Minute Deletions Research Articles

On post irradiation modification of biological effects of neutrons—II: Effect of 5-fluorodeoxyuridine on chromosomal aberrations in neutron irradiated seeds

Fast neutron irradiated barley and Vicia seeds were treated with 5-fluorodeoxyuridine (FUDR) at a concentration below that at which cytological disturbances are produced by FUDR alone. In Vicia, FUDR produced an increase of chromosome breaks (chiefly isolocus breaks) at all doses (40–300 rad). Sister unions and chromosome exchanges were decreased at high doses, and probably increased at low doses. The frequencies of minute deletions were increased at all doses. Corresponding effects were found in barley. The mutation rates studied in this material were not affected by FUDR. Especially with respect to the increased frequencies of breaks the action of FUDR is similar to that previously described for Myleran. The effect is discussed in view of a possible modification of the rejoining of radiation induced breaks.

STUDIES IN ULTRAVIOLET MUTAGENESIS IN DROSOPHILA INVOLVING TREATMENT OF INSEMINATED FEMALES.

IN the study of mutagenesis, the employment of ultraviolet light as a mutagenic agent has certain advantages as compared with X rays. For example, relatively few of the mutant changes induced by ultraviolet belong to the class of minute deletions which are difficult to distinguish phenotypically or cytologically from point mutations and more seem to be actual point mutations than in the case of X rays, as first clearly shown by STADLER (1941 ) . Ultraviolet is therefore a very suitable agent in studies which touch on the intimate nature of the mutagenic process. Moreover, it is probable that ultraviolet induced mutations are more like the naturally occurring ones than are the X-ray induced, since chromosomal rearrangements are comparatively rare among both the ultraviolet induced (MULLER and MACKENZIE 1939) and the spontaneous, but not the X-ray. The relative similarity between ultraviolet-induced and spontaneous mutations may be due in part at least to the fact that a photon of ultraviolet as a rule has only enough energy to sensitize chemically a single atom or group of atoms, resulting in a very localized effect, as contrasted to the much greater genetic damage often resulting from X rays and other high energy radiation. Because of the localized action of ultraviolet, the percent of fractionals among the mutations induced by ultraviolet might offhand be expected to be relatively high. However, ultraviolet light (UV) has very low penetrating power, and therefore can be effectively used only on genetic material which is close to the outside surface of the organism, as in the case of microorganisms and the pollen of plants. In Drosophila, it is effective on the polar cap (the early germ track, present at one end of the developing egg). But the statistical error of the mutation rates resulting from treatment of the polar cap is high because mutagenic hits are multiplied in step with the multiplication of the germ cells in the course of development, and unless the experiments are conducted on a very large scale, a few ‘‘lucky’’ hits resulting in large clusters might cause large deviations from the actual rates. Treatment of the mature sperm cells would circumvent this statistical error. Moreover, in order to detect fractional mutations, as revealed by mosaicism among the progeny, it is necessary to apply the treatment to sperm cells or other post-meiotic stages. However, as above implied, the sperm cannot be effectively treated with UV when applied to the surface of a mature Drosophila male. The mutation rate induced early in the present studies by this method was only between .5 and 1

The origination of chromatin deficiencies as minute deletions subject to insertion elsewhere

The origination of chromatin deficiencies as minute deletions subject to insertion elsewhere