LOCALIZATION OF THE LAMPBRUSH LOOP PAIR NOOSES ON THE Y-CHROMOSOME OFDROSOPHILA-HYDEI BY FLUORESCENCE IN-SITU HYBRIDIZATION

We have used fluorescence in situ hybridization to map the positions o f the different repetitive DNA sequences from the region forming the l ampbrush loop pair Nooses on the Y chromosome of Drosophila hydei. Thi s region harbours a megabase cluster of tandemly organized repeats of the Y-specific ay1 family and a megabase cluster of tandem repeats of the related Y-specific YsI family. In addition, ay1 repeats also occur in short blocks that are interspersed by other repetitive DNA sequenc es that we call Y-associated, since they have additional copies on oth er chromosomes. Using specific probes for ay1, YsI and Y-associated DN A sequences, we show that there is one large proximal cluster of YsI r epeats and one, more distally located, large cluster of ay1 repeats. T he Y-chromosomal copies of the Y-associated sequences are located in t he most distal part of the ay1 cluster. This is consistent with the ju xtaposition of ay1 and Y-associated sequences in more than 300 kb of c loned genomic DNA. Since both ay1 and Y-associated sequences have been shown to be transcribed in the Nooses, the lampbrush loop is formed i n a distal region of the short arm of the Y chromosome, adjacent to th e terminally located nucleolus organizer region. The clusters of homog eneous ay1 and YsI repeats are of no functional significance for the f ormation of the lampbrush loop.