Comparison between the molecular characteristics and the potential activity of X and Y nucleolar organizers from various Drosophila melanogaster laboratory lines.

The molecular characteristics of nucleolar organizers from X and Y chromosomes of different Drosophila melanogaster lines have previously been studied (17). By analysis of appropriate genetic crosses we show in the present study that the X and Y chromosomes of these lines can confer different degrees of resistance on an inhibitor of ribosomal RNA synthesis (beta exotoxin or thuringiensin) present in the thermostable supernatant of Bacillus thuringiensis cultures. None of the lines studied gives rise to any particular phenotype under normal culture conditions; variations in the degree of supernatant resistance of these lines provide a relative measure of what can be called the potential activity of the nucleolar organizers of the different X and/or Y chromosomes. The potential activity of the Y nucleolar organizers is found to be generally higher than that of the X organizers. This result can be correlated with the fact that the number of uninterrupted ribosomal units is much greater on the Y chromosomes than on the Xs. Significant variations in potential activity have been shown to occur among the X as also among the Y nucleolar organizers. Comparison between the molecular characteristics of the nucleolar organizers and their level of activity shows that among the different ribosomal units, only those devoid of insertion interfere with the level of activity. However, some of our results could also indicate that not all the uninterrupted units have the same level of activity; this level could be related to the size of the nontranscribed spacer of the ribosomal units.(ABSTRACT TRUNCATED AT 250 WORDS)