The complete mitochondrial genome of the nudibranch Roboastra europaea (Mollusca: Gastropoda) supports the monophyly of opisthobranchs

The complete nucleotide sequence (14,472 bp) of the mitochondrial genome of the nudibranch Roboastra europaea (Gastropoda: Opisthobranchia) was determined. This highly compact mitochondrial genome is nearly identical in gene organization to that found in opisthobranchs and pulmonates (Euthyneura) but not to that in prosobranchs (a paraphyletic group including the most basal lineages of gastropods). The newly determined mitochondrial genome differs only in the relative position of the trnC gene when compared with the mitochondrial genome of Pupa strigosa, the only opisthobranch mitochondrial genome sequenced so far. Pupa and Roboastra represent the most basal and derived lineages of opisthobranchs, respectively, and their mitochondrial genomes are more similar in sequence when compared with those of pulmonates. All phylogenetic analyses (maximum parsimony, minimum evolution, maximum likelihood, and Bayesian) based on the deduced amino acid sequences of all mitochondrial protein-coding genes supported the monophyly of opisthobranchs. These results are in agreement with the classical view that recognizes Opisthobranchia as a natural group and contradict recent phylogenetic studies of the group based on shorter sequence data sets. The monophyly of opisthobranchs was further confirmed when a fragment of 2,500 nucleotides including the mitochondrial cox1, rrnL, nad6, and nad5 genes was analyzed in several species representing five different orders of opisthobranchs with all common methods of phylogenetic inference. Within opisthobranchs, the polyphyly of cephalaspideans and the monophyly of nudibranchs were recovered. The evolution of mitochondrial tRNA rearrangements was analyzed using the cox1+rrnL+nad6+nad5 gene phylogeny. The relative position of the trnP gene between the trnA and nad6 genes was found to be a synapomorphy of opisthobranchs that supports their monophyly.     

The mitochondrial genome of Priapulus caudatus Lamarck (Priapulida: Priapulidae)

We sequenced and annotated the complete mitochondrial (mt) genome of the priapulid Priapulus caudatus in order to provide a source of phylogenetic characters including an assessment of gene order arrangement. The genome was 14,919 bp in its entirety with few, short non-coding regions. A number of protein-coding and tRNA genes overlapped, making the genome relatively compact. The gene order was: cox1, cox2, trnK, trnD, atp8, atp6, cox3, trnG, nad3, trnA, trnR, trnN, rrnS, trnV, rrnL, trnL(yaa), trnL(nag), nad1, -trnS(nga), -cob, -nad6, trnP, -trnT, nad4L, nad4, trnH, nad5, trnF, -trnE, -trnS(nct), trnI, -trnQ, trnM, nad2, trnW, -trnC, -trnY; where '-' indicates genes transcribed on the opposite strand. The gene order, although unique amongst Metazoa, shared the greatest number of gene boundaries and the longest contiguous fragments with the chelicerate Limulus polyphemus. The mt genomes of these taxa differed only by a single inversion of 18 contiguous genes bounded by rrnS and trnS(nct). Other arthropods and nematodes shared fewer gene boundaries but considerably more than the most similar non-ecdysozoan.     

Evolution of gastropod mitochondrial genome arrangements

Background  

Gastropod mitochondrial genomes exhibit an unusually great variety of gene orders compared to other metazoan mitochondrial genome such as e.g those of vertebrates. Hence, gastropod mitochondrial genomes constitute a good model system to study patterns, rates, and mechanisms of mitochondrial genome rearrangement. However, this kind of evolutionary comparative analysis requires a robust phylogenetic framework of the group under study, which has been elusive so far for gastropods in spite of the efforts carried out during the last two decades. Here, we report the complete nucleotide sequence of five mitochondrial genomes of gastropods (Pyramidella dolabrata, Ascobulla fragilis, Siphonaria pectinata, Onchidella celtica, and Myosotella myosotis), and we analyze them together with another ten complete mitochondrial genomes of gastropods currently available in molecular databases in order to reconstruct the phylogenetic relationships among the main lineages of gastropods.     

Phylogenetic relationships and evolution of pulmonate gastropods (Mollusca): new insights from increased taxon sampling

Phylogenetic relationships among higher clades of pulmonate gastropods are reconstructed based on a data set including one nuclear marker (complete ribosomal 18S) and two mitochondrial markers (partial ribosomal 16S and Cytochrome oxidase I) for a total of 96 species. Sequences for 66 of these species are new to science, with a special emphasis on sampling the Ellobiidae, Onchidiidae, and Veronicellidae. Important results include the monophyly of Systellommatophora (Onchidiidae and Veronicellidae) as well as the monophyly of Ellobiidae (including Trimusculus, Otina, and Smeagol). Relationships within Ellobiidae, Onchidiidae, and Veronicellidae are evaluated here for the first time using molecular data. Present results are compared with those from the recent literature, and the current knowledge of phylogenetic relationships among pulmonate gastropods is reviewed: despite many efforts, deep nodes are still uncertain. Identification uncertainties about early fossils of pulmonates are reviewed. Impacts of those phylogenetic and fossil record uncertainties on our understanding of the macro-evolutionary history of pulmonates, especially transitions between aquatic and terrestrial habitats, are discussed.     

The mitochondrial nad2 gene as a novel marker locus for phylogenetic analysis of early land plants: a comparative analysis in mosses

The mitochondrial nad2 gene is established as a novel marker locus for phylogenetic analyses among early land plants. The potential of this gene for phylogenetic resolution was checked with a broad taxon sampling of 42 mosses (Bryopsida, including the enigmatic genus Takakia) to allow both a comparative analysis with the recently explored nad5 gene and the fusion of independent data sets. The mitochondrial gene sequences provide valuable phylogenetic information on the relationships of classically defined orders and their respective monophylies. The more rapidly diverging sequences of a group I intron in nad5 and of a group II intron in nad2 add information for fine resolution. Although both genes provide phylogenetic information in the same taxonomic range (above family level), the combined sequence alignment results in an approximate doubling in the number of nodes with significant bootstrap support (>90). According to our data, Buxbaumiales are a paraphyletic taxon in a key position between the earliest branching taxa (Sphagnales, Takakiales, Andreaeales, Polytrichales, and Tetraphidales) and all other orders, possibly to be placed in the subclass Bryidae. A dichotomy in the latter recalls two previously suggested superorders Hypnanae and Dicrananae. Both genes independently question the monophyly of the orders Dicranales and Neckerales and reject the inclusion of the genera Schistostega, Timmia, and Encalypta among Eubryales.     

Exclusive conservation of mitochondrial group II intron nad4i548 among liverworts and its use for phylogenetic studies in this ancient plant clade

Liverworts occupy a pivotal position in land plant (embryophyte) phylogeny as the presumed earliest-branching major clade, sister to all other land plants, including the mosses, hornworts, lycophytes, monilophytes and seed plants. Molecular support for this earliest dichotomy in land plant phylogeny comes from strikingly different occurrences of introns in mitochondrial genes distinguishing liverworts from all other embryophytes. Exceptionally, however, the nad5 gene--the mitochondrial locus hitherto used most widely to elucidate early land plant phylogeny--carries a group I type intron that is shared between liverworts and mosses. We here explored whether a group II intron, the other major type of organellar intron, would similarly be conserved in position across the entire diversity of extant liverworts and could be of use for phylogenetic analyses in this supposedly most ancient embryophyte clade. To this end, we investigated the nad4 gene as a candidate locus possibly featuring different introns in liverworts as opposed to the non-liverwort embryophyte (NLE) lineage. We indeed found group II intron nad4i548 universally conserved in a wide phylogenetic sampling of 55 liverwort taxa, confirming clade specificity and surprising evolutionary stability of plant mitochondrial introns. As expected, intron nad4i548g2 carries phylogenetic information in its variable sequences, which confirms and extends previous cladistic insights on liverwort evolution. We integrate the new nad4 data with those of the previously established mitochondrial nad5 and the chloroplast rbcL and rps4 genes and present a phylogeny based on the fused datasets. Notably, the phylogenetic analyses suggest a reconsideration of previous phylogenetic and taxonomic assignments for the genera Calycularia and Mylia and resolve a sister group relationship of Ptilidiales and Porellales.     

Phylogenetic relationships among amphisbaenian reptiles based on complete mitochondrial genomic sequences

Complete mitochondrial genomic sequences are reported from 12 members in the four families of the reptile group Amphisbaenia. Analysis of 11,946 aligned nucleotide positions (5797 informative) produces a robust phylogenetic hypothesis. The family Rhineuridae is basal and Bipedidae is the sister taxon to the Amphisbaenidae plus Trogonophidae. Amphisbaenian reptiles are surprisingly old, predating the breakup of Pangaea 200 million years before present, because successive basal taxa (Rhineuridae and Bipedidae) are situated in tectonic regions of Laurasia and nested taxa (Amphisbaenidae and Trogonophidae) are found in Gondwanan regions. Thorough sampling within the Bipedidae shows that it is not tectonic movement of Baja California away from the Mexican mainland that is primary in isolating Bipes species, but rather that primary vicariance occurred between northern and southern groups. Amphisbaenian families show parallel reduction in number of limbs and Bipes species exhibit parallel reduction in number of digits. A measure is developed for comparing the phylogenetic information content of various genes. A synapomorphic trait defining the Bipedidae is a shift from the typical vertebrate mitochondrial gene arrangement to the derived state of trnE and nad6. In addition, a tandem duplication of trnT and trnP is observed in Bipes biporus with a pattern of pseudogene formation that varies among populations. The first case of convergent rearrangement of the mitochondrial genome among animals demonstrated by complete genomic sequences is reported. Relative to most vertebrates, the Rhineuridae has the block nad6, trnE switched in order with the block cob, trnT, trnP, as they are in birds.     

Complete DNA sequence of the mitochondrial genome of the sea-slug, Aplysia californica: conservation of the gene order in Euthyneura

We have sequenced and characterized the complete mitochondrial genome of the sea slug, Aplysia californica, an important model organism in experimental biology and a representative of Anaspidea (Opisthobranchia, Gastropoda). The mitochondrial genome of Aplysia is in the small end of the observed sizes of animal mitochondrial genomes (14,117 bp, NCBI Accession No. NC_005827). The Aplysia genome, like most other mitochondrial genomes, encodes genes for 2 ribosomal subunit RNAs (small and large rRNAs), 22 tRNAs, and 13 protein subunits (cytochrome c oxidase subunits 1-3, cytochrome b apoenzyme, ATP synthase subunits 6 and 8, and NADH dehydrogenase subunits 1-6 and 4L). The gene order is virtually identical between opisthobranchs and pulmonates, with the majority of differences arising from tRNA translocations. In contrast, the gene order from representatives of basal gastropods and other molluscan classes is significantly different from opisthobranchs and pulmonates. The Aplysia genome was compared to all other published molluscan mitochondrial genomes and phylogenetic analyses were carried out using a concatenated protein alignment. Phylogenetic analyses using maximum likelihood based analyses of the well aligned regions of the protein sequences support both monophyly of Euthyneura (a group including both the pulmonates and opisthobranchs) and Opisthobranchia (as a more derived group). The Aplysia mitochondrial genome sequenced here will serve as an important platform in both comparative and neurobiological studies using this model organism.     

Mitogenomic phylogeny of Trochoidea (Gastropoda: Vetigastropoda): New insights from increased complete genomes

Increased mitochondrial (mt) genomes can provide more sets of genome‐level characteristics for resolving deeper phylogeny. Limited information with respect to the Trochoidea mitochondrial genome organization is available; besides, monophyly and internal relationships of the superfamily still remain a matter of discussion. To resolve the monophyly and internal phylogenetic controversies of Trochoidea and expand our understanding for mt genomic characteristic evolution among Trochoidea, the phylogenetic trees were reconstructed using 13 newly sequenced complete mt genomes and 35 genomes from GenBank, and both the maximum likelihood and Bayesian inference analyses were highly supported. Vetigastropoda phylogenetic analyses recovered the monophyly of Trochoidea. Trochoidea phylogenetic analyses and genetic distances supported the non‐monophyly of Tegulidae and Tegula, indicating that the taxonomic status of several genera (Rochia, Tectus and Cittarium) should be revised and Tegula, Omphalius and Chlorostoma should be placed as a same genus. The close affinity between Tectus virgatus and Rochia was also revealed. Three‐nucleotide insertion in nad1, nine‐nucleotide insertion and six‐nucleotide deletion in nad5 are detected in Tegulidae, Tectus and Rochia, respectively. Gene orders within Trochoidea are stable, with gene rearrangements exclusive to tRNA genes observed. Homoplasious convergences because of trnT rearrangement display translocation in Turbinidae and reversion in Trochidae and Calliostomatida. For trnE and trnG, we identify 11 arrangement types, suggesting that the gene rearrangement history needs to be further evaluated. Our study emphasizes the importance of mt genomes in resolving phylogenetic relationships within Trochoidea. In addition, the mt genomic characters would contribute new insights into the classification of Trochoidea.     

Molecular phylogeny of the nutmeg shells (Neogastropoda, Cancellariidae)

Cancellariidae, or nutmeg shells, is a family of marine gastropods that feed on the body fluids and the egg cases of marine animals. The 300 or so living species are distributed worldwide, mostly on soft bottoms, from intertidal to depths of about 1000 m. Although they are a key group for the understanding of neogastropod evolution, they are still poorly known in terms of anatomy, ecology and systematics. This paper reports the first mitochondrial multi-gene phylogenetic hypothesis for the group. Data were collected for 50 morphospecies, representative of 22 genera belonging to the three currently recognized subfamilies. Sequences from three genes (12S, 16S and COI) were analyzed with Maximum Likelihood analysis and Bayesian Inference, both as single gene datasets and in two partitioned concatenated alignment. Largely consistent topologies were obtained and discussed with respect to the traditional subfamilial arrangements. The obtained phylogenetic trees were also used to produce Robinson-Foulds supertrees. Our results confirmed the monophyly of the subfamily Plesiotritoninae, while Admetinae and Cancellariinae, as currently conceived, were retrieved as polyphyletic. Based on our findings we propose changes to the systematic arrangement of these subfamilies. At a lower taxonomic rank, our results highlighted the rampant homoplasy of many characters traditionally used to segregate genera, and thus the need of a critical re-evaluation of the contents of many genera (e.g. Nipponaphera, Merica, Sydaphera, Bivetia), the monophyly of which was not recovered.     

Metabolic rate depression in a marine pulmonate snail: pre-adaptation for a terrestrial existence?

Summary Terrestrial and freshwater pulmonate snails exhibit a marked depression of aerobic metabolism during estivation. This is an adaptation for existence in periodically harsh environments and, though marine gastropods may undergo anaerobic metabolism, they have not been shown to adaptively depress aerobic metabolic rate. We compared the metabolic response to progressive aerial exposure of two intertidal gastropod limpets, a prosobranch and a pulmonate. The prosobranch Patella granularis maintained a constant heart rate until shortly before death. In contrast, the pulmonate Siphonaria oculus underwent facultative depression of heart rate, accompanied by a decline in oxygen consumption. Both heart rate and oxygen consumption returned to normal levels on reimmersion in water. Metabolic rate depression is energy conserving, and may account for the ability of S. oculus to extend higher up the shore than P. granularis, into areas where food availability is low. S. oculus is a primitive, marine pulmonate, periodically subject to harsh conditions, and its capacity for metabolic rate depression may represent a pre-adaptation for life on land.     

A new species of Haplosporidium Caullery & Mesnil, 1899 in the marine false limpet Siphonaria lessonii (Gastropoda: Siphonariidae) from Patagonia

A new species of Haplosporidium Caullery & Mesnil, 1899 parasitising the pulmonate gastropod Siphonaria lessonii Blainville in Patagonia, Argentina, is described based on morphological (scanning and transmission electron microscopy) and sequence (small subunit ribosomal RNA gene) data. Different stages of sporulation were observed as infections disseminated in the digestive gland. Haplosporidium patagon n. sp. is characterised by oval or slightly subquadrate spores with an operculum that is ornamented with numerous short digitiform projections of regular height, perpendicular to and covering its outer surface. The operculum diameter is slightly larger than the apical diameter of the spore. Neither the immature nor mature spores showed any kind of projections of the exosporoplasm or of the spore wall. Regarding phylogenetic affinities, the new species was recovered as sister to an undescribed species of Haplosporidium Caullery & Mesnil, 1899 from the polychaete family Syllidae Grube from Japanese waters. The morphological characters (ornamentation of the operculum, spore wall structure, shape and size of spores, and the lack of spore wall projections) corroborate it as an as yet undescribed species of Haplosporidium and the first for the phylum in marine gastropods of South America. Siphonaria lessonii is the only known host to date.     

The complete mitochondrial genome of the black coral Chrysopathes formosa (Cnidaria:Anthozoa:Antipatharia) supports classification of antipatharians within the subclass Hexacorallia

Black corals comprise a globally distributed shallow- and deep-water taxon whose phylogenetic position within the Anthozoa has been debated. We sequenced the complete mitochondrial genome of the antipatharian Chrysopathes formosa to further evaluate its phylogenetic relationships. The circular mitochondrial genome (18,398 bp) consists of 13 energy pathway protein-coding genes and two ribosomal RNAs, but only two transfer RNA genes (trnM and trnW), as well as a group I intron within the nad5 gene that contains the only copies of nad1 and nad3. No novel genes were found in the antipatharian mitochondrial genome. Gene order and genome content are most similar to those of the sea anemone Metridium senile (subclass Hexacorallia), with differences being the relative location of three contiguous genes (cox2-nad4-nad6) and absence (from the antipatharian) of a group I intron within the cox1 gene. Phylogenetic analyses of multiple protein-coding genes support classifying the Antipatharia within the subclass Hexacorallia and not the subclass Ceriantipatharia; however, the sister-taxon relationships of black corals within Hexacorallia remain inconclusive.     

The mitochondrial genomes of Euphausia pacifica and Thysanoessa raschii sequenced using 454 next-generation sequencing, with a phylogenetic analysis of their position in the Malacostracan family tree

Euphausiid krill play a critical role in coastal and oceanic food webs, linking primary producers to upper trophic levels. In addition, some species support commercial fisheries worldwide. Despite their ecological importance, the genetics of these important species remain poorly described. To improve our understanding of the genetics of these ecological links, we sequenced the mitochondrial genomes of two species of North Pacific krill, Euphausia pacifica and Thysanoessa raschii, using long-range PCR and 454 GS Junior next-generation sequencing technology. The E. pacifica mitogenome (14,692 + base pairs (bp)) encodes 13 protein-coding genes (PCGs), two ribosomal RNA (rRNA) genes, and at least 22 transfer RNA (tRNA) genes. The T. raschii mitogenome (14,240 + bp) encodes 13 PCGs, two rRNA genes, and at least 19 tRNA genes. The gene order in both species is similar to that of E. superba. Comparisons between Bering Sea and Yellow Sea E. pacifica revealed a total of 644 variable sites. The most variable protein-coding gene were atp8 (7.55 %, 12 of 159 sites variable), nad4 (6.35 %, 85 variable sites) and nad6 (6.32 %, 33 variable sites). Phylogenetic analyses to assess the phylogenetic position of the Euphausiacea, using the concatenated nucleic acid sequences of E. pacifica and T. raschii along with 46 previously published malacostracan mitogenomes, support the monophyly of the order Decapoda and indicate that the Euphausiacea share a common ancestor with the Decapoda. Future research should utilize this sequence data to explore the population genetics and molecular ecology of these species.     

Molecular mechanisms of extensive mitochondrial gene rearrangement in plethodontid salamanders

Extensive gene rearrangement is reported in the mitochondrial genomes of lungless salamanders (Plethodontidae). In each genome with a novel gene order, there is evidence that the rearrangement was mediated by duplication of part of the mitochondrial genome, including the presence of both pseudogenes and additional, presumably functional, copies of duplicated genes. All rearrangement-mediating duplications include either the origin of light-strand replication and the nearby tRNA genes or the regions flanking the origin of heavy-strand replication. The latter regions comprise nad6, trnE, cob, trnT, an intergenic spacer between trnT and trnP and, in some genomes, trnP, the control region, trnF, rrnS, trnV, rrnL, trnL1, and nad1. In some cases, two copies of duplicated genes, presumptive regulatory regions, and/or sequences with no assignable function have been retained in the genome following the initial duplication; in other genomes, only one of the duplicated copies has been retained. Both tandem and nontandem duplications are present in these genomes, suggesting different duplication mechanisms. In some of these mitochondrial DNAs, up to 25% of the total length is composed of tandem duplications of noncoding sequence that includes putative regulatory regions and/or pseudogenes of tRNAs and protein-coding genes along with the otherwise unassignable sequences. These data indicate that imprecise initiation and termination of replication, slipped-strand mispairing, and intramolecular recombination may all have played a role in generating repeats during the evolutionary history of plethodontid mitochondrial genomes.     

The mitochondrial genome of <Emphasis Type="Italic">Euphausia superba</Emphasis> (Prydz Bay) (Crustacea: Malacostraca: Euphausiacea) reveals a novel gene arrangement and potential molecular markers

Euphausiid krill are dominant organisms in the zooplankton population and play a central role in marine ecosystems. In this paper, we described the gene organization, gene rearrangement and codon usage in the mitochondrial genome of Euphausia superba Dana 1852 (sampling from Prydz Bay, PB). The mitochondrial genome of E. superba is more than 15,498 bp in length (partial non-coding region was not determined). Translocation of four tRNAs (trnL ₁ , trnL ₂ , trnW and trnI) and duplication of one tRNA (trnN) were founded in the mitochondrial genome of E. superba when comparing its genome with the pancrustacean ground pattern. To investigate the phylogenetic relationship within Malacostraca, phylogenetic trees based on currently available malacostracan mitochondrial genomes were built with the maximum likelihood and the Bayesian models. All analyses based on nucleotide and amino acid data strongly support the monophyly of Stomatopoda, Penaeidae, Caridea, and Brachyura, which is consistent with previous research. However, the taxonomic position of Euphausiacea within Malacostraca is unstable. From comparing the mitochondrial genome between E. superba (PB) and E. superba (sampling from Weddell Sea, WS), we found that nad2 gene contains maximal variation with 61 segregating sites, following by nad5 gene which has 12 segregating sites. Thus, nad2 and nad5 genes may be used as potential molecular markers to study the inherit diversity among different E. superba groups, which would be helpful to the exploitation and management of E. superba resources.     

Multiple rearrangements in mitochondrial genomes of Isopoda and phylogenetic implications

In this study, we analyse the evolutionary dynamics and phylogenetic implications of gene order rearrangements in five newly sequenced mitochondrial (mt) genomes and four published mt genomes of isopod crustaceans. The sequence coverage is nearly complete for four of the five newly sequenced species, with only the control region and some tRNA genes missing, while in Janira maculosa only two thirds of the genome could be determined. Mitochondrial gene order in isopods seems to be more plastic than that in other crustacean lineages, making all nine known mt gene orders different. Especially the asellote Janira is characterized by many autapomorphies. The following inferred ancestral isopod mt gene order exists slightly modified in modern isopods: nad1, tnrL1, rrnS, control region, trnS1, cob, trnT, nad5, trnF. We consider the inferred gene translocation events leading to gene rearrangements as valuable characters in phylogenetic analyses. In this first study covering major isopod lineages, potential apomorphies were identified, e.g., a shared relative position of trnR in Valvifera. We also report one of the first findings of homoplasy in mitochondrial gene order, namely a shared relative position of trnV in unrelated isopod lineages. In addition to increased taxon sampling secondary structure, modification in tRNAs and GC-skew inversion may be potentially fruitful subjects for future mt genome studies in a phylogenetic context.     

Mitogenomic phylogeny of the Naticidae (Gastropoda: Littorinimorpha) reveals monophyly of the Polinicinae

The Naticidae is a species-rich family of predatory marine gastropods with substantial interspecific morphological diversity. The classification of the Naticidae has been traditionally based on morphology data, but the phylogenetic relationships within the family are debated due to conflicting molecular results, especially regarding the monophyly of subfamilies Polinicinae and Naticinae. To further resolve the phylogenetic controversies within the Naticidae, we undertake a phylogenetic approach using 14 newly sequenced complete or nearly complete (only lacking a control region) mitochondrial genomes. Both the maximum likelihood and Bayesian inference analyses supported monophyly of the Polinicinae, but paraphyly of the Naticinae due to the placement of the enigmatic genus Notocochlis. The ancestral character reconstruction suggests that the operculum, a character that currently defines the two subfamilies, evolved from an ancestor with a calcareous operculum in the evolutionary history of naticids. In addition, the chronogram estimates that naticids was originated in late Triassic (about 227 million years ago), consistent with previous hypotheses. Our study highlights the importance of using complete mitochondrial genomes while reconstructing phylogenetic relationships within the Naticidae. The evolution scenario of the naticid operculum contributes new insights into the classification of Naticidae.     

New clades of euthyneuran gastropods (Mollusca) from 28S rRNA sequences

Recent morphological and molecular results on phylogeny of euthyneuran gastropods, which include opisthobranchs and pulmonates, have greatly diminished previous supposed resolution of their phylogenetic relationships. In addition to recent morphological results, sequences of the D1 and D2 domains of the 28S rRNA are here analyzed by parsimony for 31 euthyneuran species. The molecular and previous morphological data sets were not congruent according to an ILD test, and morphological and molecular data could not be analyzed simultaneously. Consequently Bremer's Combinable Component Consensus was used to obtain a new tree, with the following supported molecular results: monophyly of a new clade of opisthobranchs including actively swimming Euthyneura, i.e., pelagic Gymnosomata and Thecosomata plus benthic Anaspidea; first molecular confirmation of monophylies of Hygrophila, including Chilina, Acteonoidea, and Sacoglossa, which include both shell-bearing species and slugs; and new confirmation of the monophyly of Stylommatophora. Morphological characters which support the new clades obtained here are discussed.