The first three mitochondrial (mt) genomes of endosymbiotic turbellarian flatworms are characterised for the rhabdocoels Graffilla buccinicola, Syndesmis echinorum and S. kurakaikina. Interspecific comparison of the three newly obtained sequences and the only previously characterised rhabdocoel, the free-living species Bothromesostoma personatum, reveals high mt genomic variability, including numerous rearrangements. The first intrageneric comparison within rhabdocoels shows that gene order is not fully conserved even between congeneric species. Atp8, until recently assumed absent in flatworms, was putatively annotated in two sequences. Selection pressure was tested in a phylogenetic framework and is shown to be significantly relaxed in this and another protein-coding gene: cox1. If present, atp8 appears highly derived in platyhelminths and its functionality needs to be addressed in future research. Our findings for the first time allude to a large degree of undiscovered (mt) genomic plasticity in rhabdocoels. It merits further attention whether this variation is correlated with a symbiotic lifestyle. Our results illustrate that this phenomenon is widespread in flatworms as a whole and not exclusive to the better-studied neodermatans.

Department of Life Sciences Natural History Museum Cromwell Road London SW7 5BD United Kingdom

Hasselt University Centre for Environmental Sciences Research Group Environmental Biology Agoralaan Gebouw D B 3590 Diepenbeek Belgium

Hasselt University Centre for Environmental Sciences Research Group Zoology Biodiversity and Toxicology Agoralaan Gebouw D B 3590 Diepenbeek Belgium

Hasselt University Centre for Environmental Sciences Research Group Zoology Biodiversity and Toxicology Agoralaan Gebouw D B 3590 Diepenbeek Belgium; Laboratory of Biodiversity and Evolutionary Genomics University of Leuven Charles Deberiotstraat 32 B 3000 Leuven Belgium; Zoology Unit Finnish Museum of Natural History University of Helsinki P O Box 17 Helsinki FI 00014 Finland; Department of Botany and Zoology Faculty of Science Masaryk University Kotlářská 2 611 37 Brno Czech Republic

School of Biological Sciences University of Auckland New Zealand

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