Abstract
BackgroundWith more than 100000 living species, mollusks are the second most diverse metazoan phylum. The current taxonomic classification of mollusks recognizes eight classes (Neomeniomorpha, Chaetodermomorpha, Polyplacophora, Monoplacophora, Cephalopoda, Gastropoda, Bivalvia, and Scaphopoda) that exhibit very distinct body plans. In the past, phylogenetic relationships among mollusk classes have been contentious due to the lack of indisputable morphological synapomorphies. Fortunately, recent phylogenetic analyses based on multi-gene data sets are rendering promising results. In this regard, mitochondrial genomes have been widely used to reconstruct deep phylogenies. For mollusks, complete mitochondrial genomes are mostly available for gastropods, bivalves, and cephalopods, whereas other less-diverse lineages have few or none reported.ResultsThe complete DNA sequence (14662 bp) of the mitochondrial genome of the chaetodermomorph Scutopus ventrolineatus Salvini-Plawen, 1968 was determined. Compared with other mollusks, the relative position of protein-coding genes in the mitochondrial genome of S. ventrolineatus is very similar to those reported for Polyplacophora, Cephalopoda and early-diverging lineages of Bivalvia and Gastropoda (Vetigastropoda and Neritimorpha; but not Patellogastropoda). The reconstructed phylogenetic tree based on combined mitochondrial and nuclear sequence data recovered monophyletic Aplacophora, Aculifera, and Conchifera. Within the latter, Cephalopoda was the sister group of Gastropoda and Bivalvia + Scaphopoda.ConclusionsPhylogenetic analyses of mitochondrial sequences showed strong among-lineage rate heterogeneity that produced long-branch attraction biases. Removal of long branches (namely those of bivalves and patellogastropods) ameliorated but not fully resolved the problem. Best results in terms of statistical support were achieved when mitochondrial and nuclear sequence data were concatenated.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-014-0197-9) contains supplementary material, which is available to authorized users.
Highlights
With more than 100000 living species, mollusks are the second most diverse metazoan phylum
The first studies analyzing the relative phylogenetic position of Monoplacophora [19,20] rendered a surprising result in recovering the group as closely related to Polyplacophora, forming the taxon Serialia, which is in disagreement with most morphological evidence [21]
A study [22] based on 79 ribosomal protein genes recovered the monophyly of the phylum and of all five mollusk classes included in the analyses (Neomeniomorpha, Monoplacophora, and Scaphopoda were missing)
Summary
With more than 100000 living species, mollusks are the second most diverse metazoan phylum. Mollusks are the second largest animal phylum with more than 100000 described extant species and are grouped into eight classes: Solenogastres or Neomeniomorpha, Caudofoveata or Chaetodermomorpha, Polyplacophora, Monoplacophora, Bivalvia, Gastropoda, Cephalopoda, and Scaphopoda [1,2,3]. The latest phylogenetic studies based on seven housekeeping genes [23], and on genomic-scale data sets spanning 308 [24] and 1185 [25] genes, respectively, recovered monophyletic Mollusca, Aplacophora, and Aculifera, rejecting the Testaria hypothesis One of these studies included Monoplacophora, which was placed within Conchifera, providing no support for the Serialia hypothesis [25]. While both studies support a basal position of Cephalopoda (+Monoplacophora in [25]), one favors a clade composed of Gastropoda and Bivalvia (the so-called Pleistomollusca [24]) whereas the other groups together Gastropoda and Scaphopoda [25]
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