Genetics, Vol 123, 327-336, Copyright © 1989

INVESTIGATIONS

The Genetics of the Dras3-Roughened-ecdysoneless Chromosomal Region (62B3-4 to 62D3-4) in Drosophila melanogaster: Analysis of Recessive Lethal Mutations

T. J. Sliter, V. C. Henrich, R. L. Tucker and L. I. Gilbert
Department of Biology, University of North Carolina, Chapel Hill, North Carolina 27599-3280

The genetic organization of interval 62B3-4 to 62D3-4 on the Drosophila third chromosome was investigated. The region (designated DRE) includes four known loci: Roughened (R; 3-1.4), defined by a dominant mutation disrupting eye morphology; the nonvital locus Aprt, structural gene for adenine phosphoribosyltranferase; Dras3, a homolog of the vertebrate ras oncogene; and l(3)ecdysoneless (l(3)ecd), a gene that has been implicated in the regulation of larval molting hormone (ecdysteroid) synthesis. Overlapping chromosomal deletions of the region were generated by {gamma}-ray-induced reversion of the R mutation. Recessive lethal mutations were isolated based upon failure to complement the recessive lethality of Df(3L)R(R2), a deletion of the DRE region that removes 16-18 polytene chromosome bands. A total of 117 mutations were isolated following ethyl methanesulfonate and {gamma}-ray mutagenesis. These and two additional mutations define 13 lethal complementation groups. Mutations at two loci were recovered at disproportionately high rates. One of these loci is preferentially sensitive to radiation-induced mutational alterations. Additionally, an unusually low recovery rate for cytologically detectable rearrangement breakpoints within the {gamma}-ray-sensitive locus suggests that an interval of the DRE region closely linked to the R locus may be dominantly sensitive to position effects. Lethal phase analysis of mutant hemizygotes indicates that a high proportion of DRE-region loci (11 of 13) are necessary for larval development. Mutations in five loci cause predominantly first-instar larval lethality, while mutations in four other loci cause predominantly second-instar lethality. Mutations in two loci cause late-larval lethality associated with abnormal imaginal disc development. A temperature-sensitive allele of one newly identified complementation group blocks ecdysteroid-induced pupariation. This developmental block is overcome by dietary 20-hydroxyecdysone, suggesting that a second locus in the region in addition to l(3)ecd may play a role in the regulation of late larval ecdysteroid levels.

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