Heterochromatic Self-Association, a Determinant of Nuclear Organization, Does Not Require Sequence Homology in Drosophila

Corresponding author: Amy K. Csink, Carnegie Mellon University, 4400 Fifth Ave., Pittsburgh, PA 15213., csink{at}andrew.cmu.edu (E-mail)

Communicating editor: R. S. HAWLEY

Chromosomes of higher eukaryotes contain blocks of heterochromatin that can associate with each other in the interphase nucleus. A well-studied example of heterochromatic interaction is the brown^Dominant (bw^D) chromosome of D. melanogaster, which contains an 1.6-Mbp insertion of AAGAG repeats near the distal tip of chromosome 2. This insertion causes association of the tip with the centric heterochromatin of chromosome 2 (2h), which contains megabases of AAGAG repeats. Here we describe an example, other than bw^D, in which distally translocated heterochromatin associates with centric heterochromatin. Additionally, we show that when a translocation places bw^D on a different chromosome, bw^D tends to associate with the centric heterochromatin of this chromosome, even when the chromosome contains a small fraction of the sequence homology present elsewhere. To further test the importance of sequence homology in these interactions, we used interspecific mating to introgress the bw^D allele from D. melanogaster into D. simulans, which lacks the AAGAG on the autosomes. We find that D. simulans bw^D associates with 2h, which lacks the AAGAG sequence, while it does not associate with the AAGAG containing X chromosome heterochromatin. Our results show that intranuclear association of separate heterochromatic blocks does not require that they contain the same sequence.

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