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Genotypes of 290 cultivars from ten countries (four continents) of spring wheat Triticum aestivum , were analyzed through the identification of alleles at the gliadin-encoding (Gli) loci. The group of cultivars bred in one country during the 20th century might be characterized by its level of genetic diversity (H) and a specific set of alleles. Strong differences between genotypes of cultivars bred in different countries were confirmed by cluster analysis of genetic distances. Genotypes of cultivars bred in regions of the same country might differ, but they always composed a single cluster and strongly differed from genotypes from other countries or regions. Therefore, world polymorphism of spring wheat germplasm of the 20th century is highly structured and differentiated between countries and their regions. This differentiation seems to be mainly a result of natural selection acting in different ways in different eco-climatic conditions of breeding and propagation. At a regional scale, statistically significant temporal changes in frequency of some gliadin alleles and a tendency to temporal decrease of overall genetic diversity were discovered in cultivars bred in each of seven regions studied in Australia, Canada and the former USSR. In regions of Canada (provinces of Manitoba and Ontario) and of the former USSR (Saratov), a statistically significant decay of the genetic diversity of cultivars bred during the 20th century was discovered. However, the influence of breeders' selection for end-use wheat quality on the wheat genetic diversity was the least, if any. • Gliadin genotypes of 299 world-wide T. aestivum cultivars and accessions are compared. • Genetic distances between groups of cultivars from different countries are very long. • The 20th-century's wheat polymorphism is highly structured among countries and regions. • There was a statistically confirmed genetic erosion of polymorphism in two regions. • Differentiation of polymorphism is not necessarily a result of breeders' selection for quality. [ABSTRACT FROM AUTHOR]

High genetic diversity (H = 0.85 ± 0.03) of spring common wheats (290 cultivars from nine countries, four continents) was discovered through the identification of alleles at the gliadin-encoding (Gli) loci. There were no indications of the existence of genetic erosion (narrowing of polymorphism with time) either for wheat germplasm studied throughout the 20th century, or in the group of cultivars bred in any country. At the same time, there was a significant change of the frequency of occurrence, in wheat germplasm of Australia, Canada, Mexico, Nordic countries, Spain, and the former USSR, of certain gliadin alleles related (as shown earlier) to end-use wheat quality, mainly to dough strength. The increase in frequency, in genotypes of groups of more recent cultivars, of gliadin alleles positively influencing dough strength (Gli-B1b , Gli-B1d , and Gli-B2c) caused a loss of genetic diversity at the corresponding Gli loci. It is suggested that these alleles may serve as an indirect targets of breeders' selection for dough quality. Mechanisms of the involvement of some allelic variants of the Gli loci in the manipulation of dough quality are discussed. • Alleles at the Gli-1 and Gli-2 loci have been identified for 290 world-wide cultivars. • The 20th-century's spring common wheat polymorphism is very high (H = 0.85). • There are no signs of genetic erosion for wheat germplasm throughout the 20th century. • Gli alleles, known to relate to quality, are indirect targets of breeders' selection. • Selection for quality-related Gli alleles reduces genetic diversity at some Gli loci. [ABSTRACT FROM AUTHOR]