The heterochromatic rolled gene of Drosophila melanogaster is extensively polytenized and transcriptionally active in the salivary gland chromocenter.

This paper reports a cytogenetic and molecular study of the structural and functional organization of the Drosophila melanogaster chromocenter. The relations between mitotic (constitutive) heterochromatin and alpha- and beta-heterochromatin are not fully understood. In the present work, we have studied the polytenization of the rolled (rl) locus, a 100-kb genomic region that maps to the proximal heterochromatin of chromosome 2 and has been previously thought to contribute to alpha-heterochromatin. We show that rolled undergoes polytenization in salivary gland chromosomes to a degree comparable to that of euchromatic genes, despite its deep heterochromatic location. In contrast, both the Bari-1 sequences and the AAGAC satellite repeats, located respectively the left and right of rl, are severely underrepresented and thus both appear to be alpha-heterochromatic. In addition, we found that rl is transcribed in polytene tissues. Together, the results reported here indicate that functional sequences located within the proximal constitutive heterochromatin can undergo polytenization, contributing to the formation of beta-heterochromatin. The implications of this finding to chromocenter structure are discussed.