[Emergence of a recessive lethal mutation in derivatives of an unstable X(Z)-chromosome from Drosophila melanogaster].

Xz chromosome isolated from a natural population of Drosophila melanogaster is characterized by spontaneous mutability of the genes yellow, white, and signed and the appearance of chromosomal rearrangements. In mutant lines derived from the line carrying the Xz chromosome that had one, two, or three unstable visible mutations (markers), the rate of appearance of sex-linked lethal mutations was analyzed. This rate was shown to increase with an increase in the number of markers in a line. This phenomenon, termed "marker induction", might explain the phenotypic homogeneity of natural Drosophila populations. Spontaneous lethal mutations were mapped, and their nonrandom distribution along the Xz chromosome was shown. Along with common "hot spot" of lethal mutations, the derivatives for the Xz chromosomes had their own specific sites for lethal mutations. In some cases, the appearance of lethal mutations was accompanied by the formation of inversions in the Xz chromosome. The lethal destabilization of the Xz derivatives, caused by selection for accumulation of visible mutations, is associated with an increase in the number of hot spots for lethal mutations. Presumably, these hot spots are hot sites for the transposition of mobile genetic elements.