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The heterochromatic Responder (Rsp) locus of Drosophila melanogaster is the target of the two distorter loci Sd and E(SD). Rsp is located in a specific heterochromatic region of the second chromosome and is made up of AT-rich satellite sequences whose abundance is related to its sensitivity to the distorter chromosomes. Here we report that a cluster of Rsp sequences is also located in the third chromosome. The third-chromosome cluster has the same flanking sequences as the clone originally used to identify the Rsp elements, and one of the flanking sequences is a rearranged 412 retrotrsansposon. The presence of a second, unlinked Rsp-sequence cluster makes re-interpretation necessary for some earlier experiments in which segregation of the third chromosome had not been followed and raises interesing possibilities for the origin of the Rsp locus.

The heterochromatic Responder (Rsp) locus of Drosophila melanogaster is the target of the two distorter loci Sd and E(SD). Rsp is located in a specific heterochromatic region of the second chromosome and is made up of AT-rich satellite sequences whose abundance is related to its sensitivity to the distorter chromosomes. Here we report that a cluster of Rsp sequences is also located in the third chromosome. The third-chromosome cluster has the same flanking sequences as the clone originally used to identify the Rsp elements, and one of the flanking sequences is a rearranged 412 retrotrsansposon. The presence of a second, unlinked Rsp-sequence cluster makes re-interpretation necessary for some earlier experiments in which segregation of the third chromosome had not been followed and raises interesing possibilities for the origin of the Rsp locus.