BIODIVERSITY IN AGROECOSYSTEMS - CHAPTER 14 pdf

Hawtin CONTENTS Introduction Sources of Genetic Diversity in Crops The Need to Conserve Diversity Approaches to Conservation Ex Situ Conservation In Situ Conservation Integrated Conserva

CHAPTER 14

Conserving and Using Crop Plant Biodiversity in Agroecosystems Wanda W Collins and Geoffrey C Hawtin

CONTENTS

Introduction

Sources of Genetic Diversity in Crops

The Need to Conserve Diversity

Approaches to Conservation

Ex Situ Conservation

In Situ Conservation

Integrated Conservation Strategies

Approaches to Breeding

Formal Systems

Informal Systems

Linking Conservation and Plant Breeding

Conclusions

References

INTRODUCTION

Biodiversity is the variability among living organisms from all sources, including terrestrial, marine and other aquatic ecosystems, and the ecological complexes of which they are a part The interaction of various forms of biodiversity creates and shapes the environment in which we live; it also creates and sustains the agroeco-systems on which we depend for food and other basic needs Diversity within an ecosystem enables that ecosystem to survive and be productive and to produce an

enormous range of products and services Agrobiodiversity is that component of biodiversity that is important to agriculture and agroecosystems It helps ensure sustainability, stability, and productivity of production systems regardless of the level

of complexity of the ecosystem in which it occurs, and, in the final analysis, it contributes to social welfare of the population through its contributions to poverty alleviation and sustainable food security

Diversity at the agroecosystem level contributes to greater food security, helps increase employment opportunities, and increases local or national self-reliance by allowing a variety of enterprises, based on products and services, to develop on a national, regional, or community scale A diversity of crop and animal species, at the community, farm, or field level adds to social and economic stability through reducing reliance on a single enterprise Such diversity can also lead to a more efficient use of natural resources, for example, through providing greater opportu-nities for nutrient recycling (Carroll et al., 1990) Species diversity can also provide

a buffering effect against losses to diseases and pests or adverse weather conditions Diseases and insects that are major problems in large, single-crop species plantings become less of a problem and cause less damage when additional crop species are added to the system (Alexander and Bramel-Cox, 1991) Even at the field level, diversity generated through planting crop mixtures can reduce losses to pests and diseases The net result of these types of utilization of crop diversity is resilience and sustainability of agroecosystems

Within any particular species, genetic diversity is the variation which is most important It is the variation that enables that species to adapt to new ecosystems and environments through natural and/or human selection Genetic diversity within

a cultivated crop species at the field or farm level helps diminish the risk of losses through diseases or pests, and provides opportunities to exploit different features of the microenvironment through, for example, the presence of diverse growth habits and rooting patterns (Smith and Zobel, 1991) Such factors can contribute both to greater stability and, in many circumstances, greater productivity Both multilines and variety mixtures have been used effectively for this purpose in grain crops (Matson et al., 1997)

Modern agroecosystems often rely on crop species which are more uniform (i.e., less genetically diverse) than those in traditional agroecosystems However, the conservation of the existing genetic diversity of species is critical to the modern farmer as well as the traditional farmer Genetic diversity provides the reservoir of genes for future crop improvement by farmers and professional plant breeders The ability to continue to rely on uniform, high-yielding crop species in modern agro-ecosystems depends on the constant new identification and use of genes that are, or have been, found in the genetically diverse crops of the traditional agroecosystems Similarly, useful genetic diversity can be found in wild relatives of crop species and this diversity must also be conserved and appropriately used for improving crop performance

Thus, diversity is important to agriculture at all levels and in all agroecosystems While recognizing the importance to agriculture of diversity at all these levels, this chapter will focus on genetic diversity within crops — the genetic resources that lie

at the heart of sustainable agricultural development and provide the basis for the continued evolution and adaptation of crops

SOURCES OF GENETIC DIVERSITY IN CROPS

Genetic variation within a crop gene pool can be found within and among professionally bred varieties, landraces or farmers’ varieties, and nondomesticated relatives In addition, new genetic variation can be introduced through mutations and the transfer of genes from different gene pools Commercially released varieties aim to combine genes for high productivity with those required to meet different needs and environments They contain a wealth of useful genes and gene combina-tions and normally form the basis for further professional plant-breeding efforts Landraces and farmers’ varieties tend to be genetically heterogeneous and have

proved to be an excellent source of genes for, inter alia, adaptive characters and

disease and pest resistance They are still widely grown, especially in marginal environments where they may be more stable, and even more productive, than many modern varieties Landraces of many minor crop species are also still commonly grown as, in general, they have received relatively little attention from plant breeders and have been less subject to replacement by modern varieties

Wild, nondomesticated relatives of crops frequently provide useful sources of

genes For example, a wild rice, Oryza nivara, was used to introduce resistance to

grassy stunt virus in cultivated rice (Khush and Beachell, 1972) In Africa and India,

cassava (Manihot esculenta) yields increased up to 18 times after genes from wild

Brazilian cassava, conferring disease resistance, were incorporated into local vari-eties (Prescott-Allen and Prescott-Allen, 1982) In the U.S., disease-resistant, wild

Asian species of sugarcane (Saccharum sp.) helped to save the U.S sugar cane

industry from collapse (Prescott-Allen and Prescott-Allen, 1982) Many other cases that have benefited agriculture in all parts of the world can be cited, as well

In addition to these sources of genetic diversity, new DNA sequences can be created or introduced into crop species For example, mutations are a source of new diversity and can be induced by chemical mutagens or ionizing radiation And with modern genetic engineering techniques, all organisms, at least in theory, can contain potentially useful genes which could be transferred between crops and induced to express themselves These new genes then become integrated into the plant genome and are passed from generation to generation

THE NEED TO CONSERVE DIVERSITY

Conservation of genetic resources is essential, both to ensure that professional breeders continue to have access to the genes and gene complexes needed for current and future crop improvement and to enable farmers to continue to select and modify their crops in response to changing environments and circumstances Plant breeders very effectively used genetic resources, coupled with higher levels of inputs, to meet

the demands of food shortages in the 1960s and 1970s The result was the Green Revolution However, the lessons learned from that period have contributed to the current emphasis on long-term sustainability of production systems and the protec-tion of the natural resource base, while at the same time maintaining an adequate and healthy food supply This places new demands on plant breeders to continue to increase productivity in socially and environmentally appropriate ways Plant breed-ers will respond to that demand; biodivbreed-ersity is the means for achieving their goals Potentially valuable genes and gene combinations, which might at present be unknown or undiscovered, can provide the means to fewer external inputs in crop production systems, lower levels of environmentally toxic pesticides, and internal resilience of agroecosystems Conserving these needed genetic resources for future use in the face of technical and political obstacles can be an enormous challenge The number of higher plant species is estimated to be between 300,000 and 500,000, of which approximately 250,000 have so far been identified (Wilson, 1988; Heywood, 1995) Of these, about 30,000 are edible and an estimated 7000 have been cultivated or collected by humans for food (Wilson, 1992) Despite this, the crops that “feed the world,” by providing 95% of dietary energy or protein, number only 30 (McNeely and Wachtel, 1988) Just three of them — rice, wheat, and maize

— account for almost 60% of the plant-derived calories in the human diet Much of the genetic diversity of major tropical food crops is now believed to

be reasonably secure in gene banks, especially of those species having orthodox seeds such as rice, wheat, sorghum, and maize For some crops, which require conserving bulky vegetative organs or living tissues other than seeds, there are often fewer accessions maintained Cassava is a crop of world importance, but there are

only 23,000 accessions; and yam (Dioscorea spp.), an important staple crop in Africa,

is represented by only 11,500 accessions There are still specific gaps in collections even for crops which are well represented For example, there are over 300,000

accessions of rice in storage, but there is still need for conservation of O sativa

from Madagascar, Mozambique, South Asia, and Southeast Asia (FAO, 1996) The situation is greatly different for most minor crops and those species that are vegetatively propagated or produce seeds that cannot easily be stored for extended

periods at subzero temperatures Crops such as taro (Colocasia esculenta), yams, rice beans (Vigna umbellata), and breadfruit form part of the staple diet of millions

of the world poor, yet relatively little work has been done either to conserve or improve them In addition, there are many underutilized species of vegetables, fruits, and other species, including nondomesticated plants, which contribute to nutrition and dietary diversification in millions of tropical households These species often have been largely ignored because they are of little commercial value or because they are at little risk of being lost The result is that very little is known of the diversity, distribution, and characteristics of such species and so conservation and maintenance efforts are minimal

Concern about the lack of knowledge and attention to conservation and enhance-ment was expressed by many countries in the regional and subregional meetings held in preparation for the Food and Agricultural Organization (FAO) International Technical Conference on Plant Genetic Resources, held in Leipzig, Germany in June

1996 For example, representatives of countries in west and central Africa identified

a large number of underutilized species which are important to the livelihoods of local populations These included 7 species of cereals, 8 of legumes, 4 of roots and tubers, 8 of oil crops, 31 of fruits and nuts, 17 vegetables and spices, 4 of beverages,

38 of medicinal plants, and 44 genera of forages.1

The situation for minor and underutilized species could be improved in the future,

as some gene banks have agreed to accept regional responsibility for long-term ex

situ storage of some minor crops The National Bureau of Plant Genetic Resources

in India has, for example, accepted responsibility for rice bean, moth bean (V.

aconitifolia), okra (Abelmoschus esculentus), and amaranth (Amaranthus spp.), and

the Institute of Plant Breeding in the Philippines has accepted responsibility for winged beans (FAO, 1996)

Because of the gaps in collections of both major and minor crops, the added factor of genetic erosion increases the urgency to conserve diversity The diversity can be immense within the relatively small number of plant species which supply most of the world energy and protein For example, the International Rice Research

Institute gene bank contains about 80,000 accessions of rice But much of the rice

diversity may already have been lost from farmers’ fields and may now only exist

in gene banks By 1982, the rice variety IR36 was grown on about 11 million ha in Asia and had replaced many local varieties (Plucknett et al., 1987) Although there are varied reasons for the loss of genetic diversity, there is widespread agreement that one major reason is replacement of local crop varieties by new cultivars At present, few quantitative data exist to define the extent and rate of genetic erosion

of crops and their wild relatives (Ceccarelli et al., 1992) Farmers in traditional systems will routinely and intentionally discard components of local crop varieties

as a normal part of their management practices (Wood and Lenné, 1997)

The report State of the World’s Plant Genetic Resources (FAO, 1996) also lists

a number of other causes of such genetic erosion, including:

• Changes in agricultural systems and the abandonment of traditional crops in favor

of new ones;

• Overgrazing and excessive harvesting;

• Deforestation and land clearance, which is cited as being the most frequent cause

of genetic erosion in Africa;

• Adverse environmental conditions such as drought and flooding;

• The introduction of new pests and diseases;

• Population pressure and urbanization;

• War and civil strife; and

• Policy legislation (for example, until recently the cultivation of farm landraces was discouraged in Europe).

Regardless of the reasons for disappearance of local crop varieties and their wild relatives, the need to conserve that germplasm must be considered in conservation strategies

There are additional needs which must be recognized in considering the necessity

to conserve genetic resources, such as the high concentration of global collections

1 See http://www.icppgr.fao.org/srm/srm-syn/caf/E3.html.

for many export crops and commodities concentrated in a small number of countries For example, Zaire maintains over 80% of the global oil palm accessions It is critical that more attention be given to managing these collections, to safety duplication, and to establishing new collections, especially in areas of diversity In addition, some tropical countries need to consolidate national collections to comply with the Con-vention on Biological Diversity, which emphasizes in-country conservation of

indig-enous genetic resources, both in situ and ex situ Many tropical and subtropical

countries report problems in maintaining collections because of the lack of suitable drying facilities and unreliable electricity supplies (FAO, 1996) This situation is particularly serious in coastal regions having 60 to 90% relative humidity Further constraints to conservation in the majority of developing countries are the lack of adequate human and financial resources and infrastructure

Overall, the efforts to conserve the most important world agrobiodiversity are impressive and commendable However, as noted above, there is still much to be done and many questions to be answered about the most effective and efficient conservation strategies The need exists, as well, to ensure that agrobiodiversity is not just conserved but also fully utilized to serve agroecosystem needs

APPROACHES TO CONSERVATION

Conservation can be broadly considered in two ways: ex situ and in situ Ex situ

conservation involves removing reproductive plant material from its natural setting for maintenance in seed or tissue banks or plantations Because of the finite nature

of any living plant material, ex situ conservation also requires regeneration of the

reproductive material at given storage conditions and at species-dependent intervals

In situ conservation is accomplished by protecting plant material in the site in which

it naturally occurs For most wild relatives this is in nature preserves or in wild stands For landraces, or traditional farmer varieties, it occurs in the fields in which farmers grow those varieties (on-farm conservation) or in the communities in which they are grown

Ex Situ Conservation

Conservation in ex situ gene banks ensures that stored material is readily

acces-sible; can be well documented, characterized, and evaluated; and is relatively safe from external threats When material is stored in this way, plant evolution is effec-tively frozen at the time of storage

Of the main ex situ methods of conservation, the most common is the storage

of dried seeds in gene banks at low temperatures For recalcitrant seeds, such as those of many tropical perennial species, and vegetatively propagated germplasm,

such as Musa, cassava, or potatoes, other methods are needed These include con-servation as living collections in field gene banks or in vitro either as living plantlets,

as plant tissue on appropriate media, often under conditions of slow growth, or by

cryopreservation at very low temperatures, generally using liquid nitrogen Genetic

resources are also conserved as frozen or freeze-dried pollen Increasing use is being made of isolated genomic DNA banks for the storage of germplasm (Adams, 1997) and for most major crop species as cDNA inserts in microbial species such as yeast and bacteria DNA nucleotide sequence data for functional genes are a relatively new mode of conservation of genetic resources From these data, polynucleotides can be synthesized in the laboratory

For breeding programs, the screening of very large numbers of accessions for specific traits can be expensive and time-consuming Attention has been focused in recent years on the development of core collections as a mechanism to facilitate their use (Hodgkin et al., 1995) Core collections are collections that aim to represent most of the diversity spectrum of the parent collection with a manageable number

of accessions, thus improving access to the whole collection In setting up a core collection, hierarchical approaches may be used, frequently with geographic origin

as one of the primary levels of discrimination Specific adaptive traits (such as maturity groups) can also be used to help stratify collections

To be most useful to breeders, germplasm collections should be well docu-mented Accurate passport data, including site descriptions, are useful as a basis for correlating origins with environmental parameters Characterization data include information on traits that are simply inherited and stably expressed in a wide range

of environments, such as major morphological features These types of data assist

in discriminating among accessions and provide information on major adaptive features (e.g., phenological characteristics) Evaluation data involve characters important in crop production, such as yield and its components, resistance to dis-eases and insect pests, flowering time, and plant height, and are perhaps the most useful overall in the search for special adaptive traits, especially if originating in diverse environments

With respect to landraces, geographic origin and local knowledge can provide very valuable leads to possible sources of genes Farmers typically have a good knowledge about the attributes of their varieties — e.g., phenology, reaction to prevalent pests and diseases, and suitability for growing on the different soil types found in the vicinity Local knowledge is only rarely sought during collecting, and greater efforts are needed to record such information (Guarino and Friis-Hansen, 1995) It has often been argued (IPGRI, 1993) that such information is under as great or greater threat as the germplasm itself and data collection forms for plant genetic resource collectors which provide for notes on ethnobotanical information that should be obtained during collecting missions have been developed (Eyzagu-irre, 1995)

With recent advances in computer science, not only are germplasm documenta-tion systems becoming more powerful and user-friendly, but also data exchange and the sharing of information among different systems are becoming easier One exam-ple of how the new technologies are being applied is the information system under development by the International Agricultural Research Centers of the Consultative Group on International Agricultural Research (CGIAR) Collectively, these centers maintain over 500,000 germplasm accessions of most of the major world food and forage crops The information system is known as SINGER (System-wide

Informa-tion Network on Genetic Resources) and is available for internaInforma-tional access through the Internet.2

In Situ Conservation

In situ techniques allow the conservation of greater inter- and intraspecific genetic

diversity than is possible in ex situ facilities They also permit continued evolution

and adaptation to take place, whether in the wild or on-farm where human selection also plays a critical role For some species, such as many tropical trees, it is the only feasible method of conservation Sustaining habitats indefinitely due to hazards

such as extreme weather conditions, pests, and diseases is a major concern for in

situ conservation Difficulties in mapping, characterizing, evaluating, and accessing

genetic resources in situ are evident.

As with ex situ conservation, the method adopted depends on the nature of the

species Traditional crop cultivars may be conserved on-farm, while undomesticated relatives of food crops may require the setting aside of reserves Agroforestry species, and other plants which require little maintenance, can be conserved by developing and maintaining sustainable harvesting practices and involving local communities, while forest genetic resources are usually maintained in forest reserves and in areas under specially designed management regimens

One of the first steps for in situ conservation of target species or populations is

to determine their status in the area where they exist It is also necessary to determine the factors known to threaten the survival of the species and its vulnerability at various stages of its life cycle In the case of species threatened by extinction, the minimum viable population size in the target area needs to be determined This concept implies that a population in a given habitat cannot persist if the number of organisms is reduced below a certain threshold The Species Survival Commission Steering Committee of the World Conservation Union (IUCN) has recently devel-oped new categories for threatened species based on population sizes, fragmentation, and population viability analysis (IUCN, 1994) With the growing availability and use of techniques for crossing plants which are distantly related and for transferring genes from non related genera or even kingdoms, the search for useful genes has been broadened This has resulted in an increase in activities devoted to the collection and maintenance of crop wild relatives (Ingram and Williams, 1987) This, in turn,

has led to a greater realization of the value of in situ techniques for ensuring the

conservation of a large range of potentially useful genes for future use in breeding Once considered primarily the domain of environmentalists and conservationists,

in situ conservation is now also becoming of increasing interest to those concerned

with crop improvement (Hodgkin, 1993) However, even though there is this growing

interest in the in situ conservation of genetic resources, most current in situ programs

target the preservation of ecosystems (often areas of outstanding natural beauty) or particular species (generally endangered animals or plants) rather than the intraspe-cific genetic diversity of plant species of potential interest for agriculture

Options for in situ conservation range from nature reserves from which all human

intervention is excluded, through national parks in which economic activities with

a potential to disturb the natural ecosystems are carefully regulated, to the imple-mentation of special management regimes in areas used primarily for agriculture and forestry The identification of specific areas in which a deliberate attempt is made to increase and maintain intraspecific diversity of key species is another approach (Krugman, 1984) which is being tried in Turkey and in Mexico The Man and Biosphere (MAB) program of the United Nations Economic, Social and Cultural

Organization is perhaps the largest coordinated global attempt to establish in situ

reserves, one of the objectives of which is the conservation of natural areas and the genetic materials they contain Under the MAB program, more than 250 biosphere reserves have been established around the world

As more attention is paid to in situ conservation, more innovative approaches

are developed For example, locally based conservation (Qualset et al., 1997) seeks

to conserve biological entities at the farm, community, or regional level Local issues such as traditional and cultural behavior and knowledge play a large part in the conservation effort and are thus conserved themselves

Integrated Conservation Strategies

While plant breeders can readily access germplasm maintained in ex situ collec-tions, it is far more difficult to do so in the case of material conserved in situ.

Nevertheless, the amount of inter- and intraspecific diversity that can be conserved

ex situ is a very small proportion of the total potentially useful variation And, for

technical reasons, some domesticated and many wild species are very difficult to

conserve ex situ Thus, to provide a comprehensive conservation program for any particular species, strategies must include both ex situ and in situ approaches.

The comprehensive conservation of crop gene pools, which often comprise both domesticated and wild forms, may require a combination of different methods, each covering a different part of the gene pool, to enable the total to be conserved in the most cost-effective and efficient way possible Bretting and Duvick (1997) use the

terms static conservation and dynamic conservation (roughly comparable to ex situ and in situ, respectively) to denote the purpose of the conservation programs rather

than the location They recommend close collaboration between static conservation, which serves to safeguard genetic resources outside the evolutionary context, and dynamic conservation, which seeks to safeguard genetic resources in nature In dynamic conservation, the potential for evolution of the resources is conserved as well as the cultural and agroecosystem properties that evolve along with them The two are not mutually exclusive but are seen to be integral parts of a continuum of conservation

The choice of appropriate strategies to protect and conserve the full range of diversity in a crop species and its relatives depends on technical factors such as reproductive biology and the nature of storage organs or propagules It also depends

on the availability of human, financial, and institutional resources to sustain a course

of action once it is chosen Such combinations of approaches are often referred to

as integrated conservation strategies and are based on the unique complementarity

of strengths and weaknesses between different approaches with respect to a single crop (Hawtin, 1994)

APPROACHES TO BREEDING

Approaches to crop improvement generally fall into one or two broadly defined systems:

• Formal systems, in which modern science is brought to bear on crop improvement

by institutions such as government plant-breeding stations, university departments, and private breeding companies, with the aim of producing cultivars for wide, often commercial, distribution to farmers.

• Informal systems, in which farmers and local communities, mainly in developing countries, breed and select cultivars primarily to meet their own needs and circum-stances.

Both systems coexist in many regions and each depends, to a greater or lesser extent,

on the other

Formal Systems

Formal systems of crop improvement normally aim to produce high-yielding cultivars that are broadly adapted across a wide range of agroecosystems Special attention is given to breeding cultivars with specific resistances to pests and diseases and tolerances to abiotic stresses Quality characteristics are generally determined

by the preferences of large consumer groups, often in importing countries, or by the demands of processors New cultivars must meet legal requirements for distinctness, uniformity, and stability to be officially registered Plant breeding is an expensive process, and there is an ever-growing need to show financial returns on investment

in crop improvement from the public as well as private sectors Under these circum-stances, it is perhaps not surprising that the majority of modern cultivars are widely grown or high-value crops and tend to have a relatively narrow genetic base

In the formal sector, breeding for adaptation will continue to be concerned with improving adaptability in existing environments, extending the areas in which indi-vidual crops are grown, or seeking improved stability across a range of environments New variation will be sought and traditional cultivars and wild relatives will continue

to provide the necessary variation so long as their conservation is secured Improved knowledge of the distribution of diversity and of the effect of specific environmental variables on that distribution will improve our capacity to locate desired characters Improved understanding of the significance and nature of coadapted gene complexes will enable breeders to use adapted germplasm with much greater efficiency

In formal systems, breeders are generally concerned with adaptation in one of three ways: