Lengths of transcribed rDNA repeating units in spermatocytes of Drosophila hydei: only genes without an intervening sequence are expressed.

In Drosophila hydei a 5 kb long intervening sequence (ivs) which is limited to only one of the three nucleolus organizers occurs in the 28S coding region of the rDNA (Kunz and Glätzer, 1979). This raises the possibility to study transcription of repeated genes with long intervening sequences. Three genotypes of males were studied each having only one defined nucleolus organizer. Spermatocytes of single males were spread in a solution of low salt and examined by electron microscopy. The length distributions of the repeating units, matrix units, and the apparent spacer units did not reveal two classes with a 5 kb length difference. The data suggest that in D. hydei the 5 kb long intervening sequence is not transcribed and that, in fact, the entire gene which contains an intervening sequence remains silent in primary spermatocytes. There is evidence that genes with an intervening sequence are not intermingled with "normal" genes at random.