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Allopolyploids are usually considered to have originated fromspecies-hybrids in which there was little or no meiotic chromosome pairing because of the structural dissimilarities of the chromosome sets of the parental species. The same structural distinctions are alsoheld to be responsible for the absence of multivalents in the derivedpolyploids, since they would limit heterogenetic associations (Waddington, 1950) between chromosomes of the different basic sets. Indeed, it has been thought that the differential affinity which causes eachchromosome to pair with its identical partner may often be so great that only bivalents are formed. However, an examination of many of the frequently quoted examples of allopolyploid behaviour suggests that the direct origin of polyploids free from heterogenetic multivalents, with their depressing influence on fertility and stability, must be rare. Indeed, the cytologicallydiploid behaviour which many natural polyploid species display must often have been acquired subsequent to the development of polyploidy. Hitherto there has been no evidence to show how the cytological diploidisation of a natural polyploid has been achieved. However, recent experimental results with the hexaploid common wheat, Triticumestivum, illustrate the basis of diploidisation in an apparently typical allopolyploid species, and at the same time give an insight into the cytogenetic functioning of a polyploid (Riley and Chapman, i958b). The purpose of the present paper is to collect, extend and discuss the evidence so far available on the diploidisation of wheat.

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