Contrasting response of euchromatin and heterochromatin to translocation in polytene chromosomes of Drosophila melanogaster

Studies on Feulgen-DNA content in the polytene chromosomes of D. melanogaster T(1:4)w^m258-21 heterozygotes showed that when the euchromatic region 3D₁-E₂ is located next to the heterochromatic breakpoint it contains less DNA than in the non-translocated homologue (Hartmann-Goldstein and Cowell, 1976). In contrast to the region adjacent to the breakpoint, region 3C_1-10, which contains intercalary heterochromatin, shows more DNA in the translocated than in the non-translocated chromosome. Transposition may induce morphologically euchromatic regions containing putatively underreplicated sequences to undergo additional replication cycles. Region 2E₁-3A₄, distal to 3C₁ and at some distance from the heterochromatic breakpoint is apparently unaffected. Extended replication and reduced DNA content in regions which have undergone chromosomal rearrangement could be accounted for by varying degrees of blockage of replication in individual strands of the polytene chromosome.