Molecular phylogeny of leeches: Congruence of nuclear and mitochondrial rDNA data sets and the origin of bloodsucking

Complete 18S rDNA sequences and sequences of domain III of mitochondrial 12 S rDNA were obtained to assess phylogenetic relationships among major supra generic taxa of leeches and the possibly closely related clitellate taxa Br anchiobdellida and Acanthobdellida. The monophyly of the families Erpobdell idae, Piscicolidae, and Glossiphoniidae, the suborders Erpobdelliformes and Hirudiniformes, and the order Arhynchobdellida have been confirmed by pars imony and maximum likelihood phylogenetic analysis of separate and combined data sets. Both the nuclear 18S rDNA sequences and the mitochondrial 12S r DNA sequences were consistent in not supporting a monophyletic order Rhynch obdellida, represented by the families Piscicolidae and Glossiphoniidae. A topology with the Piscicolidae as the first branch in the leech tree follow ed by the Glossiphoniidae received the highest support in terms of taxonomi c, character: and outgroup congruence. According to this topology, the puta tive apomorphies of the Rhynchobdellidae (e.g. the proboscis) can be parsim oniously explained as plesiomorphies already present in the ancestral leech . This common ancestor was probably a bloodsucking leech with a proboscis r ather than an unspecialized ectocommensal. as suggested by previous hypothe ses. During the course of leech evolution, a reduction of the proboscis cou ld have taken place in predatory arhynchobdellid ancestors to enable swallo wing of larger prey. A second gain of sanguivory by the jawed Hirudiniforme s could have been facilitated by pre-adaptations to ectoparasitic blood fee ding. The 18S rDNA analysis further indicates a close relationship between the clitellate groups Branchiobdellida and Acanthobdellida, although this r elationship is not strongly supported.