Insertion sites of the transposable element mariner are fixed in the genome of Drosophila sechellia

The abundance of the transposable element mariner differs dramatically in the genomes of the closely related species Drosophila simulans, D. mauritiana, D. sechellia, and D. melanogaster. Natural populations of D. simulans and D. mauritiana have 1-10 and 20-30 copies per diploid genome, respectively, and the insertion sites are polymorphic. The element has not been found in D. melanogaster. In this paper we show that D. sechellia, a species endemic to the Seychelles Islands, contains only two mariner elements that are at fixed sites in the genome. One element, inserted in chromosome 2R at 51A1-2, contains three deletions and is missing much of the 3' end. The other element, inserted in chromosome 3L at 64A10-11, is the full length of 1286 bp. Although the sequence of the full-length element is polymorphic in populations of D. sechellia, at least some of the sequences are closely related to elements from D. simulans and D. mauritiana that are known to be active. However, judging from the progeny of crosses between D. sechellia and D. simulans, the biological activity of the full-length D. sechellia element appears to be low, either because of the nucleotide sequence of the element or because of its position in the genome, or both.