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.

Expression of six3 and otx in Solenogastres (Mollusca) supports an ancestral role in bilaterian anterior‐posterior axis patterning

“…The animals spawned fertilized eggs, which developed via free‐swimming larvae into metamorphosized juveniles, so that the entire developmental sequence could be followed. For a detailed account on collection, culture, and rearing see Redl, Scherholz, Todt, Wollesen, & Wanninger () and Redl, Scherholz, Wollesen, Todt, & Wanninger (). The age of the larvae is given in days posthatching (dph), whereby 0–1 dph is used for larvae ranging from newly hatched to an age of 24 hr.…”

“…RNA extraction and sequencing of the transcriptome was done as described in Redl et al (). The obtained paired‐end reads of an average read length of 100 bp were subsequently filtered (rRNA removal).…”

“…Two assembled transcriptomes were generated, one comprising 163,084 contigs and approximately 145 Mbp, the other comprising 217,838 contigs and approximately 173 Mbp. The assemblies were analyzed and mined for the six3 and otx orthologs according to the description in Redl et al (). Primers with the following sequences were designed for amplifying parts of the genes (in case of six3 , the part between the primers spans the whole conserved region of the gene, in otx it includes a major part of it):…”

The evolution of molluscs

“…To this end, some molluscan species have already been demonstrated to be particularly amenable to becoming true laboratory models, including the gastropods Ilyanassa obsoleta (Goulding & Lambert, ) and Crepidula fornicata (Henry & Lyons, ). Moreover, protocols for comparative studies into morphogenesis and gene expression are available for various representatives distributed all over the molluscan tree, including aplacophorans (Redl et al, , , ; Scherholz et al, ), polyplacophorans (Fritsch et al, ; Wollesen et al, , ; Fritsch, Wollesen, & Wanninger, ), scaphopods (Wollesen et al, ), bivalves (Kakoi et al, ; Kin et al, ; Wollesen et al, ; Pavlicek, Schwaha, & Wanninger, ), and cephalopods (Wollesen et al, , , ). Newly obtained sequence data especially for the notoriously undersampled aplacophorans, polyplacophorans, scaphopods, and monoplacophorans should further aid in achieving a better and more stable resolution of molluscan inter‐ and intrarelationships (particularly within Conchifera) as an important prerequisite for evolutionary inferences.…”

Brain regionalization genes are co-opted into shell field patterning in Mollusca

“…Paired-end reads of an average read length of 100 bp were obtained and were subsequently filtered (rRNA removal; see refs 15 and 16 for details on transcriptome assembly). Adapter and low quality sequences were trimmed, normalized, and assembled de novo into contigs with the assembler Trinity 18 or IDBA-tran, Version 1.1.1 in case of W .…”