The Relation of Oxygen Posttreatment and Heterochromatin to X-Ray-Induced Chromosome Aberration Frequencies in Crepis Capillaris.

EVERAL studies have now demonstrated that postirradiation oxygen treatS ments can markedly increase the initial X-ray damage in barley seeds (ADAMS, NILAN AND GUNTHARDT 1955; NILAN 1955; ADAMS AND NILAN 1958; CALDECOTT et al. 1957; CURTIS, CALDECOTT AND KONZAK 1957). One of the more striking of these after effects is the marked increase in the frequencies of chromosome aberrations (ADAMS, NILAN AND GUNTHARDT 1955; ADAMS AND NILAN 1958). The present investigation was undertaken to determine if the influence of oxygen after X-irradiation was random or differential in its action on the location of chromosome breaks in seeds. In other words, does oxygen increase chromosome aberrations in particular regions of the chromosomes? Such a study required a detailed analysis of the chromosome aberrations that was not possible with barley. Therefore, Crepis capillaris (2n = 6) with its readily identified chromosome arms was chosen as the experimental organism. In addition to providing data on the original objective, the study also disclosed some new information concerning chromosome breakage in Crepis. Here the increased chromosome rejoining in heterochromatic regions is of particular importance.