Induced Loss of a Ring and a Telomeric Chromosome in Drosophila Melanogaster.

ING chromosomes are potentially useful for studying the process of rejoining R after chromosome breakage, since a certain portion of the restitutions should produce dicentric ring chromosomes or interlocked rings rather than structurally normal chromosomes. The relevance of ring chromosomes studies to problems of induced chromosome breakage in Drosophila melanogaster sperm has been recognized for some time (e.g., BAUER 1942; CATCHESIDE and LEA 1945; BAKER and VON HALLE 1955). However, the interpretation of many aspects of this work has been hampered by two factors: a comparable telomeric3 chromosome was not available for comparison with the ring-X and marked Y chromosomes were not used with the result that loss of the X chromosome could not be detected directly. The study being reported consists of a re-examination of the process of induced ring chromosome loss4 coupled with the evidence this provides on the mechanism of the oxygen effect on chromosome breakage and rejoining. These data furnish no evidence against the hypothesis that the greater sensitivity of the ring chromosome to induce loss is caused by its ring nature. The data suggest, however, that some novel mechanism may underlie the formation of dominant lethals in the sex chromosomes of Drosophila.