Managing Global Genetic Resources Agricultural Crop Issues and Policies (1993) / Chapter Skim
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5. The Science of Managing Genetic Resources
5. The Science of Managing Genetic Resources
Pages 153-172
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From page 153... ...
In addition, base collections of many of the major crops have now grown so large and diffuse that they may inhibit, rather than promote, effective management of genetic resources and their use by plant breeders. Because of increased emphasis on the collection of wild and weedy relatives of crop plants and the continuing development of new elite gene combinations by plant breeders, collections that are already large may quickly grow progressively larger.
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From page 154... ...
There are special problems for wild species, including seed dormancy, seed shattering, and high variability in flowering time and seed production. Seed regeneration is thus costly in terms of resources and time, while the risks of genetic drift (change in allelic frequencies due to sampling accidents in small populations)
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From page 155... ...
Breeding systems determine to a considerable extent the patterns of ecotypic differentiation and, consequently, dictate subsequent regeneration procedures. Complete self-pollination is the most extreme form of inbreeding.
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From page 156... ...
Principles Involved in Maintaining Genetic Integrity During Regeneration The initial supply of seeds generally is inadequate for distribution and must itself be multiplied. The Liabilities of seeds or other propagules held in storage eventually fall below acceptable levels.
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From page 157... ...
Fortunately, with most inbreeders, minimal isolation, such as that provided by planting accessions in rows sufficiently separated to prevent physical contact between different accessions, virtually eliminates interaccession outcrossing. In other cases, for example, when bees serve as pollen vectors, techniques such as caging or larger separation distances may be necessary.
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From page 158... ...
Alternative methods use insect-proof cages, pollen-proof glasshouse chambers for wind-pollinated species, or hand-pollination. Population Size in Relation to Inbreeding Depression and Drift Genetic integrity during seed regeneration is also threatened by
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From page 159... ...
For amenable species, such as maize and sunflower, pollination control by hand crossing and storage of equal amounts of maternal seed can maintain effective population size at, or above, the nominal population number and, thus, minimize the effects of drift and selective shift. For many crop species, hand-pollination is impractical, and an effective population size can only be maintained by ensuring that random mating is achieved as efficiently as possible by natural pollen vectors (wind or insect)
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From page 160... ...
In general, random drift causes rare alleles, whether adaptive or not, to drift out of small populations, whereas selection tends to eliminate the less adaptive alleles and to increase the frequency of alleles that are better adapted in the regeneration environment. Evidence of marked phenotypic changes is now well documented in outbred populations, especially when multiplication is performed in populations growing in regions that differ from their origins (Breese, 1989~.
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From page 161... ...
At the same time, it should be remembered that few populations are completely selfpollinated in all environments, so that minimal isolation may require that accessions are sufficiently separated to avoid physical contact. When maintained as bulks, a doubling of the effective population size (Ne)
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From page 162... ...
Rice workers have arrived at a consensus, and the standardized descriptors (International Board for Plant Genetic Resources-International Rice Research Institute Rice Advisory Committee, 1980) are widely used.
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From page 163... ...
The Genetic Evaluation and Utilization Program of the International Rice Research Institute is an example of such multidisciplinary collaboration and interdisciplinary interaction. Effective evaluation experiments often lead to more refined and rewarding research.
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From page 164... ...
At the International Rice Research Institute, wild rices are grown in large pots to control their spread, to ensure their identity, and to obtain harvests from ratooned plants. Verification of Evaluation Data Disease- and insect-susceptible accessions may be recorded as such without retesting, whereas populations identified as resistant or tolerant need to be retested under more precisely controlled conditions to establish the validity of the preliminary evaluation.
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From page 165... ...
The use of standardized descriptors and descriptor states (International Rice Research Institute, 1975) has proved its usefulness in international evaluation programs.
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From page 166... ...
. Germplasm banks of the People's Republic of China and the International Rice Research Institute use a 50 percent threshold level, whereas the U.S.
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From page 167... ...
When countries heavily depend on particular crops, germplasm collections of those species acquire strategic importance, and larger national collections are seen as insurance against an uncertain future, especially in the light of the growing controversy surrounding the ownership and control of crop genetic resources. The major problem in redundancy is not within collections, but between them.
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From page 168... ...
If resistance is found in the core collection, then the breeder can use that resistance immediately in the breeding program as well as screen additional accessions from the now identified geographic areas held in the base collection. The core subset concept also has advantages for curators.
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From page 169... ...
. This can be done rationally only when reasonable knowledge about the materials in question is available (International Rice Research Institute, 1991~.
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From page 170... ...
For large collections, computer-based multivariate methods such as numerical cluster analysis, principal component analysis, or network analysis might be helpful to order the collection into related groups. RECOMMENDATIONS Neither the reduction of collection size by elimination of accessions nor the use of core subsets is universally accepted (see, for example, Brown, 1989a,b; Chang, 1989; Marshall, 1989; National Research Council, 1991a)
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From page 171... ...
each regeneration event places accessions at risk from clerical and other human errors, and in danger of genetic shift due to selection or genetic drift; and (3) effective population size is bound to vary with each regeneration event, and over several generations will be much closer to the smallest single value.
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From page 172... ...
The requisite expertise derives in part from direct experience beyond a training course or university degree. The development of trained professionals and germplasm workers is an imperative for sustaining the future of genetic resources and making advances in agriculture.
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