HIGH COPY NUMBER OF HIGHLY SIMILAR MARINER-LIKE TRANSPOSONS IN PLANARIAN (PLATYHELMINTHE) - EVIDENCE FOR A TRANS-PHYLA HORIZONTAL TRANSFER

Several DNA sequences similar to the mariner element were isolated and characterized in the platyhelminthe Dugesia (Girardia) tigrina. They were 1,288 bp long, flanked by two 32 bp-inverted repeats, and contain ed a single 339 amino acid open-reading frame (ORF) encoding the trans posase. The number of copies of this element is approximately 8,000 pe r haploid genome, constituting a member of the middle-repetitive DNA o f Dugesia tigrina. Sequence analysis of several elements showed a high percentage of conservation between the different copies. Most of them presented an intact ORF and the standard signals of actively expresse d genes, which suggests that some of them are or have recently been fu nctional transposons. The high degree of similarity shared with other mariner elements from some arthropods, together with the fact that thi s element is undetectable in other planarian species, strongly suggest s a case of horizontal transfer between these two distant phyla.