The key role of heterochromatin in the phenotypic manifestation of the In(1)sc⁸ inversion disrupting the achaete-scute complex in Drosophila melanogaster

The achaete-scute complex (AS-C) is a locus approximately 90 kbp in length, containing multiple en hancers. The local expression of the achaete and scute genes in proneural clusters of Drosophila melanogaster imaginal discs results in the formation of a well-defined pattern of macrochaetae in adult flies. A wide variety of easily analyzed phenotypes, along with the direct connection between individual regulatory elements and the development of specific setae make this locus a classic model in developmental genetics. One classic AS-C allele is sc8, which arose as a result of the In(1) sc8 inversion. One breakpoint of this inversion lies between the ac and sc genes, while the second is in the pericentromeric heterochromatin of chromosome X, within satellite block 1.688. The heterochromatic position of the breakpoint raised the question of whether position effect variegation contributes to the disruption of normal locus function in the In(1)sc8 flies. However, conflicting results were obtained. Previously, we found that a secondary inversion, In(1)19EHet, arose spontaneously in one of the stocks of the In(1)sc8 BDSC line, transferring most of the heterochromatin from the ac gene to the 19E region of the X chromosome. Here, we demonstrate that the In(1)19EHet inversion leads to complete rescue of the number of posterior supraalar (PSA) and partial rescue of the number of dorsocentral (DC) macrochaetes observed in the original In(1)sc8 line. The same rescue of the macrochaetes pattern was observed when the In(1)sc8 inversion was introduced into a strain with the Su(var)3-906 position effect modifier. Combining the inversion with the Rif1¹ mutation, a conserved factor determining late replication and underreplication, does not restore the normal pattern of bristles. Our data indicate that the phenotype of flies carrying the In(1) sc8 inversion, associated with a disturbance in bristle development, is determined by the effect of heterochromatin on the distal part of the locus. This model can be used to test the influence of various factors on the position effect variegation caused by heterochromatin. Another phenotypic manifestation of In(1)sc8, a decreased proportion of males in the offspring, was independent of the proximity of the distal part of AS-C to heterochromatin and was not affected by the Rif11 mutation.

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