Why did theTc1-like elements of mollusks acquired the spliceosomal introns?

Transposable elements are the DNA sequences capable of transpositions within the genome and, thus, exerting a considerable influence on the genome functioning and structure and providing the source of new genes. Transposable elements are classified into retrotransposons and the DNA transposons. IS630/Tc1/mariner superfamily of DNA transposons is one of the most diverse groups broadly represented among the eukaryotes. We identified a new group of Tc1 -like elements in the mollusks, which we named TLEWI . These DNA transposons are characterized by the low copy number, the lack of terminal inverted repeats and the presence of DD36E signature and the spliceosomal introns in transposase sequence. Their prevalence among the mollusks is limited to subclass Pteriomorpha (Bivalvia). Since TLEWI possess the features of domesticated TE and the structure similar to the eukaryotic genes, which is not typical for the DNA transposons, we consider the hypothesis of co-optation of TLEWI gene by the bivalves.