The Behavior of a S Unstable Ring Chroaiosome of Drosophila R.ielanogaster'3

ING-SHAPED chromosomes provide an opportune situation for the analysis R of some aspects of chromosome mechanics. In particular, sister-strand crossing over eludes detection in rod chromosomes but its consequences in ring chromosomes are manifested by the formation of anaphase bridges. Thus RICCLIKTOCK (1938) and SCHWARTZ (1953) have presented cytological evidence for the occurrence of exchanges between sister chromatids of ring chromosomes in mitotic and meiotic divisions in maize. On the other hand, genetic studies on the ring-X chromosomes of Drosophila melanogasler apparently deny the occurrence of meiotic sister-strand exchange (MORGAN 1933; NOVITSKI 195.5), and although ring-X chromosomes do experience somatic loss more frequently than their rod-X homologs (MORGAN 1926; BROWN and HANNAH 1952), they are described as being stable in comparison with maize rings. This report is concerned with an atypical ring-X chromosome characterized by frequent loss and dominant lethality. Results will be presented which suggest that the instability of this chromosome is mediated through anaphase bridge formation and that this process is controlled by come heterochromatic element in the centric region of the chromosome.