Relative abundance of various rDNA repeat types in polytene nuclei of Drosophila melanogaster

The relative abundance of the five predominant X chromosome nucleolus organizer repeat types was determined in polytene nuclei. Previous studies showed that the major type I insert containing class of repeats was disproportionately underreplicated when compared with the insert-free class of repeats in these nuclei. Our results demonstrate that this disproportionate underreplication also occurs for both the minor type I and type II insert containing repeats. There was not an exclusive polytenization of the insert-free repeats for this wild-type X nucleolus organizer region. A repeat type lacking either insert type and having an unusually long nonstrancribed spacer was identified and found to be preferentially replicated in polytene nuclei. Preliminary mapping studies indicate that this class of repeats has a unique location, in the distal portion of either the X nucleolus organizer region or the centric heterochromatin. Their similarity to the common insert-free repeats includes preferential polytenization and generalized DNase I sensitivity, which suggests that they are transcriptionally active.