A different level of X-chromosomal transcription in an In(1)BM2 (reinverted) strain and in its hyperploid derivatives resolves an X-coded regulatory activity for dosage compensation in Drosophila
The transcriptional competence of the A'-chromosome of a mutant strain of Drosophila melanogaster, [in(l)BM 2 (reinverted)], and of hyperploid derivatives with different additional segments of the A'-chromosome has been examined. The single X in the mutant male shows twice as much puffiness and RNA synthesis as does that in the normal male, revealing a level of A'-coded activity in addition to the normal male and female levels. Feulgen cytophotometry reveals no duplication of DNA content in the
... DNA content in the mutant X. When duplication for the segments 1A-3E, 9A-20F, 11A-20F and 16A-20F of the A'-chromosome are combined in the male with the mutant chromosome, the super-hyperactivity of the mutant X is completely abolished. In combination with the B s . Y duplication, which contains 16A7-B2, the two-fold activity is also completely suppressed. The mutant chromosome can appear in three discrete manifestations, namely, highly flabby, intermittently flabby and normal, suggesting a leaky nature of the mutant. The effect is also temperature-sensitive. Our results suggest that there may be a modulator gene complex (M + ) in the 16A7-B2 region as well as regulators elsewhere on the X, which in combination influence the hyperactivity of the male X in Drosophila. We suggest that the In(l)BM 2 (reinverted) chromosome carries a hypomorphic mutation of M + (M m ). The results presented here and earlier data on various A'-chromosomal and autosomal hyperploids are discussed in the light of a model for dosage compensation in Drosophila. https://www.cambridge.org/core/terms. https://doi.