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The cloning of polyhomeotic, a complex Drosophila locus required for segment determination and cuticular differentiation Freeman, John Douglas


The polyhomeotic (ph) locus of Drosophila melanogaster has been characterized genetically. Early studies showed that ph is a member of the Polycomb (Pc) group. These genes have similar phenotypes and are required for normal segment determination. Recent analyses of amorphic ph mutations show that the ph locus is complex, has a strong maternal effect and plays a role in cuticular development. To test the function of ph at the molecular level, the cloning of the ph locus was undertaken. One strain had been shown to contain a P element insertion near ph. A genomic library was prepared from this strain and a recombinant phage containing this P element insertion was isolated by transposon tagging. The DNA flanking the insertion was used as a starting point for a chromosomal walk. A series of overlapping phage spanning 170 kilobases was isolated. Southern blot analysis was used to determine the locations of important deficiency breakpoints within the region covered by the walk. A distance of approximately 35 kb was shown to separate the two deficiency breakpoints which include ph. This interval was found to contain rearrangements in four of the seven ph alleles which were examined by Southern blot analysis. The interval also contains a repeated sequence. The relationship between the genetic and molecular structure of ph is discussed.

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