Transpositions of the bobbed locus in Drosophila melanogaster

Due to the complete absence of ribosomal DNA (genetic symbol Ybb(-)), the X bb(-) chromosome of Drosophila is lethal both in homozygous conditions and in compound with the Ybb(-) chromosome. However, in the cross between the C (1)RM/Ybb(-) females and the Xbb(-)/B(S)Ybb(+) males, characterized by the development of lethal Xbb(-)/Ybb(-) zygotes, two fertile males were detecte d. These males possessed all the markers of the Xbb(-) chromosome but lacke d the Y chromosome B-S marker. Genetic analysis of their progeny showed tha t genes responsible for restoration of viability and fertility of these exc eptional mates were associated with the X chromosome. The crossover tests s howed that in one case these genes were tightly linked to the w locus (the bb(AM7) allele), and in the second case they were located 12.6 map units to the right of the Tu locus (the bb(AM7) allele). It has also been shown tha t the bb locus was transposed to the X chromosome within the short arm of Y chromosome. Transposition of the B(S)Ybb(+) chromosome-specific rDNA seque nces to the X chromosome was confirmed by means of Southern blotting. These data indicate that replacement of the bb locus is realized by transpositio n rather than recombination.