Molecular biological and cytological study of the phylogenically unique characters in the acoel Platyhelminthes

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1. The developmental process in the acoel flatworm Convoluta naikaiensis was examined by light microscopy and scanning electron microscopy. The early cell lineage was established. The cleavage patter was unique and different from those in other turbellarians. In the last of the cleavage stage, blastomeres internalize both from the animal and vegetal ends.2. The surface structures were compared among 9 turbellarian groups. In the acoel flatworm, the epidermal cells and the underlying muscle cells were not separated by any basal structure but were in direct contact with each other, forming a muscle-epidermis complex. This structure was unique in the acoel flatworm, not found in other turbellarian orders examined. The muscle-epidermis complex was formed at an early embryonic stage and the basal structure never appeared between the epidennis and the muscle during the course of the muscle-epidermis complex. This fact suggests that the muscle-epidermis complex does not results from degeneration.3. Mitochondrial codons in the mitochondrial genes (CO1 and cytochrome b ) were compared among 7 turbellarian orders. The genetic code was different between the acoel flatworm and other 6 turbellarian groups. In the 6 orders except the acoel flatworm, AAA and ATA code aspartic acid and isoleucine, respectively, but in the acoel flatworm, AAA and ATA codes lysine and methionine.4. Detailed re-analysis of the molecular phylogy on the basis of the 18s ribosomal DNA confirmed the previous result that the acoel flatworm diverged before other turbellarians were specified.5. The results obtained in the present project collectively suggest that the acoel flatworms are very unique animals belonging to a phylum different from the Platyhelminthes.