Abstract
The occurrence of the various highly repetitive DNA fractions in Drosophila hydei DNA identified by hybridization across gradients was studied in tissues of different levels of polyteny (fat body, about 128 C; salivary glands, about 1024 C). It is shown that the repetitive fraction located in the X chromosome and some other repetitive DNA fractions diminish with increasing polyteny. The Y chromosome of D. hydei is cytologically not detectable in polytene cells. Since it carries only a small proportion of highly repetitive DNA sequences, other DNA sequences must be excluded from polytenization. Some minor highly repetitive DNA fractions, on the other hand, achieve a level of polyteny similar to that of the DNA in euchromatin. They may be located in intercalary heterochromatin or in β-heterochromatin. Also a correlation with the nucleolus organizer region, located in under-replicated heterochromatin, might be possible. Exonuclease-treated salivary gland DNA is able to produce significant amounts of “folded” circles in re-association, which is an expression of internal sequence repeats in the molecules. Hence, the polytenized highly repetitive DNA sequences could also be interspersed between other DNA sequences.
Published Version
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