CONTROLLING element is the name given by MCCLINTOCK to those chromosomal entities associated with inordinately high mutation rates at specific gene loci (cf. detailed reviews by MCCLINTOCK 1965 and 1967) . Although originally described in Zea mays, the occurrence of controlling elements has been adduced for organisms so widely separate as Escherichia coli (TAYLOR 1963) and Drosophila melanogaster (GREEN 1967) . While the induced mutation in E. coli is directly attributable to the integration of

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... the phage, mu-I, at the site of mutation, the precise nature of the mutation inducing agents of maize and Drosophila is to date obscure. Nonetheless, the presence of controlling elements in maize and Drosophila leads to a number of cytogenetic changes which parallel in many details the behavior of temperate phages involved in lysogeny. Accordingly, it is not unreasonable to extend the parallelism and speculate that controlling elements are, in fact, integrated into the chromosomes at the gene sites associated with high mutability. That controlling elements are intimately associated with specific chromosomes is indisputable; that they are integrated into and replicate concurrently with chromosomes is still an open question. Evidence supporting integration would follow if controlling elements could be mapped by conventional crossing over. In at least one instance controlling elements have been successfully mapped. NELSON (1968) has mapped three controlling element regulated mutants of the W-x (waxy) gene of maize by interallelic recombination. In each the controlling element can be assigned to a specific location on the gene map, thereby supporting the idea of integration. Evidence for a controlling element associated high mutation rate of the white ( w ) eye color gene in D. melanogaster has been previously documented (GREEN 1967) . Since independent w mutants have been mapped by interallelic crossing over (GREEN 1959; JUDD 1959) , experiments were undertaken to apply the same methods to map the Drosophila mutable gene, we. These experiments are the subject of this report. MATERIALS A N D METHODS Standard Drosophila culture methods were employed throughout. Details of each cross will be given, where appropriate, in the text. A synopsis of the mutants used with their assigned symbols and linkage relationhips is included in Table 1 . *