| Nuclear DNA and the evolution of wheat1) H. RESS and M. R. WALTERS Department of Agricultural Botany, University of Wales, Aberystwyth The analysis of nuclear DNA variation provides a useful approach to investigating ancestry. In the wheats it provides new evidence on the diploid source of genomes comprising the cultivated polyploid forms. It shows also that in this group at least, the hybridisation and subsequent polyploidy are accomplished without appreciable alteration in the DNA content of individual chromosomes. A previous report by PAI, UPADHYA, BHASKARAN and SWAMINATHAN (1961) of a chromosome diminution of the order of about 30 per cent. In the wheat polyploids in shown to be incorrect as indeed is also made clear in a subsequent report by two of the above authors (UPADHYA and SWAMINATHAN 1963). There is thus no evidence in wheat for a DNA diminution or for the associated change in the structural organisation of chromosomes inferred in polyploid Hemiptera (SCHRADER and HUGHES-SCHRADER 1956, 1958). There are, of course, impediments to beware of in applying this kind of DNA analysis to problems of ancestry. The first relates to the accuracy of the DNA determinations and hence to the precision of the analysis. In this respect a most obvious precaution is to make all preparations in as near standard conditions as possible. This means that wherever possible the material from all species or types selected for comparison should be fixed, stained and scored together in the same batch. This is particularly important in eliminating variation in FEULGEN staining, one of the chief sources of error variation. Another objection that could be made to the method and to the validity of the conclusions derived from it is that the samples or varieties of the species used may not be typical (see UPADHYA and SWAMINATHAN, loc. cit.). Short of making widespread surveys within cultivated and other species there is no certain way of telling to what extent the objection is valid. At the same time it was shown that, where tested, no significant differences in DNA amount occurred between different species known to have the same genome constitution. It is therefore a reasonable assumption that DNA differences between varieties within species are not likely to be greater, aud hence, likely to be negligible. Conclusions and summary : 1. Comparisons are described of 2C nuclear DNA amounts, measured by FEULGEN photometry, in cultivated and related species of wheat. 2. DNA amounts were the same in species of similar genome constitution investigated, viz. AA or AABB. 3. Triticum timopheevi, usually classified AAGG, has a lower nuclear DNA amount than T. durum (AABB). 4. Aegilops speltoides, on the basis of DNA comparisons, is a more likely contributor of the B genome found in the cultivated AABB tetraploids and AABBDD hexaploid than Ae. becornis or Agropyron triticeum. 5. There is no evidence of appreciable change in nuclear DNA subsequent to the hybridisation and polyploidy by which the cultivated wheats arose. References PAI, R. A., UPADEYA, M. D., BHASKAVAN, S. and SWAMINATHAN, M. S. 1961. Chromosome diminution and evolution of polyploid species in Triticum. Chromosoma 12: 398-400. SCHRADER, F. and HUGHES-SCHRADER, S. 1956. Polyploidy and fragmentation in the chromosomal evolution of various species of Thyanta (Hemiptera). Chromosoma 7: 469-480. SCHRADER, F., HUGHES-SCHRADER, S. 1958. Chromatid autonomy in Banasa (Hemiptera : Pentatomidal). Chromosoma 9 : 193 - 200. UPADHYA, M. D. and SWAMINATHAN, M. S .1963 Deoxyribonucleic acid and the ancestry of wheat. Nature 200: 713-714. |
| 1) Cited from Heredity 20, Part 1, pp 73-82. 1965, with understanding of the authors. |