| The D genome of hexaploid wheat Ralph RILEY and Victor CHAPMAN Plant Breeding Institute, Cambridge, England The sequence of investigations, originated by Kihara, Sears, Sax and others, which let to the conclusion that the D genome of hexaploid wheat was derived from Aegilops squarrosa is well known. Nevertheless as an outstanding example of phylogenetic deduction it merits reiteration. Initially, as a result of the study of meiotic chromosome pairing in hybrids, it was established that one of the genomes of the tetraploid Aegilops cylindrica was also present in the hexaploid wheat, Triticum aestivum, but not in any of the tetraploid wheat species. Thus hexaploid wheat and Ae. cylindrica had the dinkel, or D, genome in common. Subsequently one of the two genomes of Ae. cylindrica was demonstrated to be derived from Aegilops caudata. This was the C genome. Moreover the investigation of hybrids between T. aestivum and Ae. caudata showed that these two species had no common genome. Consequently when the other parent of Ae. cylindrica was found to be Ae. squarrosa; and synthetic Ae. cylindrica was produced as the tetraploid from the Ae. caudata x Ae. squarrosa hybrid, the problem of the D genome was essentially solved. More or less simultaneously, and independently, McFadden and Sears, and Kihara, were able to synthesise hexaploid wheat as the amphiploid from tetraploid wheat Ae. squarrosa crosses. The regular meiotic pairing in hybrids between the natural and synthetic hexaploids confirmed that Ae. squarrosa was indeed the source of the D genome. Hitherto, however, the final link in the chain of evidence - that dependent upon the cytological analysis of a hybrid between hexaploid wheat and Ae. squarrosa - has been lacking. However the appropriate hybrid was produced recently at Cambridge, following the pollination of 20 florets of T. aestivum Chinese Spring by Ae. squarrosa pollen. One seed was produced and its embryo was cultured by Rommel's method. The hybrid plant grew vigorously, although it was completely sterile. It was intermediate to its parents in most characters, although the rachis was non-fragile, like that of the wheat parent. Meiotic behaviour was analysed at first metaphase of meiosis and the mean pairing is shown in Table 1, and the distribution of cells with different pairing conditions in Table 2. Usually there were seven bivalents per cell and most of the bivalents were closed rings, often with three or four chiasmata. |
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