Molecular structure of a novel gypsy-Ty3-like retrotransposon (Kabuki) andnested retrotransposable elements on the W chromosome of the silkworm Bombyx mori

We previously characterized a female-specific randomly amplified polymorphi c DNA (RAPD), designated W-Kabuki, derived from the W chromosome of the sil kworm, Bombyx mori. To further analyze the W chromosome of B. mori, we obta ined a lambda phage clone which contains the W-Kabuki RAPD sequence and seq uenced the 18.1-kb DNA insert. We found that this DNA comprises a nested st ructure of at least seven elements; three retrotransposons, two retroposons , one functionally unknown insertion, and one Bombyx repetitive sequence. T he non-LTR retrotransposon BMC1, the retroposon Bm1, a functionally unknown inserted DNA (FUI), and a copia-like LTR retrotransposon (Yokozuna) are th emselves inserted into a novel gypsy-Ty3-like LTR retrotransposon, named Ka buki. Furthermore, this Kabuki element is itself inserted into another copy of Bm1. The BMC1. and Yolroz2mn elements inserted in the Kabuki sequence a re intact. Moreover, the Kabuki element is largely intact. These results su ggest that many retrotransposable elements have accumulated on the W chromo some, and these elements are expected to evolve more slowly than those on o ther chromosomes.