| The seeds were sown in the field immediately after treatment. Unfortunately,
the spring of 1959 and 1960 was too dry to give a satisfactory germination,
which condition was especially critical for the X-ray series. This even
failed completely in the 1959 year experiment. From comprehensive earlier
studies, it can, however, be stated that the relation between X-rays and
fast neutrons is representative in the 1957 year experiment. In order to condense the results as much as possible and to present only data supported by a complete progeny test, Table 1 is concentrated to the mutational events in chromosome 5A (IX) and those doses which gave the highest mutation rates in the various years and treatment series. The wheat variety used (Svaloef Rival Spring Wheat) carries three markers on the long arm of chromosome 5A. The inactivation of a proximal factor SK results in a shift from spring to winter wheat, a mutation involving B1 will cause awning and the loss of the distal factor Q wil1 give a speltoid. A terminal deficiency longer than 30 centimorgans will result in a bearded spring speltoid or, when the loss includes SK as well, a bearded winter speltoid. A simple segregation test will further reveal,whether a part or whole the chromosome is lost, and the duplication of Q will phenotypically be distinguished as a compactoid. The system obviously allows for a rough grouping from point mutations to aneuploidy, a width in scorable mutation mechanisms, which is not available in a diploid and well suited for studies on mutagenic specificity. As evident from Table 1, none of the chemical mutagens proved as efficient as X-rays and neutrons in producing mutational events in chromosome 5A of 6x vulgare wheat. As in other chromosomes of this polyploid, most of the events scored must involve more than a simple gene alteration, which is evident from comparing the relative prevalence of compactoids and speltoids against bearded normals and winter types. The two last categories includes only gene mutations or interstitual deficiencies, since here only B1 or SK, respectively, is mutated or lost. The compactoid and speltoid groups include, however, not only mutations merely related to Q but also to B1 alone or together with SK. The high correlation between induced sterility in M1 and mutation rate in M2 further favours the idea that visible mutations in common wheat are mostly associated with changes above the simple gene level. The lower efficiency of the studied chemicals to produce visible mutations in a polyploid than in a diploid organism if compared with X-rays and neutrons thus strongly supports the idea of a different mutagenic specificity. Since chromosome mutations are more likely to be deleterious, a lower efficiency of chemical mutagenesis in polyploids will not necessarily imply a lower yield of practically interesting mutations. Repeated treatments over successive generations may also overcome the phenotypic buffering against minor genetic events. |
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