Unique genome of dicyemid mesozoan: highly shortened spliceosomal introns in conservative exon/intron structure.

Dicyemids are enigmatic endoparasites, or endosymbionts, living in the renal sac of benthic cephalopod molluscs. The body of dicyemids consists of only 9-41 cells, with neither extracellular matrices nor differentiated tissues. Due to the unusually simple body organization, dicyemids have long been the subject of phylogenetic controversy. Molecular evidences suggest dicyemids are lophotrochozoans that have secondarily lost many morphological characters. We studied 40 genes of the dicyemid Dicyema japonicum and found that their spliceosomal introns are very short (mean length=26 bp). This size was shorter than that of introns of animals, such as Fugu rubripes and Oikopleura dioica which possess compact genome and introns. In the intron size, the dicyemid was nearly equal to the chlorarachniophyte Bigelowiella natans nucleomorph (18-21 bp) which has the shortest introns of any known eukaryote. Despite the short introns, the intron density (5.3 introns/gene) of the dicyemid is similar to that in model invertebrates. In addition, the exon/intron structure of the dicyemid is more similar to vertebrates than to the model invertebrates. These results suggest that the positions of the introns are possibly conserved under functional constraints.