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.
Conservation in ex situ gene banks ensures that stored material is readily accessible; can be well documented, characterized, and evaluated; and is relatively safe from external threats. When material is stored in this way, plant evolution is effectively 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 conservation 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 documented. 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 diseases 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 documentation systems becoming more powerful and user-friendly, but also data exchange and the sharing of information among different systems are becoming easier. One example 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 international access through the Internet.2
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 developed 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 plaits 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.
2 See http://www.cgiar.org/SINGER.
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 implementation 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.
While plant breeders can readily access germplasm maintained in ex situ collections, 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).
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