title: Cell Proliferation Pattern and Twist Expression in an Aplacophoran Mollusk Argue Against Segmented Ancestry of Mollusca.
abstract: The study of aplacophoran mollusks (i.e., Solenogastres or Neomeniomorpha and Caudofoveata or Chaetodermomorpha) has traditionally been regarded as crucial for reconstructing the morphology of the last common ancestor of the Mollusca. Since their proposed close relatives, the Polyplacophora, show a distinct seriality in certain organ systems, the aplacophorans are also in the focus of attention with regard to the question of a potential segmented ancestry of mollusks. To contribute to this question, we investigated cell proliferation patterns and the expression of the twist ortholog during larval development in solenogasters. In advanced to late larvae, during the outgrowth of the trunk, a pair of longitudinal bands of proliferating cells is found subepithelially in a lateral to ventrolateral position. These bands elongate during subsequent development as the trunk grows longer. Likewise, expression of twist occurs in two laterally positioned, subepithelial longitudinal stripes in advanced larvae. Both, the pattern of proliferating cells and the expression domain of twist demonstrate the existence of extensive and long-lived mesodermal bands in a worm-shaped aculiferan, a situation which is similar to annelids but in stark contrast to conchiferans, where the mesodermal bands are usually rudimentary and ephemeral. Yet, in contrast to annelids, neither the bands of proliferating cells nor the twist expression domain show a separation into distinct serial subunits, which clearly argues against a segmented ancestry of mollusks. Furthermore, the lack of twist expression during the development of the ventromedian muscle argues against homology of a ventromedian longitudinal muscle in protostomes with the notochord of chordates.
authors.affiliation: Faculty of Life Sciences, Department of Integrative Zoology, University of Vienna, Vienna, Austria.
authors.forename: Emanuel
authors.initials: E
authors.lastname: Redl
authors.affiliation: Faculty of Life Sciences, Department of Integrative Zoology, University of Vienna, Vienna, Austria.
authors.forename: Maik
authors.initials: M
authors.lastname: Scherholz
authors.affiliation: Faculty of Life Sciences, Department of Integrative Zoology, University of Vienna, Vienna, Austria.
authors.forename: Tim
authors.initials: T
authors.lastname: Wollesen
authors.affiliation: University Museum, The Natural History Collections, University of Bergen, Bergen, Norway.
authors.forename: Christiane
authors.initials: C
authors.lastname: Todt
authors.affiliation: Faculty of Life Sciences, Department of Integrative Zoology, University of Vienna, Vienna, Austria.
authors.forename: Andreas
authors.initials: A
authors.lastname: Wanninger
mesh_terms: D000818:Animals; D005075:Biological Evolution; D049109:Cell Proliferation; D007814:Larva / Q000166:cytology / Q000254:growth & development / Q000378:metabolism; D008974:Mollusca / Q000166:cytology* / Q000254:growth & development / Q000378:metabolism*; D024510:Muscle Development; D010802:Phylogeny; D051879:Twist-Related Protein 1 / Q000378:metabolism*
chemical_list: D051879:Twist-Related Protein 1