SUMMARY: A variety of heritable changes affecting colony morphology and colour are induced in a sp. by exposure of the spores to ultraviolet and gamma rays. Most of the changes are associated with instabilities which give rise to further variation during colony growth and spore formation. The instabilities persist indefinitely in most lines of descent and give rise to characteristic proportions of new variant types each having its own pattern of instability. The ultraviolet dose/effect curve shows the familiar peak and decline found for mutations in many other organisms. The present changes, however, differ from the gene mutations previously studied in that they can be induced with much greater frequencies (30-60% of the colonies being affected), and that gamma rays are as effective or more so than ultraviolet irradiation. This suggests that the initial changes, and perhaps subsequent ones as well, might be more of the nature of the chromosomal rearrangements, rather than of the gene mutations, of higher organisms. Similar instabilities arise spontaneously but with a much lower frequency.


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