Eur. J. Entomol. 92 (1): 263-270, 1995
Structure and expression of the br-c locus in Drosophila melanogaster (Diptera: Drosophilidae)
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We review data on the structure and expression of the Broad Complex, a key gene in the response to ecdysterone (20E) in Drosophila cells.
The 2B3-7 X chromosome region, containing an early (20E) inducible puff, was saturated with mutations in the genes dor, hfw (swi) and BR-C that cause disturbances in the normal ecdysterone-inducible puffing sequence. By analysis of chromosome rearrangements localized on the genetic, cytological and physical maps, the BR-C locus has been mapped exactly within the 2B3-5 puff. Homozygous mutations for BR-C result in the multiple distortions of metamorphosis and female fertility due to loss of sensitivity of the genes in this chromosomal region to ecdysteroids.
Using interstrain larval ovary transplantation, the causes of female sterility were studied in females mutant for the BR-C. Transplantation of BR-C ovaries into females heterozygous for a dominant sterility mutation followed by genetic analysis of the progeny demonstrated that the transplanted ovaries were capable of normal function. The sterility of the BR-C mutant females is therefore associated with a somatic extra-ovarial defect rather than a germ line malfunction.
According to cytogenetic and molecular analysis, the BR-C occupies more than 115 kb and contains at least three regulatory elements. One of these elements (in position 146.5-161 kb) may respond to 20E and this results in puff activation in the salivary gland cells. The second element (at 99-120 kb) is located upstream in the first exon and is necessary for transcription activation, while the third element (to the left of 99 kb) is necessary for normal fertility.
The strongest BR-C mutations result in underdevelopment of almost all ecdysterone-inducible puffs. In animals homozygous for the weaker BR-C mutations (rbp group) which survive until organ formation in the pupa, the larval salivary glands do not histolyse until at least 72 hour after pupariation (Restifo & White, 1992). Analysis of polytene chromosomes has shown that in the rbp4 mutant all ecdysterone-inducible puffs are extremely reduced in 0, 3, and 10 h prepupae. Some of the puffs including the 2B3-5 are active until 24 h pupae.
Keywords: Development, Drosophila, genes, ecdysterone, puffs
Accepted: January 3, 1995; Published: February 24, 1995 Show citation
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