Exchanges at the bobbed locus of Drosophila melanogaster.

I N this paper evidence will be presented that crossing over at the bobbed (bb) locus of Drosophih melanogaster generates a variety of mutant alleles. Some of these alleles are distinguishable from one another and their parental alleles in the degree of their effect as judged by bristle size, abdominal etching and viability. These observations are in harmony with the genetic organization of the bobbed locus proposed by RITOSSA, ATWOOD and SPIEGELMAN ( 1966). Their paper presents evidence which identifies the bobbed locus with the nucleolus organizer ( N O ) , that segment of DNA which codes for ribosomal RNA. Their model proposes that the r-DNA is intercalated in the chromosome as a linear array of tandem repeats. The various bb mutants of different strengths are considered as phenotypic manifestations of different sized deletions of the region. Bobbed mutants may frequently arise and revert because tandem repeats present opportunities for unequal pairing and subsequent crossing over between homologous chromosomes or intrachromosomal exchange will generate products of unequal lengths. On the other hand, the data presented here for bb are incompatible with the “master-slave” organization for genetic units in general that has been proposed by CALLAN (1967) and elaborated by WHITEHOUSE (1967). The appearance of the paper by RITossA et al. stimulated the experiments presented here, since two X ray-induced mutants obtained for other purposes proved to be eminently suitable for the detection of exchanges at the bb locus. A sketch of the method and summary of the results have appeared in abstract form (SCHALET 1967).