Managing Global Genetic Resources Agricultural Crop Issues and Policies (1993) / Chapter Skim
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4. The Science of Collecting Genetic Resources
4. The Science of Collecting Genetic Resources
Pages 131-152
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From page 131... ...
Base collection materials are not intended for distribution except to replace materials that have been lost from back-up or active collections. Base collections include the most comprehensive sample of the entire genetic 131
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From page 132... ...
Alternative methods of long-term storage, such as cryopreservation of in vitro cultures, are needed (see Chapter 7~. A global network of germplasm collections of various crops has been initiated with the guidance and help of the Food and Agriculture Organization of the United Nations and the International Board for Plant Genetic Resources, by designating different agencies that serve the base and back-up collections for principal crop species.
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From page 133... ...
The Science of Collecting Genetic Resources / 133 Seeds are counted, packaged, and sealed in foil-laminated moisture-proof bags for storage at the National Seed Storage Laboratory. Credit: U.S.
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From page 134... ...
Modern breeders, however, infrequently turn directly to exotic materials in active collections for potentially useful variability. Rather, they usually obtain their exotic variability indirectly from genetically enhanced populations, breeding stocks, or both into which potentially useful alleles have been introgressed.
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From page 135... ...
In this model, the expected numbers and frequencies of alleles are a function of the effective population size and the rate at which mutation produces novel neutral alleles at any locus. When either or both of these is small, it is expected that most neutral alleles will be either very common or very rare within populations but that, as population size or mutation rates increase, a higher proportion of neutral alleles will occur at intermediate frequencies.
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From page 136... ...
Thus, if selection strongly favors one allele, the expected equilibrium situation within populations is that the highly favored allele is likely to be more frequent than p = .5, that fewer alleles will be present at intermediate frequencies, and that there will be more rare alleles than the number under the neutral model. Balancing selection, in contrast, is expected to lead to greater numbers of alleles present at intermediate frequencies, and if it is strong, it is expected to lead to maintenance in populations of large numbers of alleles, each at a low frequency (that is, allelic distribution profiles are expected to be more flat, platykurtic, than they are under the neutral model)
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From page 137... ...
Although the majority of loci that govern traits that can be classified into discrete categories, such as color or morphology, appear to be nearly monomorphic for a wild-type allele or a cluster of wild-type isoalleles, some loci appear to have allelic profiles featuring one or two frequent alleles plus a number of infrequent or rare alleles. Also, a few loci are known that
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From page 138... ...
In summary, studies of loci whose variants can be classified into discrete categories indicate that allelic profiles for most such loci in Drosophila and other species tend toward near monomorphism (p > .9) for a single wild-type allele, or a cluster of nearly indistinguishable wild-type isoalleles, accompanied by a number of infrequent to rare mutant alleles or clusters of mutant isoalleles.
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From page 139... ...
a single copy of this rare allele.) It follows that formulation of sampling strategies for the first and fourth classes of alleles does not lead to general guiding principles in sampling, other than the trivial principle of ignoring both classes or of collecting impractically large samples to detect the rare alleles.
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From page 140... ...
or determining the distribution of environmentally influenced genetic variation within species. Very large numbers of accessions have been accumulated in germplasm collections for many different species and species groups, and these large numbers in themselves have often been taken to indicate that collections include all or nearly all potentially useful genetic variability.
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From page 141... ...
Fourth, the management of accessions within germplasm collections has often been inadequate, with the result that there has been a decay of genetic variability within accessions. EMPIRICAL STUDIES OF ALLELIC DIVERSITY AND ENVIRONMENTALLY INFLUENCED GENETIC DIFFERENTIATION Two aspects of genetic variation are of major importance in formulating sampling strategies: (1)
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From page 142... ...
Allelic profiles were closely similar for 4 additional isozyme loci; these 4 loci differed from the 5 invariant or nearly invariant loci discussed above primarily because many of the Spanish populations were weakly polymorphic for 1 to 3 or more infrequent or rare alleles at each locus and that both the southwestern Asian and Californian populations also tended to be slightly more variable for rare or infrequent alleles. Thus, the gene pools of southwestern Asia, Spain, and California can be characterized as nearly identical in allelic compositions for these 9 invariant or weakly polymorphic loci.
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From page 143... ...
This class of alleles (Marshall and Brown's class one alleles) therefore contributed relatively little to either allelic variability within or environmentally influenced genetic differentiation among populations.
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From page 144... ...
, which were present in the total sample at a frequency of p < .01, were nearly always confined to a single race. Thus, nearly all rare alleles appear to fit into class two or class four of Marshall and Brown (1975~.
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From page 145... ...
Principal component and cluster analyses showed that variation among races was continuous and that there were no welldefined race complexes; weakly defined differentiated groups were apparent (high-elevation races, northern and northwestern races, southern and western low-elevation dent and flour maize races) , but in general, races of maize were not sharply differentiated.
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From page 146... ...
The time and resources required to travel to a site, record ecological and other relevant information about the site, send the collected materials through customs, and so on are also wasted. However, it is also wasteful of resources if the collector takes samples from single collection sites larger than those needed to capture the alleles and provide for longterm storage in germplasm banks or the distribution of accessions to users.
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From page 147... ...
substantially increases the probability of capturing potentially useful alleles if present, usually without a significant increase in the resources expended at the site. This is because a large proportion of the total time, effort, and expense involved in collecting usually goes for traveling from location to location, inspecting and choosing the sites to be sampled at each location, and recording the ecological features of the sites rather than for actually collecting seeds or other propagules from the location.
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From page 148... ...
Seed numbers are very large, and the mixing during harvesting and cleaning virtually guarantees that all large samples from a given seed source will include all potentially useful genetic variability. A field or a farm seeded from a common source is the unit of sampling.
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From page 149... ...
How many seeds or other propagules should collectors take under such ideal conditions? Assume that a sample from each collection site is to be distributed to a base collection, a back-up collection, five active collections, and 50 samples for immediate distribution to users.
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From page 150... ...
Ecological factors include slope, soil type and drainage, and differences in the Oora and fauna in the vadous human. We most important population factors are the mating system and the mobility of the species (especiaMy the mobi~ties of pollen and seeds in plants these factors often have very large effects on w~hin-population organization as well as on genetic differentiation The goal of capturing useful alleles is most likely to be met by taking stratified samples from diverse sites.
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From page 151... ...
In either case, the number of distinct sites sampled should be the maximum possible, provided that the samples taken at individual sites are large enough to capture potentially useful local genetic variability. RECOMMENDATIONS The scientific bases for the efficient collection, preservation, and distribution of plant genetic resources are all well understood.
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From page 152... ...
The size of a collected sample should, where practical, be adequate for deposition in both base and back-up collections. Duplicate samples should be supplied to host country institutions, base collections, and back-up collections.
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