ALPHA-HETEROCHROMATIN AND BETA-HETEROCHROMATIN IN POLYTENE CHROMOSOME-2 OF DROSOPHILA-MELANOGASTER

The formation of alpha and beta heterochromatin in chromosomes of Dros ophila melanogaster was studied in salivary glands (SGs) and pseudonur se cells (PNCs). In SGs of XO, XY, XYY, XX and XXY individuals the amo unts of alpha heterochromatin were similar, suggesting that the Y chro mosome does not substantially contribute to alpha heterochromatin form ation. Pericentric heterochromatin developed a linear sequence of bloc ks in PNCs, showing morphology of both alpha and beta heterochromatin. In situ hybridization with Rsp sequences (H-o clone) revealed that th e most proximal heterochromatic segment of the mitotic map (region h39 ) formed a polytenized block in PNCs. Dot analysis showed that the clo ne had a hybridization rate with PNC-DNA very close to that with DNA f rom mainly diploid head cells, whereas the homologous SG-DNA was drama tically underrepresented . A similar increase of DNA representation in PNC was found for AAGAC satellite DNA. The mitotic region h44 was fou nd not to polytenize in the SG chromosome, whereas in PNC chromosome 2 this region was partly polytenized and presented as an array of sever al blocks of alpha and beta heterochromatin. The mapping of deficienci es with proximal breakpoints in the most distal heterochromatin segmen ts h35 in arm 2L and h46 in 2R showed that the mitotic eu-heterochroma tin transitions were located in SG chromosomes distally to the polyten e 40E and 41C regions, respectively. Thus, the transition zones betwee n mitotic hetero- and euchromatin are located in banded polytene euchr omatin. A scheme for dynamic organization of pericentric heterochromat in in nuclei with polytene chromosomes is proposed.