Comparative study of molecular characteristics of X and Y nucleolar organizers of various lines of Drosophila melanogaster.

We have determined for the X chromosomes of 10 laboratory strains and the Y chromosomes of 4 of them both the total number of ribosomal units and the relative percentages of uninterrupted (ins-), type 1 (ins1: with distinction between small ins1S and large ins1L) and type 2 (ins2) interrupted ribosomal units. These studies were made with the DNA extracted from third instar larval diploid tissues (brains and imaginal discs) of X/X female lines or XNO-/Y male lineages (devoid of X ribosomal genes) whose members possess copies of the same initial X or Y chromosome. Between the X chromosomes as well as the Y chromosomes an approximately equal to 2-fold variation was observed in the total number of ribosomal genes: from approximately equal to 200 to 420 for the X chromosomes and from approximately equal to 150 to 330 for the Y ones. The Y chromosomes are devoid of insertion 1 interrupted units, but one can observe some variation in the percentage and hence the absolute numbers of uninterrupted and insertion 2 interrupted units. Among the X chromosomes a very large variation exists between the percentage and absolute number of all the ribosomal unit types; it is to be noted especially that the number of uninterrupted units, which are the only kind of ribosomal genes actively transcribed, can vary from about 20 to 140 without any differences in the development of the different strains.