Molecular and morphological evidence for a sister group relationship of the classes Armophorea and Litostomatea (Ciliophora, Intramacronucleata, Lamellicorticata infraphyl. nov.), with an account on basal litostomateans

Based solely on the localization of the cytostome, Cavalier-Smith (2004) divided the ciliate subphylum Intramacronucleata into three infraphyla: the Spirotrichia, including Armophorea and Spirotrichea; the Rhabdophora, containing exclusively Litostomatea; and the Ventrata, comprising the remaining six intramacronucleate classes. This scheme is supported largely by 18S rRNA phylogenetic analyses presented here, except for the placement of the Armophorea. We argue that this group does not belong to the Spirotrichia but forms a lineage together with the Litostomatea because the molecular sister relationship of the Armophorea and Litostomatea is supported by two morphological and morphogenetic synapomorphies: (i) plate-like arranged postciliary microtubule ribbons, forming a layer right of and between the ciliary rows and (ii) a telokinetal stomatogenesis. Thus, we unite them into a new infraphylum, Lamellicorticata, which replaces Cavalier-Smith's Rhabdophora. Further, our phylogenetic analyses consistently classify the most complex haptorian genus Dileptus basal to all other litostomateans, though morphological investigations suggest dileptids to be highly derived and possibly originating from a spathidiid ancestor. These discrepancies between molecular and morphological classifications have not as yet been investigated in detail. Thus, we propose an evolutionary scenario, explaining both the sister relationship of the Armophorea and Litostomatea, as well as the basal position of the morphologically complex dileptids.