EXTRACHROMOSOMAL ELEMENT DELTA IN DROSOPHILA MELANOGASTER. IX. INDUCTION OF DELTA-RETAINING CHROMOSOME LINES BY MUTATION AND GENE MAPPING

[Delta b], symbolized as [δ^b], is retained by S^b chromosome lines and transmitted through the females to their progeny. Transmission through the males is not directly demonstrable (Minamori 1969a). [delta r], symbolized as [δ^r], is retained by S^r chromosome lines and transmitted biparentally (Minamori 1971). The multiplication of delta is suppressed at low temperature. All descendant lines derived from S^b-carrying or S^r-carrying flies in which the presence of delta cannot be demonstrated gradually accumulate their specific delta factors over many generations (Minamori 1969b, 1972). The delta factors and the sensitive chromosomes are inseparably associated. This observation led to the assumption that delta may be a copy of a chromosomal gene or a certain agent integrated into the chromosome (Minamori 1972). This assumption was examined in the present study by experiments designed to induce delta-retaining sensitive chromosomes, and to map the gene(s) responsible for delta-retention and/or for sensitivity to the killing action of delta factor. One sensitive chromosome which retained [δ^b] (S^b chromosome) was obtained in the presence of [δ^b] out of 2492 insensitive chromosomes which retained no delta; in addition one S^b chromosome was obtained in the presence of [δ^r] out of 2131 insensitives. The latter finding suggests that S^b might be induced by a mutation caused by [δ^b] or [δ^r], but not by integration of either delta into the chromosome. Four S^b chromosomes and one sensitive chromosome which retained [δ^r] (S^r chromosome) were obtained out of 1970 insensitives when males carrying the chromosome were fed an alkylating mutagen, ethyl methane sulfonate (EMS). The location of delta-retaining genes was examined by crossing-over experiments employing eight S^b and five S^r chromosomes. The genes on these chromosomes were found to be located in the same region or near one another. The gene for [δ^b], symbolized as Da^b, and the gene for [δ^r], symbolized as Da^r, are assumed to be multiple alleles of a locus at 2-24.9. The sensitivity of the chromosomes was modified appreciably by recombination; hence, the genes controlling this trait are assumed to be a polygenic system. The findings obtained in this study lead to the hypothesis that delta may be produced by a chromosomal gene (Da) and transmitted extrachromosomally.