREILLE KHAIRALLAH, TIMOTHY REEVES, JEAN-MARCEL RIBAUT, BENT SKOVMAND,
SUKETOSH} TABA, AND MARILYN WARBURTON
International Maize and Wheat Improvement Center (CIMMYT), Lisboa 27, Apartado. Postal 6-641, 06600 Mexico City, Mexico
ABSTRACT To feed a world population growing by up to
160 people per minute, with >90% of them in developing
countries, will require an astonishing increase in food pro-
duction. Forecasts call for wheat to become the most impor-
tant cereal in the world, with maize close behind; together,
these crops will account for ~80% of developing countries'
cereal import requirements. Access to a range of genetic
diversity is critical to the success of breeding programs. The
global effort to assemble, document, and utilize these re-
sources is enormous, and the genetic diversity in the collec-
tions is critical to the world's fight against hunger. The
introgression of genes that reduced plant height and increased
disease and viral resistance in wheat provided the foundation
for the "Green Revolution" and demonstrated the tremendous
impact that genetic resources can have on production. Wheat
hybrids and synthetics may provide the yield increases needed
in the future. A wild relative of maize, Tripsacum, represents
an untapped genetic resource for abiotic and biotic stress
resistance and for apomixis, a trait that could provide devel-
oping world farmers access to hybrid technology. Ownership
of genetic resources and genes must be resolved to ensure
global access to these critical resources. The application of
molecular and genetic engineering technologies enhances the
use of genetic resources. The effective and complementary use
of all of our technological tools and resources will be required
for meeting the challenge posed by the world's expanding
demand for food.
Today, on the eve of a new millennium, we are approaching a
critical era in the evolution of our planet and species we are
in a race between growing population and food production.
This era was cast in Paul Ehrlich's The Population Bomb (1),
perhaps prematurely, as a time when population would out-
pace the earth's resources, including its capacity to produce
food. The threat of the Malthusian crisis forecast by Ehrlich
appears to have diminished as we have witnessed a slowdown
in the rate of population growth. But the challenge of feeding
a world population growing by up to 160 people every minute
(>90% of them in developing countries) remains daunting. It
is forecast that, by 2050, world population will increase from
the current level of ~6 billion to >8 billion people. Feeding
this population will require an astonishing increase in food
production. In fact, it has been estimated that the world will
need to produce as much food during the next 50 years as was
produced since the beginning of agriculture 10,000 years ago
(2~! Today, it appears more likely that a population/food crisis
may be born, not from an exponentially increasing world
population (though in some of the world's poorest regions,
population growth remains exceedingly high), but from an
ill-founded sense of complacency about food production.
PNAS is available online at www.pnas.org.
Our staggering requirement for food must be viewed in the
context of statistics that indicate that the area available for
food production has, essentially, remained constant since 1960
(3~. Despite some new land being brought into cultivation, soil
erosion and urbanization have offset these gains. In addition,
less resources (both human and financial) are being devoted to
overcoming major production constraints. Financial support
for agricultural research has decreased for the last several years
and is expected to continue its slow decline as most developed
nations continue to focus on domestic issues rather than
addressing the multitude of problems facing the world's de-
veloping nations.
How will we feed the world in the coming years? For the
foreseeable future, conventional agriculture will be our pri-
mary response, with cereal grains playing a pivotal role. The
International Food Policy Research Institute has predicted
that, by the year 2020, almost 96% of the world's rice con-
sumption, two-thirds of the world's wheat consumption, and
almost 60% of the world's maize consumption will be in
developing countries. Forecasts call for wheat to surpass rice
in its apical role in feeding the poor of those nations. It will
likely become the most important cereal in the world, with
maize close behind; together, these crops will account for
~80% of the cereal import requirements of developing coun-
tries. Many economists stress, however, that increased pro-
duction in developing countries will be essential for achieving
food security. Maize and wheat are each expected to have an
annual global demand of ~775 million tons eacht and will be
of critical consequence in the race between crop production
and population growth . This paper focuses on the potential of
genetic resources, particularly those of maize and wheat, to
help meet the continually expanding demand for these major
grains. It will indicate how such resources have contributed in
the past, and how they may advance our efforts in the future.
The Role of the Consultive Group on International
Agricultural Research (CGIAR) in Preserving Genetic
Resources
Simply stated, plant breeding depends on the correct combi-
nation of specific alleles at the 50-60,000 genetic loci present
in a plant's genome. The knowledge of where these alleles are
best found and the combination and evaluation of these into a
Abbreviations: CGIAR, Consultative Group on International Agri-
cultural Research; CIMMYT, Centro Internacional de Mejoramiento
de Maiz y Trigo; FAO, Food and Agriculture Organization; IPR,
intellectual property rights.
*To whom reprint requests should be addressed at: International
Maize and Wheat Improvement Center (CIMMYT), Aptdo 370, P.O.
Box 60326, Houston, TX 77205. e-mail: D.Hoisington@CGIAR.org.
TRosegrant, M. W., Sambilla, M. A., Gerpacio, R. V. & Ringler, C.,
Illinois World Food and Sustainable Agriculture Program Confer-
ence, May 27, 1997, Urbana-Champaign, IL.
5937
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OCR for page 5940
OCR for page 5942
OCR for page 5943
Representative terms from entire chapter:
genetic diversity
5938 Colloquium Paper: Hoisington et al.
single species can be considered the "art" of breeding. Obvi-
ously, access to a wide range of genetic diversity is critical to
the success of any breeding program.
The work of the 16 centers that collectively form CGIAR
represents the largest concerted effort toward collecting, pre-
serving, and utilizing global agricultural resources. Together,
the centers hold nearly 600,000 samples of the estimated 6
million accessions stored globally (Table 1~. The remaining
germplasm are stored in other international, regional, and
national gene banks, many of which collaborate closely with
the CGIAR centers. If one considers that the Food and
Agriculture Organization (FAO) has estimated that the total
number of unique accessions globally are on the order of 1-2
million, the CGIAR centers account for an estimated 30-60%
of the world's unique holdings under long-term conservation.
Given that the CGIAR has focused its efforts on the crops of
highest significance in world agriculture, this proportion could
be even greater.
The materials in the CGIAR gene banks include traditional
varieties and landraces, nondomesticated species, advanced
cultivars, breeding lines, and genetic stocks. The effort re-
quired to assemble, document, and maintain these collections
is enormous but well justified as the genetic diversity present
in the gene banks represents a critical component in the world's
fight against hunger.
The International Center for Maize and Wheat Improve-
ment, better known by its Spanish name of Centro Interna-
cional de Mejoramiento de Maiz y Trigo (CIMMYT), has a
global mandate for improving the productivity and sustain-
ability of maize and wheat in developing countries. The
collection, documentation, and evaluation of the genetic re-
sources of maize and wheat are a critical part of meeting this
mandate. CIMMYT's newly established Genetic Resource
Center contains ~120,000 accessions of wheat and 18,000
accessions of Latin American maize (of the 25,000-35,000
accessions in partner gene banks in Latin America). This
represents the largest collection of these two important cere-
als. It is, perhaps, the only effort that is actively pursuing the
documentation and evaluation of its collection on a routine
basis.
Before discussing several issues related to the use of these
genetic resources, it is important to mention a few examples of
their contributions to crop improvement. Although these
examples have been drawn mostly from maize and wheat, there
Table 1. Summary of CGIAR's germplasm holdings (4)
Center Total holdings
70,940
136,637
13,911
2,448
109,029
110,478
39,756
13,470
1,051
80,646
17,440
595,806
Major species
Cassava, Phaseolus, rice
Maize, wheat
Potato, sweet potato
Agroforestry species
Lentil, chickpea
Chickpea, sorghum, groundnut
Yam, rice, maize, cassava
Forage legumes and grasses
Banana, plantain
Rice
Rice
CIAT
CIMMYT
CIP
ICRAF
ICARDA
ICRISAT
IITA
ILRI
IPGRI
IRRI
WARDA
Total
IITA, International Institute for Tropical Agriculture; CIAT, Cen-
tro Internacional de Agricultura Tropical; CIP, Centro Internacional
de Papa; ICRAF, International Centre for Research in Agroforestry;
ICARDA, International Center for Agriculture in the Dry Areas;
ICRISAT, International Crops Research Institute for the Semi-Arid
Tropics; ILRI, International Livestock Research Institute; IPGRI,
International Plant Genetic Resources Institute; IRRI, International
Rice Research Institute; WARDA, West African Rice Development
Association.
Proc. Natl. Acad. Sci. USA 96 (19994
are a large number of similar examples for many other major
food crops handled by the CGIAR Centers.
Contributions of Wheat Genetic Resources
Wheat is truly global, being one of the few crops grown over
most of the world. It belongs to the genus Triticum, which
oriainated almost 10,000 years ago in the historic Fertile
Crescent, an area in the Middle East. Triticum arose from the
cross (supposedly in nature) of two diploid wild grasses to
produce tetraploid wheat, which today includes the many
cultivated durum (pasta or macaroni) wheats (
Colloquium Paper: Hoisington et al.
Rust Resistance. Some of the most devastating and universal
crop diseases are caused by fungal pathogens. Among them,
the rust pathogens are the most widespread and generally
cause the largest crop losses per season. Many genes have been
found that provide resistance to specific races of each rust
pathogen. Within wheat, leaf, yellow, and stem rusts are major
pathogens. Fungicides can provide a level of control; however,
the chemical option is often limited for many farmers, partic-
ularly in developing countries, by high costs and lack of
knowledge about application. In addition, the negative envi-
ronmental effects of chemical applications can be consider-
able.
The incorporation of host plant resistance genes into mod-
ern wheat varieties has allowed yields of resistant wheat that
have not been treated with fungicides to nearly equal those of
the same varieties under fungicide applications (6~. Many of
these varieties have incorporated single major genes that
convey resistance to specific races of the rust pathogen. Of >40
known genes for leaf rust resistance, 12 originated in species
other than T. aestivam and T. turgidum while 20 of the 41
known genes for stem rust resistance originated in species
other than T. aestivam and T. turgidum (ref. 11; also see Table
2~. Even among the genes originating from T. aestivam, many
come from landraces.
Unfortunately, many of these major genes have already been
"broken"; i.e., the specific race has mutated to become viru-
lent against the specific resistance gene. Efforts to identify and
incorporate genes that confer "durable" resistance are there-
fore preferable. Several such genes have been identified and
incorporated into modern wheat varieties. One of the most
important is Lr34, which was originally found in the cultivar
"Frontana" (131. Lr34 has been incorporated into >50% of the
wheat varieties grown in the world today; together with several
modifier genes, it has resulted in stable resistance to leaf rust
Table 2. Important genes in wheat that were found in related
species (12)
Trait
Locus Source
Disease resistance
Leaf rust
Stem rust
Stripe rust
Powdery mildew
Wheat streak mosaic
virus
Karnal bunt
Pest resistance
Hessian fly
Cereal cyst nematode
Quality traits
Grain protein
High protein
Low molecular weight
glutenins
Lr9
Lrl8
Lrl9
Lr23
Lr24
Lr25
Lr29
Lr32
Sr2
Sr22
Sr36
Yrl S
Pml2
Pm21
Pm25
Wsml
Aegilops umbellulata
Triticum timopheevi
Thinopyrum
T. turg7tdum
Ag. elongatum
Secale cereale
Ag. elongatum
T. tauschii
T. turgidum
Tritzcum monococcum
Triticum timopheev~i
Triticum dicoccoides
Aegilops speltoides
Haynaldia villosa
1. monococcum
Ag elongatum
Qqantitative T. turgidum
trait loci
H21
H23, H24
H27
Cre3 (Ccn-D1)
Quantitative
trait loci
S. cereale
T. tausch~i
Aeg~lops ventricosa
T. tauschii
T. turg~dum
T. dicoccoides
T. turgidum
Proc. Natl. Acad. Sci. USA 96 (1999' 5939
in the 1980s and 1990s. Most of these durable sources of
resistance have not come from alien sources but from cultivars
and landraces that evolved in the past to contain broad levels
of resistance to pathogens.
The cultivar "Hope," bred in the United States earlier in the
century (14), was later used by Borlaug as a source of stem rust
resistance in the Rockefeller/Mexico wheat program. The
Hope resistance was based on the Sr2 gene that, when com-
bined with other unidentified genes, produced a more durable
resistance. The Sr2 originally came from a tetraploid wheat
variety known as emmer and has since been incorporated into
many wheat varieties worldwide, providing excellent levels of
resistance. Recently, a linked molecular marker was developed
(15) that may allow more rapid identification and manipula-
tion of this important gene. It is fair to say that the incorpo-
ration of the Sr2 and Lr34 genes from genetic resources into
cultivated wheat varieties represent milestones in the grain's
genetic advancement. Most likely, the gains of the Green
Revolution could not have been made without them.
Veery Wheats. Genetic resources have contributed more
than single genes to crop improvement efforts; entire chro-
mosomal segments also have been introduced with noteworthy
results. Perhaps the most important of these is the lB/lR
translocation that was identified as a simple transfer between
rye and wheat in the former Soviet Union cultivar "Kaukaz."
The lB/lR translocation, which carries a number of genes
from rye, confers resistance to various diseases (fungal and
viral pathogens) and adaptation to marginal environments
(16~. This translocation has been deemed so important that it
has been incorporated into >60 wheat varieties, including the
prominent Veery lines, that occupy >50% of all developing
country wheat area, almost 40 million hectares (174.
Yield Potential. Yields of the major cereal crops (rice, wheat,
and maize) have increased steadily over the past years, al-
though the rate of these yield increases appears to have
slowed.! To meet cereal production demand in the next
decade, we must continue to increase yields; even more
daunting, we must increase them at an ever-increasing rate.
How will such growth be supported, particularly when the rates
of increase over the past few years appear to have declined?
For rice and wheat, the use of hybrids may be one possibility,
although it remains to be demonstrated what level of heterosis
(hybrid vigor) can be achieved in either crop. Heterosis levels
currently detected in wheat are ~10-25~o, lower than the
25-35% levels historically found in maize, one of the first
hybrid cereal crops. The reasons for wheat's lower heterosis
levels have not been determined, but one possibility is the
lower level of diversity generally found in self-pollinated crops
such as rice and wheat. Many groups continue to search for
alternatives to the existing germplasm. In wheat, it is possible
to reproduce the hybridization event that created hexaploid
wheats from a cross of tetraploid with diploid wheat. These
so-called "synthetics" represent a source of novel genetic
variation (18~. Research at CIMMYT has led to the develop-
ment of >600 new synthetic wheats, crosses between various
durum wheats and T. tauschii accessions. Many of these crosses
have produced rapid improvements in important characteris-
tics, including disease resistance, abiotic stress tolerance, and
yield.
Although much work remains to be done, the use of
molecular genetic techniques now allows us to identify the
gene segments most likely responsible for improved perfor-
mance and, thus, to focus on more directed crosses in the
future. A recent example, now under investigation at CIM-
MYT, is the role of the Lrl9-containing segment. This gene (or
segment) originally came from Agropyron elongatum and was
tRosegrant7 M. W.7 Sambilla7 M. A.7 GerpaciO7 R. V. & Ringler7 C.7
Illinois World Food and Sustainable Agriculture Program Confer-
ence7 May 277 19977 Urbana-Champaign7 IL.
5940 Colloquium Paper: Hoisington et al.
first incorporated into the wheat variety "Agatha" (11~. Yield
trial data indicates that varieties containing the Lrl 9 gene yield
at least 10% more than counterparts without Lrl 9 (Ravi Singh,
personal communication). The gene was originally transferred
for its possible role in conferring leaf rust resistance, but its
potential to increase yields may become a more important
factor for breeders, thus demonstrating the often unantici-
pated potential of these alien transfers.
Use of Genetic Resources in Maize Improvement
Unlike wheat, the use of genetic resources in maize improve-
ment has not been well documented at the global level and may
not be as great. Although ~50,000 accessions of maize exist in
germplasm banks around the world (19, 20), most of these have
never been adequately evaluated for useful traits. Reasons
cited for low utilization include lack of evaluation data,
documentation, and information; poor coordination of na-
tional policies; and poor linkages between gene banks and
breeders (21~. The untapped potential of these genetic re-
sources is indicated to some extent by the progress that U.S.
breeders achieved through a combination of plant improve-
ment and pedigree breeding. Using double and three-way
crosses, varieties were produced that helped double U.S. yields
between 1930 and 1966; by 1995, single crosses and the use of
better hybrid materials by breeders helped triple 1930 yields.
Meanwhile, there have been frequent warnings about the
genetic vulnerability of maize and the potential of exotic
germplasm to reduce the threat (22-24~. It has been estimated
that, in the U.S.,
Colloquium Paper: Hoisington et al.
(IPR) to agricultural products, even plant varieties. This is in
great contrast to the 1960s and 1970s, when such protection
was considered a detriment to global progress in plant im-
provement. With initiatives such as the 1991 strengthening of
the Union for the Protection of New Varieties of Plants
Convention and the 1993 Multilateral Trade Negotiating
Rounds in the General Agreement on Tariffs and Trade, IPR
was widened to include inventions and breeding technology.
Looking to better protect genetic resources, FAO estab-
lished the International Undertaking on Plant Genetic Re-
sources in 1983. At its outset, the Undertaking subscribed to
the rule of free and unrestricted interchange of germplasm and
recognized all plant genetic resources as the "heritage for
mankind." The Undertaking was modified in 1989 and 1991 to
include resolutions regarding compensation and ownership of
genetic resources.
In 1992, the Convention on Biological Diversity officially
recognized the sovereign rights of individual nations over
biological diversity and the resources within their territories.
The Treaty has been ratified by most developing countries and
many developed countries but has yet to be ratified by the U.S.
A result of the Convention has been a modification of the
agreement between FAO and the CGIAR centers. In 1994,
each CGIAR Center signed an agreement with FAO putting
their genetic resources "in trust" under the auspices of the
FAO. These agreements describe the roles and responsibilities
of the centers as trustees and include articles that state: (i) The
Center shall hold the designated germplasm in trust for the
benefit of the international community, in particular, devel-
oping countries; (ii) neither the Center, or any recipient, will
seek IPR protection over the designated germplasm or related
information; and (iii) the Center will undertake to make
samples of the designated germplasm and related information
available directly to users for the purpose of scientific research,
plant breeding, or genetic resource conservation without re-
striction.
Hawtin and Reeves (37) provide an excellent review of the
historic aspects and current status of IPR in the CGIAR
centers. The authors point out that several important issues
must still be resolved, such as the meaning of the phrase
"germplasm and related information" and the definition of
when a variety is sufficiently different from the original in-trust
germplasm from which it was derived (the issue of derived
varieties). These are under study, and it is expected that,
following the next round of negotiations, some, if not all, of
these issues will be clarified. What effects these agreements
will have on the use of genetic resources is unclear. The
interpretation of related information, in particular, will have
implications for IPR options. If the interpretation restricts IPR
over materials such as genes obtained from genetic resources
held in trust, these important resources will continue to be
incorporated in conventional ways, but the application of
biotechnology may be more limited.
Molecular Approaches to Utilization of Genetic Resources
There is a multitude of examples illustrating the use of genetic
resources to improve modern plant varieties, but one can
readily posit that biotechnology could substantially enhance
this use. Molecular genetics now allows the routine genetic
analysis of nearly any characteristic of interest. As these traits
are better understood, the underlying genes can be isolated
and used to identify corresponding genes in a wide range of
genetic resources.
Molecular genetics has already had a tremendous impacts on
plant breeding. Progress in the development of new PCR-
based marker systems (amplified fragment length polymor-
phisms and microsatellites) has opened the vast majority of
plant genomes to investigation. Linkage maps for many species
have been developed and often have been combined with
Proc. Natl. Acad. Sci. USA 96 (1999' 5941
suitable phenotypic data to identify genomic regions contain-
ing the genes coding for specific traits of interest. Many of
these regions have been further characterized (e.g., by map-
based cloning) and even manipulated (via genetic engineering
and marker-assisted selection) in breeding programs (38~.
Undoubtedly, these efforts will continue and will even be
enhanced by newer molecular techniques such as DNA arrays
(39) and automation.
DNA markers also have been used extensively to charac-
terize germplasm, a process popularly known as fingerprinting,
to evaluate the genetic relationships among accessions (genetic
diversity) and provide important information in the areas of
ecology, population genetics, and evolution. Within wheat,
several studies have reported the application of molecular
markers within Asiatic wheat landraces (40~. Molecular
marker technology has advanced our understanding of genetic
resources more than any other type of genetic data.
A major hurdle, however, must be overcome soon: how to
store, access, and analyze the vast amounts of data that will be
produced by using molecular markers. Databases, such as the
International Crop Information System, are under develop-
ment to store pedigree and performance information for most
of the crop species within the CGIAR. Efforts are also
underway to expand the capability of the system to include
molecular genetic and diversity information. This data will
allow scientists to more exactly determine "relatedness" be-
tween accessions and even trace specific genomic segments
through pedigrees. The potential uses of this information
capability for gene identification and verification are enor-
mous and point to the importance of this area. Fortunately,
computing hardware and software systems are keeping pace
with advances in molecular technology so that appropriately
large-scale and real-time applications should be feasible.
Bioinformation has been called the next revolution in the
world; it is certainly of major import to our utilization of
genetic resources.
Genetic Resources: What Are the Potential Impacts?
Agriculture before the 18th century completely depended on
landraces for new varieties. During the industrial revolution,
the entire nature and practice of agriculture was transformed
forever. The discovery of genetics by Mendel in the mid-19th
century and the subsequent work of other plant geneticists
provided the knowledge base that made dramatic increases in
agricultural productivity possible. Undeniably, these discov-
eries have led to vast improvements in agriculture, but they also
have led to a decline in the genetic diversity of the crops in
many farmers' fields. Landraces and traditional varieties have
been replaced by less diverse modern cultivars and hybrids.
What lies ahead? A tremendous challenge now awaits
us an increased demand for food production spurred on by
an increasing world population, a decreasing natural resources
base, and declining investments in agricultural research. How
will this challenge be met?
Genetic resources will be fundamental to our efforts to
improve agricultural productivity. These resources, fortunately
stored in gene banks around the world, evolved an assortment
of alleles needed for resistance and tolerance to the diseases,
pests, and harsh environments found in their natural habitats.
Consumer characteristics enter the picture when one considers
that farmers over the years have selected their preferred
varieties based on yield, color, texture, and taste. Many of these
combinations cannot be easily duplicated artificially, even with
the help of modern molecular techniques. What should be
alarming is the fact that many of these valuable genetic
resources are essentially "sitting on the shelf" in what have
been dismissively termed "gene morgues." The conservation
of a resource only becomes important if the resource has or
5942 Colloquium Paper: Hoisington et al.
Proc. Natl. Acad. Sci. USA 96 (1999'
farmer (often the net consumer) produce additional food in his
or her own fields may be the surest and quickest way to increase
food security in these countries
Can genetic resources contribute to maintaining the rate of
yield increase for the major crop species (or even increase it)
without requiring significantly higher inputs (human and fi-
nancial)? Molecular genetic studies indicate that self-
pollinating species generally have lower levels of molecular
diversity (as measured by the level of molecular polymor-
phism). Controversy still exists as to the exact relationship
between molecular diversity and yield potential, although
studies with maize indicate that at least among related mate-
rials, molecular diversity is positively and significantly corre-
lated with yield (43, 44J. Based on these studies, measures of
molecular diversity could be used to identify new germplasm
sources that, when crossed with existing varieties, would result
in enhanced yields. Work at CIMMYT using synthetic wheat
clearly indicates that this strategy is extremely promising.
The array of resources at our disposal, together with new
biotechnology techniques, provides us with a healthy measure
of optimism for meeting the world's future food requirements.
One technique or resource alone will not suffice; rather, the
effective and complementary use of all of our technological
tools and materials will be required to meet this enormous
challenge. We should not, however, let our growing capabilities
lull us into complacency. We cannot afford to wait on this
issue- we must move quickly and effectively. The world must
commit itself now to feeding its future generations to ensure
that, indeed, there is enough time for us to produce plants that
can provide food for all humankind.
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