| Genomes A and D of hexaploid wheat are clearly identified and recognized
but the identity of genome B is not yet firmly established, in spite of
amount of accumulated evidence. Correlatively, the following question arises:
which is the source of mildew susceptibility as found in cultivated wheat?
It is well known that mildew susceptibility is almost a general condition in all cultivated wheat varieties. Inversely, most diploid wheats are resistant to this pathogen. Ae. squarrosa (genome D contributor) possesses some susceptible strains, though differences between varieties strangulata and the others resemble those found by HIRATSUKA (1959) with puccinia graminis and P. triticina; the question of which Ae. squarrosa variety is the most probable ancestor, remains unsolved. Even considering that this would account for hexaploid susceptibility, the one corresponding to tetraploid condition would still remain unexplained. It would be difficult to assume that a previous non-existing factor in a tetraploid, could arise in it, transmitted through the hexaploid which has been, in turn, partly built by the said tetraploid. Furthermore, some authors consider (e.g. SARKAR and STEBBINS 1956) that "the genes for different characters in the emmers (tetraploids) are all or mostly derived from the same genes existing in the ancestral diploids" and not from mutation. Section Sitopsis has been considered contributor of genome B because of cytological as well as morphological reasons. But it has not been yet conclusively shown which is the most probable donor species. Within the four (or three if Ae. sharonensis is included in Ae. longissima) species represented in Table 1, two were discarded for different reasons; Ae. speltoides (SARKAR and STEBBINS 1956) and Ae. bicornis (SEARS 1956) remain as probable constituents of cultivated wheat. The latter would have additional support from its susceptible reaction to mildew as compared with the resistance found in all Ae. speltoides strains tested in this work. |
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