Abstract
ABSTRACTThe 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.
Highlights
Seriality, that is, a repetitive arrangement of morphological structures along the anterior–posterior axis, is a common feature across the Metazoa
Journal of Experimental Zoology Part B: Molecular and Developmental Evolution published by Wiley Periodicals, Inc
Cell Proliferation Patterns in W. argentea and G. pellucida In early larvae, cells proliferate at the highest rate in a pair of often kidney-shaped areas, which run along most of the longitudinal axis and encompass the area where the foregut is about to be formed (Figs. 1A and 2B and B )
Summary
That is, a repetitive arrangement of morphological structures along the anterior–posterior axis, is a common feature across the Metazoa. Only few taxa include representatives that possess a concerted seriality of several organ systems with involvement of the mesoderm, a situation, which is often called segmentation sensu stricto, or metamerism These taxa, which include the Annelida, Kinorhyncha, Panarthropoda, and Chordata, occupy very distant positions on the tree of life, with the Annelida belonging to the Lophotrochozoa, the Kinorhyncha and the Panarthropoda to the Ecdysozoa, and the Chordata to the Deuterostomia, respectively. Despite their phylogenetic distance, similarities do exist in the way as to how some representatives of these groups form their “segments”, including the expression. Journal of Experimental Zoology Part B: Molecular and Developmental Evolution published by Wiley Periodicals, Inc
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