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

Abstract

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 brownDominant (bwD) 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 bwD, in which distally translocated heterochromatin associates with centric heterochromatin. Additionally, we show that when a translocation places bwD on a different chromosome, bwD 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 bwD allele from D. melanogaster into D. simulans, which lacks the AAGAG on the autosomes. We find that D. simulans bwD 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.

Chromatin Structure and “DNA Sequence View”: The Role of Satellite DNA in Ectopic Pairing of the Drosophila X Polytene Chromosome

“…The association does not require similar sequences such as (aagag) n satellite to be located within the contacting areas. Presumably, their associations are mediated by proteins that recognize the general features of heterochromatin, such as specific histone modifications, repetitiveness, late replication and low activity [39]. Nevertheless, the restricted homology at the IH areas containing DNA breaks may be important for ectopic contact formations.…”