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

The fossil record of freshwater Gastropoda - a global review

Gastropoda are an exceptionally successful group with a rich and diverse fossil record. They have conquered land and freshwater habitats multiple times independently and have dispersed across the entire globe. Since they are important constituents of fossil assemblages, they are often used for palaeoecological reconstruction, biostratigraphic correlations, and as model groups to study morphological and taxonomic evolution. While marine faunas and their evolution have been a common subject of study, the freshwater component of the fossil record has attracted much less attention, and a global overview is lacking. Here, I review the fossil record of freshwater gastropods on a global scale, ranging from their origins in the late Palaeozoic to the Pleistocene. As compiled here, the global fossil record of freshwater Gastropoda includes 5182 species in 490 genera, 44 families, and 12 superfamilies over a total of ~340 million years. Following a slow and poorly known start in the late Palaeozoic, diversity slowly increased during the Mesozoic. Diversity culminated in an all-time high in the Neogene, relating to diversification in numerous long-lived (ancient) lakes in Europe. I summarise well-documented and hypothesised freshwater colonisation events and compare the patterns found in freshwater gastropods to those in land snails. Furthermore, I discuss potential preservation and sampling biases, as well as the main drivers underlying species diversification in fresh water on a larger scale. In that context, I particularly highlight the importance of long-lived lakes as islands and archives of evolution and expand a well-known concept in ecology and evolution to a broader spectrum: scale-independent ecological opportunity.

The Coral Triangle and Strait of Malacca are two distinct hotspots of mangrove biodiversity

Knowledge of the biogeography of marine taxa has lagged significantly behind terrestrial ecosystems. A hotspot of marine biodiversity associated with coral reefs is known in the Coral Triangle of the Indo-West Pacific, but until now there was little data with which to evaluate broad patterns of species richness in the coastal fauna of ecosystems other than coral reefs. This data is critically needed for fauna with low functional redundancy like that of mangroves, that are vulnerable to habitat loss and rising sea levels. Here we show that the diversity of mangrove fauna is characterized by two distinct hotspots in the Indo-West Pacific, associated with two habitat types: fringe mangroves in the Coral Triangle, and riverine mangroves in the Strait of Malacca, between the west coast of Peninsular Malaysia and Sumatra. This finding, based on a family of slugs of which the systematics has been completely revised, illustrates an unexpected biogeographic pattern that emerged only after this taxon was studied intensively. Most organisms that live in the mangrove forests of Southeast Asia remain poorly known both taxonomically and ecologically, and the hotspot of diversity of onchidiid slugs in the riverine mangroves of the Strait of Malacca indicates that further biodiversity studies are needed to support effective conservation of mangrove biodiversity.

Complete mitochondrial genomes of the slugs Deroceraslaeve (Agriolimacidae) and Ambigolimaxvalentianus (Limacidae) provide insights into the phylogeny of Stylommatophora (Mollusca, Gastropoda)

In this study, we sequenced two complete mitogenomes from Deroceraslaeve and Ambigolimaxvalentianus. The mitogenome of Ambigolimaxvalentianus represented the first such data from the family Limacidae. The lengths of the mitogenomes of Deroceraslaeve and Ambigolimaxvalentianus were 14,773 bp and 15,195 bp, respectively. The entire set of 37 mitochondrial genes were identified for both mitogenomes. Compared with the mitogenome of Achatinafulica, the trnP_trnA tRNA cluster was rearranged in both Deroceraslaeve and Ambigolimaxvalentianus. The secondary structures of tRNA and rRNA genes for the two species were predicted. Phylogenetic analyses based on amino acid sequences supported (1) monophyly of Stylommatophora, (2) division of Stylommatophora into the 'achatinoid' clade (i.e., the suborder Achatinina) and the 'non-achatinoid' clade (i.e., the suborder Helicina), (3) placement of the Orthurethra in the 'non-achatinoid' clade, and (4) monophyly of each of the superfamilies Helicoidea, Urocoptoidea, Succineoidea, Arionoidea, Pupilloidea and Limacoidea. The exemplars of Helicidae, Philomycidae and Achatinellidae displayed many more mitochondrial gene rearrangements than other species of Stylommatophora.

The Taming of Smeagol? A New Population and an Assessment of the Known Population of the Critically Endangered Pulmonate Gastropod Smeagol hilaris (Heterobranchia, Otinidae)

The genus Smeagol consists of five named species of air-breathing marine slugs (restricted to southern Australia and New Zealand) and three undescribed taxa from southern Japan. Only one species, S. hilaris, is known to be from New South Wales (NSW), and it previously had a known distribution limited to one site, Merry Beach on the south coast. This diminutive invertebrate is classified as critically endangered in NSW due to its extremely restricted distribution and concern about its historically declining numbers. Accordingly, the aims of this study were to survey the known population of S. hilaris at Merry Beach and to explore other potentially suitable sites, using a visual census method, to determine if further populations or species exist in NSW. The resulting quantitative surveys of the known population and a new population at Storm Bay, Kiama, NSW, are reported here. DNA barcoding of a ~650 bp segment of the mitochondrial cytochrome c oxidase I (COI) gene for several individuals from each population confirmed the conspecificity among the two populations. The population at Merry Beach was found to remain viable, while the discovery of the new population of S. hilaris represents a doubling of the known global populations of this species. Details of the highly-specialised niche habitat occupied by Smeagol in New South Wales and recommendations for ongoing management are documented.

A small slug from a tropical greenhouse reveals a new rathouisiid lineage with triaulic tritrematic genitalia (Gastropoda: Systellommatophora)

A small slug found in the tropical greenhouse of the Science Museum (MUSE) of Trento (Italy) turned out to be a species of the little-known systellommatophoran family Rathouisiidae. We undertook detailed comparative anatomical and molecular studies using specimens of the MUSE slug, Rathouisia sinensis, and sequences of other systellommatophoran species deposited in GenBank to conduct a systematic and phylogenetic assessment. Analysis of the genitalia of the MUSE slug and R. sinensis revealed an unusual triaulic tritrematic structure: two separate female ducts – one for egg release (oviduct), the other for intake of allosperm (vagina) – and a separate male duct for autosperm release. Analysis of the nucleotide sequences of several mitochondrial (COI, 16S rDNA) and nuclear (18S rDNA, ITS2 flanked by 5.8S and 28S rDNA fragments, H3) gene fragments supported assignation of the MUSE slug to Rathouisiidae, but also its distinction from the other rathouisiid genera Atopos, Granulilimax, Rathouisia and an undescribed genus from the Ryukyu Islands (Japan). Therefore, we decided to describe the MUSE slug as a new species in a new genus: Barkeriella museensis gen. et sp. nov. The species is certainly an alien introduced into the tropical greenhouse of MUSE, but its origin is unknown and calls for further investigation.

Extreme mitochondrial DNA divergence underlies genetic conflict over sex determination

Cytoplasmic male sterility (CMS) is a form of genetic conflict over sex determination that results from differences in modes of inheritance between genomic compartments.^1-3 Indeed, maternally transmitted (usually mitochondrial) genes sometimes enhance their transmission by suppressing the male function in a hermaphroditic organism to the detriment of biparentally inherited nuclear genes. Therefore, these hermaphrodites become functionally female and may coexist with regular hermaphrodites in so-called gynodioecious populations.³ CMS has been known in plants since Darwin's times⁴ but is previously unknown in the animal kingdom.^5-8 We relate the first observation of CMS in animals. It occurs in a freshwater snail population, where some individuals appear unable to sire offspring in controlled crosses and show anatomical, physiological, and behavioral characters consistent with a suppression of the male function. Male sterility is associated with a mitochondrial lineage that underwent a spectacular acceleration of DNA substitution rates, affecting the entire mitochondrial genome-this acceleration concerns both synonymous and non-synonymous substitutions and therefore results from increased mitogenome mutation rates. Consequently, mitochondrial haplotype divergence within the population is exceptionally high, matching that observed between snail taxa that diverged 475 million years ago. This result is reminiscent of similar accelerations in mitogenome evolution observed in plant clades where gynodioecy is frequent,^9,10 both being consistent with arms-race evolution of genome regions implicated in CMS.^11,12 Our study shows that genomic conflicts can trigger independent evolution of similar sex-determination systems in plants and animals and dramatically accelerate molecular evolution.

Phylogenomic resolution of the root of Panpulmonata, a hyperdiverse radiation of gastropods: new insight into the evolution of air breathing

Transitions to terrestriality have been associated with major animal radiations including land snails and slugs in Stylommatophora (>20 000 described species), the most successful lineage of 'pulmonates' (a non-monophyletic assemblage of air-breathing gastropods). However, phylogenomic studies have failed to robustly resolve relationships among traditional pulmonates and affiliated marine lineages that comprise clade Panpulmonata (Mollusca, Gastropoda), especially two key taxa: Sacoglossa, a group including photosynthetic sea slugs, and Siphonarioidea, intertidal limpet-like snails with a non-contractile pneumostome (narrow opening to a vascularized pallial cavity). To clarify the evolutionary history of the panpulmonate radiation, we performed phylogenomic analyses on datasets of up to 1160 nuclear protein-coding genes for 110 gastropods, including 40 new transcriptomes for Sacoglossa and Siphonarioidea. All 18 analyses recovered Sacoglossa as the sister group to a clade we named Pneumopulmonata, within which Siphonarioidea was sister to the remaining lineages in most analyses. Comparative modelling indicated shifts to marginal habitat (estuarine, mangrove and intertidal zones) preceded and accelerated the evolution of a pneumostome, present in the pneumopulmonate ancestor along with a one-sided plicate gill. These findings highlight key intermediate stages in the evolution of air-breathing snails, supporting the hypothesis that adaptation to marginal zones played an important role in major sea-to-land transitions.

Comparative mitogenomics of freshwater snails of the genus Bulinus, obligatory vectors of Schistosoma haematobium, causative agent of human urogenital schistosomiasis

Among the snail genera most responsible for vectoring human-infecting schistosomes, Bulinus, Biomphalaria, and Oncomelania, the former is in many respects the most important. Bulinid snails host the most common human blood fluke, Schistosoma haematobium, responsible for approximately two-thirds of the estimated 237 million cases of schistosomiasis. They also support transmission of schistosomes to millions of domestic and wild animals. Nonetheless, our basic knowledge of the 37 Bulinus species remains incomplete, especially with respect to genome information, even including mitogenome sequences. We determined complete mitogenome sequences for Bulinus truncatus, B. nasutus, and B. ugandae, and three representatives of B. globosus from eastern, central, and western Kenya. A difference of the location of tRNA-Asp was found between mitogenomes from the three species of the Bulinus africanus group and B. truncatus. Phylogenetic analysis using partial cox1 sequences suggests that B. globosus is a complex comprised of multiple species. We also highlight the status of B. ugandae as a distinct species with unusual interactions with the S. haematobium group parasites deserving of additional investigation. We provide sequence data for potential development of genetic markers for specific or intraspecific Bulinus studies, help elucidate the relationships among Bulinus species, and suggest ways in which mitogenomes may help understand the complex interactions between Schistosoma and Bulinus snails and their relatives.

Global diversification and evolutionary history of onchidiid slugs (Gastropoda, Pulmonata)

Many marine species are specialized to specific parts of a habitat. In a mangrove forest, for instance, species may be restricted to the mud surface, the roots and trunks of mangrove trees, or rotting logs, which can be regarded as distinct microhabitats. Shifts to new microhabitats may be an important driver of sympatric speciation. However, the evolutionary history of these shifts is still poorly understood in most groups of marine organisms, because it requires a well-supported phylogeny with relatively complete taxon sampling. Onchidiid slugs are an ideal case study for the evolutionary history of habitat and microhabitat shifts because onchidiid species are specialized to different tidal zones and microhabitats in mangrove forests and rocky shores, and the taxonomy of the family in the Indo-West Pacific has been recently revised in a series of monographs. Here, DNA sequences for onchidiid species from the North and East Pacific, the Caribbean, and the Atlantic are used to reconstruct phylogenetic relationships among Onchidella species, and are combined with new data for Indo-West Pacific species to reconstruct a global phylogeny of the family. The phylogenetic relationships of onchidiid slugs are reconstructed based on three mitochondrial markers (COI, 12S, 16S) and three nuclear markers (28S, ITS2, H3) and nearly complete taxon sampling (all 13 genera and 62 of the 67 species). The highly-supported phylogeny presented here suggests that ancestral onchidiids most likely lived in the rocky intertidal, and that a lineage restricted to the tropical Indo-West Pacific colonized new habitats, including mudflats, mangrove forests, and high-elevation rainforests. Many onchidiid species in the Indo-West Pacific diverged during the Miocene, around the same time that a high diversity of mangrove plants appears in the fossil record, while divergence among Onchidella species occurred earlier, likely beginning in the Eocene. It is demonstrated that ecological specialization to microhabitats underlies the divergence between onchidiid genera, as well as the diversification through sympatric speciation in the genera Wallaconchis and Platevindex. The geographic distributions of onchidiid species also indicate that allopatric speciation played a key role in the diversification of several genera, especially Onchidella and Peronia. The evolutionary history of several morphological traits (penial gland, rectal gland, dorsal eyes, intestinal loops) is examined in relation to habitat and microhabitat evolutionary transitions and that the rectal gland of onchidiids is an adaptation to high intertidal and terrestrial habitats.

Origin and significance of two pairs of head tentacles in the radiation of euthyneuran sea slugs and land snails

The gastropod infraclass Euthyneura comprises at least 30,000 species of snails and slugs, including nudibranch sea slugs, sea hares and garden snails, that flourish in various environments on earth. A unique morphological feature of Euthyneura is the presence of two pairs of sensory head tentacles with different shapes and functions: the anterior labial tentacles and the posterior rhinophores or eyestalks. Here we combine molecular phylogenetic and microanatomical evidence that suggests the two pairs of head tentacles have originated by splitting of the original single tentacle pair (with two parallel nerve cords in each tentacle) as seen in many other gastropods. Minute deep-sea snails of Tjaernoeia and Parvaplustrum, which in our phylogeny belonged to the euthyneurans’ sister group (new infraclass Mesoneura), have tentacles that are split along much of their lengths but associated nerves and epidermal sense organs are not as specialized as in Euthyneura. We suggest that further elaboration of cephalic sense organs in Euthyneura closely coincided with their ecological radiation and drastic modification of body plans. The monotypic family Parvaplustridae nov., superfamily Tjaernoeioidea nov. (Tjaernoeiidae + Parvaplustridae), and new major clade Tetratentaculata nov. (Mesoneura nov. + Euthyneura) are also proposed based on their phylogenetic relationships and shared morphological traits.

Chromosome Diversity and Evolution in Helicoide a (Gastropoda: Stylommatophora): A Synthesis from Original and Literature Data

Simple Summary

The superfamily Helicoidea is a large and diverse group of Eupulmonata. The superfamily has been the subject of several molecular and phylogenetic studies which greatly improved our knowledge on the evolutionary relationships and historical biogeography of many families. In contrast, the available karyological information on Helicoidea still results in an obscure general picture, lacking a homogeneous methodological approach and a consistent taxonomic record. Nevertheless, the available karyological information highlights the occurrence of a significant chromosomal diversity in the superfamily in terms of chromosome number (varying from 2n = 40 to 2n = 62), chromosome morphology and the distribution of different karyological features among different taxonomic groups. Here we performed a molecular and a comparative cytogenetic analysis on of 15 Helicoidea species of three different families. Furthermore, to provide an updated assessment of the chromosomal diversity of the superfamily we reviewed all the available chromosome data. Finally, superimposing all the chromosome data gathered from different sources on the available phylogenetic relationships of the studied taxa, we discuss the overall observed chromosome diversity in Helicoidea and advance a hypothesis on its chromosomal evolution.

Abstract

We performed a molecular and a comparative cytogenetic analysis on different Helicoidea species and a review of all the available chromosome data on the superfamily to provide an updated assessment of its karyological diversity. Standard karyotyping, banding techniques, and Fluorescence in situ hybridization of Nucleolus Organizer Region loci (NOR-FISH) were performed on fifteen species of three families: two Geomitridae, four Hygromiidae and nine Helicidae. The karyotypes of the studied species varied from 2n = 44 to 2n = 60, highlighting a high karyological diversity. NORs were on a single chromosome pair in Cernuella virgata and on multiple pairs in four Helicidae, representing ancestral and derived conditions, respectively. Heterochromatic C-bands were found on pericentromeric regions of few chromosomes, being Q- and 4′,6-diamidino-2-phenylindole (DAPI) negative. NOR-associated heterochromatin was C-banding and chromomycin A₃ (CMA₃) positive. Considering the available karyological evidence on Helicoidea and superimposing the chromosome data gathered from different sources on available phylogenetic inferences, we describe a karyotype of 2n = 60 with all biarmed elements as the ancestral state in the superfamily. From this condition, an accumulation of chromosome translocations led to karyotypes with a lower chromosome number (2n = 50–44). This process occurred independently in different lineages, while an augment of the chromosome number was detectable in Polygyridae. Chromosome inversions were also relevant chromosome rearrangements in Helicoidea, leading to the formation of telocentric elements in karyotypes with a relatively low chromosome count.

Assessment of mitochondrial genomes for heterobranch gastropod phylogenetics

Background

Heterobranchia is a diverse clade of marine, freshwater, and terrestrial gastropod molluscs. It includes such disparate taxa as nudibranchs, sea hares, bubble snails, pulmonate land snails and slugs, and a number of (mostly small-bodied) poorly known snails and slugs collectively referred to as the “lower heterobranchs”. Evolutionary relationships within Heterobranchia have been challenging to resolve and the group has been subject to frequent and significant taxonomic revision. Mitochondrial (mt) genomes can be a useful molecular marker for phylogenetics but, to date, sequences have been available for only a relatively small subset of Heterobranchia.

Results

To assess the utility of mitochondrial genomes for resolving evolutionary relationships within this clade, eleven new mt genomes were sequenced including representatives of several groups of “lower heterobranchs”. Maximum likelihood analyses of concatenated matrices of the thirteen protein coding genes found weak support for most higher-level relationships even after several taxa with extremely high rates of evolution were excluded. Bayesian inference with the CAT + GTR model resulted in a reconstruction that is much more consistent with the current understanding of heterobranch phylogeny. Notably, this analysis recovered Valvatoidea and Orbitestelloidea in a polytomy with a clade including all other heterobranchs, highlighting these taxa as important to understanding early heterobranch evolution. Also, dramatic gene rearrangements were detected within and between multiple clades. However, a single gene order is conserved across the majority of heterobranch clades.

Conclusions

Analysis of mitochondrial genomes in a Bayesian framework with the site heterogeneous CAT + GTR model resulted in a topology largely consistent with the current understanding of heterobranch phylogeny. However, mitochondrial genomes appear to be too variable to serve as good phylogenetic markers for robustly resolving a number of deeper splits within this clade.

Systematics and geographical distribution of Galba species, a group of cryptic and worldwide freshwater snails

Cryptic species can present a significant challenge to the application of systematic and biogeographic principles, especially if they are invasive or transmit parasites or pathogens. Detecting cryptic species requires a pluralistic approach in which molecular markers facilitate the detection of coherent taxonomic units that can then be analyzed using various traits (e.g., internal morphology) and crosses. In asexual or self-fertilizing species, the latter criteria are of limited use. We studied a group of cryptic freshwater snails (genus Galba) from the family Lymnaeidae that have invaded almost all continents, reproducing mainly by self-fertilization and transmitting liver flukes to humans and livestock. We aim to clarify the systematics, distribution, and phylogeny of these species with an integrative approach that includes morphology, molecular markers, wide-scale sampling across America, and data retrieved from GenBank (to include Old World samples). Our phylogenetic analysis suggests that the genus Galba originated ca. 22 Myr ago and today comprises six species or species complexes. Four of them show an elongated-shell cryptic phenotype and exhibit wide variation in their genetic diversity, geographic distribution, and invasiveness. The remaining two species have more geographically restricted distributions and exhibit a globose-shell cryptic phenotype, most likely phylogenetically derived from the elongated one. We emphasize that no Galba species should be identified without molecular markers. We also discuss several hypotheses that can explain the origin of cryptic species in Galba, such as convergence and morphological stasis.

Genetic assessment of the variation and distribution of the species of Salinator (Panpulmonata: Amphibolidae) in south-eastern Australia

Amphibolidae is one of the most abundant families of gastropods in estuarine environments of south-eastern Australia. However, the range limits of the species of Salinator, one of the family’s two genera in the region, remain unclear partly due to uncertainty of identifications based solely on shell morphology. Insufficient data have been collected to address questions regarding the genetic variability of any of the species of Salinator. Here, DNA sequences from a segment of the cytochrome c oxidase subunit I and 28S ribosomal RNA genes were collected to investigate the distribution and variation of the three Salinator species in the region, these being S.fragilis, S.rhamphidia and S.tecta. The results demonstrate a large range extension in S.rhamphidia and suggest that S.tecta may have limited distribution in Tasmania. In contrast to previously-studied estuarine Mollusca in the south-eastern coasts of the mainland and Tasmania, S.rhamphidia has regional differentiation. There is evidence of genetic disequilibrium within S.fragilis, suggesting that it may presently comprise contributions from two distinct sets of populations.

Systematic revision of the genus Peronia Fleming, 1822 (Gastropoda, Euthyneura, Pulmonata, Onchidiidae)

The genus Peronia Fleming, 1822 includes all the onchidiid slugs with dorsal gills. Its taxonomy is revised for the first time based on a large collection of fresh material from the entire Indo-West Pacific, from South Africa to Hawaii. Nine species are supported by mitochondrial (COI and 16S) and nuclear (ITS2 and 28S) sequences as well as comparative anatomy. All types available were examined and the nomenclatural status of each existing name in the genus is addressed. Of 31 Peronia species-group names available, 27 are regarded as invalid (twenty-one synonyms, sixteen of which are new, five nomina dubia, and one homonym), and four as valid: Peronia peronii (Cuvier, 1804), Peronia verruculata (Cuvier, 1830), Peronia platei (Hoffmann, 1928), and Peronia madagascariensis (Labbé, 1934a). Five new species names are created: P. griffithsi Dayrat & Goulding, sp. nov., P. okinawensis Dayrat & Goulding, sp. nov., P. setoensis Dayrat & Goulding, sp. nov., P. sydneyensis Dayrat & Goulding, sp. nov., and P. willani Dayrat & Goulding, sp. nov. Peronia species are cryptic externally but can be distinguished using internal characters, with the exception of P. platei and P. setoensis. The anatomy of most species is described in detail here for the first time. All the secondary literature is commented on and historical specimens from museum collections were also examined to better establish species distributions. The genus Peronia includes two species that are widespread across the Indo-West Pacific (P. verruculata and P. peronii) as well as endemic species: P. okinawensis and P. setoensis are endemic to Japan, and P. willani is endemic to Northern Territory, Australia. Many new geographical records are provided, as well as a key to the species using morphological traits.

From Persian Gulf to Indonesia: interrelated phylogeographic distance and chemistry within the genus Peronia (Onchidiidae, Gastropoda, Mollusca)

The knowledge of relationships between taxa is essential to understand and explain the chemical diversity of the respective groups. Here, twelve individuals of the panpulmonate slug Peronia persiae from two localities in Persian Gulf, and one animal of P. verruculata from Bangka Island, Indonesia, were analyzed in a phylogenetic and chemotaxonomic framework. Based on the ABGD test and haplotype networking using COI gene sequences of Peronia specimens, nine well-supported clades were found. Haplotype network analysis highlighted a considerable distance between the specimens of P. persiae and other clades. Metabolomic analysis of both species using tandem mass spectrometry-based GNPS molecular networking revealed a large chemical diversity within Peronia of different clades and localities. While P. persiae from different localities showed a highly similar metabolome, only few identical chemical features were found across the clades. The main common metabolites in both Peronia species were assigned as polypropionate esters of onchitriols and ilikonapyrones, and osmoprotectant amino acid-betaine compounds. On the other hand, the isoflavonoids genistein and daidzein were exclusively detected in P. persiae, while cholesterol and conjugated chenodeoxycholic acids were only found in P. verruculata. Flavonoids, bile acids, and amino acid-betaine compounds were not reported before from Onchidiidae, some are even new for panpulmonates. Our chemical analyses indicate a close chemotaxonomic relation between phylogeographically distant Peronia species.

Tracking parallel adaptation of shell morphology through geological times in the land snail genus Pupilla (Gastropoda: Stylommatophora: Pupillidae)

Changing environmental conditions force species either to disperse or to adapt locally either genetically or via phenotypic plasticity. Although limits of plasticity can be experimentally tested, the predictability of genetic adaptation is restricted due to its stochastic nature. Nevertheless, our understanding of evolutionary adaptation has been improving in particular through studies of parallel adaptation. Based on molecular phylogenetic inferences and morphological investigations of both recent and fossil shells we tracked the morphological changes in three land snails, Pupilla alpicola, Pupilla loessica and Pupilla muscorum. These species differ in habitat requirements as well as historical and extant distributions with P. alpicola and P. loessica being more similar to each other than to P. muscorum. Therefore, we hypothesized, that the three species reacted independently and individually to the conditions changing throughout the Pleistocene, but expected that changes within P. alpicola and P. loessica would be more similar compared to P. muscorum. Indeed, intraspecific shell shape differences across time were similar in P. alpicola and P. loessica, suggesting that similar niche shifts have led to similar transformations in parallel. In contrast, extant P. muscorum populations were practically identical in shape to their ancestors. They have probably tracked their ecological niches through time.

Egg perivitelline fluid proteome of a freshwater snail: Insight into the transition from aquatic to terrestrial egg deposition

RATIONALE

Proteins from the egg perivitelline fluid (PVF) are assumed to play critical roles in embryonic development, but for many groups of animals their identities remain unknown. Identifying egg PVF proteins is a critical step towards understanding their functions including their roles in evolutionary transition in habitats.

METHODS

We applied proteomic and transcriptomic analysis to investigate the PVF proteome of the eggs of Pomacea diffusa, an aerial ovipositing freshwater snail in the family Ampullariidae. The PVF proteins were separated with the sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) method, and proteomic analysis was conducted using an LTQ Velos ion trap mass spectrometer coupled with liquid chromatography. Comparison of PVF proteomes and evolution analyses was performed between P. diffusa and other ampullariids.

RESULTS

In total, 32 egg PVF proteins were identified from P. diffusa. They were categorized as PV1-like subunits, immune-responsive proteins, protein degradation, signaling and binding, transcription and translation, metabolism, oxidation-reduction and proteins with unknown function. Interestingly, the proteome includes a calcium-binding protein important in forming the hard eggshell that enabled the terrestrial transition. However, it does not include PV2, a neurotoxic protein that was assumed to be present in all Pomacea species.

CONCLUSIONS

The PVF proteome data from P. diffusa can help us better understand the roles that reproductive proteins played during the transition from underwater to terrestrial egg deposition. Moreover, they could be useful in comparative studies of the terrestrialization in several groups of animals that occurred independently during their evolution.

The complete mitogenome of Arion vulgaris Moquin-Tandon, 1855 (Gastropoda: Stylommatophora): mitochondrial genome architecture, evolution and phylogenetic considerations within Stylommatophora

Stylommatophora is one of the most speciose orders of Gastropoda, including terrestrial snails and slugs, some of which are economically important as human food, agricultural pests, vectors of parasites or due to invasiveness. Despite their great diversity and relevance, the internal phylogeny of Stylommatophora has been debated. To date, only 34 stylommatophoran mitogenomes were sequenced. Here, the complete mitogenome of an invasive pest slug, Arion vulgaris Moquin-Tandon, 1855 (Stylommatophora: Arionidae), was sequenced using next generation sequencing, analysed and compared with other stylommatophorans. The mitogenome of A. vulgaris measures 14,547 bp and contains 13 protein-coding, two rRNA, 22 tRNA genes, and one control region, with an A + T content of 70.20%. All protein coding genes (PCGs) are initiated with ATN codons except for COX1, ND5 and ATP8 and all are ended with TAR or T-stop codons. All tRNAs were folded into a clover-leaf secondary structure except for trnC and trnS1 (AGN). Phylogenetic analyses confirmed the position of A. vulgaris within the superfamily Arionoidea, recovered a sister group relationship between Arionoidea and Orthalicoidea, and supported monophyly of all currently recognized superfamilies within Stylommatophora except for the superfamily Helicoidea. Initial diversification time of the Stylommatophora was estimated as 138.55 million years ago corresponding to Early Cretaceous. The divergence time of A. vulgaris and Arion rufus (Linnaeus, 1758) was estimated as 15.24 million years ago corresponding to one of Earth's most recent, global warming events, the Mid-Miocene Climatic Optimum. Furthermore, selection analyses were performed to investigate the role of different selective forces shaping stylommatophoran mitogenomes. Although purifying selection is the predominant selective force shaping stylommatophoran mitogenomes, six genes (ATP8, COX1, COX3, ND3, ND4 and ND6) detected by the branch-specific aBSREL approach and three genes (ATP8, CYTB and ND4L) detected by codon-based BEB, FUBAR and MEME approaches were exposed to diversifying selection. The positively selected substitutions at the mitochondrial PCGs of stylommatophoran species seems to be adaptive to environmental conditions and affecting mitochondrial ATP production or protection from reactive oxygen species effects. Comparative analysis of stylommatophoran mitogenome rearrangements using MLGO revealed conservatism in Stylommatophora; exceptions refer to potential apomorphies for several clades including rearranged orders of trnW-trnY and of trnE-trnQ-rrnS-trnM-trnL2-ATP8-trnN-ATP6-trnR clusters for the genus Arion. Generally, tRNA genes tend to be rearranged and tandem duplication random loss, transitions and inversions are the most basic mechanisms shaping stylommatophoran mitogenomes.

Molecular phylogeny of freshwater snails and limpets (Panpulmonata: Hygrophila)

We have undertaken a comprehensive molecular phylogenetic analysis of the Hygrophila based on 3112 sites of the large subunit and 5.8S ribosomal RNA genes. A clear basal division between Chilinoidea and Lymnaeoidea is observed. The monophyly of Acroloxidae, Lymnaeidae and Physidae is also well established. However, Planorbidae are not supported as a monophyletic group, because the Bulinidae cluster within Planorbidae. The Amphipepleinae within Lymnaeidae and both Planorbinae and Ancylinae within Planorbidae are strongly supported as monophyletic subfamilies. However, the Aplexinae within Physidae and the Lymnaeinae within Lymnaeidae are not recovered. A new taxonomic revision of the Hygrophila is proposed based on the findings of this molecular phylogeny, and the implications for the evolution of chirality are discussed.

Metallomics reveals a persisting impact of cadmium on the evolution of metal-selective snail metallothioneins

The tiny contribution of cadmium (Cd) to the composition of the earth's crust contrasts with its high biological significance. We suggest that in gastropod clades, the protein family of metallothioneins (MTs) has evolved to specifically detoxify Cd.

A new species and new records of Onchidium slugs (Gastropoda, Euthyneura, Pulmonata, Onchidiidae) in South-East Asia

A new species, Onchidiummelakense Dayrat & Goulding, sp. nov., is described, bringing the total to four known species in the genus Onchidium Buchannan, 1800. Onchidiummelakense is a rare species with only nine individuals found at three mangrove sites in the Andaman Islands and the Strait of Malacca (western Peninsular Malaysia and eastern Sumatra). The new species is delineated based on mitochondrial (COI and 16S) and nuclear (ITS2 and 28S) DNA sequences as well as comparative anatomy. Each Onchidium species is characterized by a distinct color and can easily be identified in the field, even in the Strait of Malacca where there are three sympatric Onchidium species. An identification key is provided. In addition, Onchidiumstuxbergi (Westerlund, 1883) is recorded for the first time from eastern Sumatra, and Onchidiumpallidipes Tapparone-Canefri, 1889, of which the type material is described and illustrated here, is regarded as a new junior synonym of O.stuxbergi.

Molecular phylogenetic analysis of Pulmonata (Mollusca: Gastropoda) on the basis of Histone-3 gene

Background

In the present study, phylogenetic relationships within Heterobranchia in particular to Pulmonata were evaluated by means of Histone-3 (H3) gene sequence information. H3 gene is a slow evolving marker and is useful in resolving the deep level relationships. This is the first study to report the phylogeny of Pulmonata with more number of representatives from the group on the basis of H3 gene.

Results

The major groups within Heterobranchia viz. Lower Heterobranchia, Opisthobranchia, and Pulmonata were non-monophyletic. A few of the pulmonate groups’ viz. Planorbidae, Lymnaeidae, Siphonariidae, Veronicellidae, and Stylommatophora were recovered as monophyletic. The concepts of Eupulmonata and Geophila were not observed in the present study.

Conclusions

The present study was undertaken with an objective to study the phylogeny of Pulmonata reconstructed on the basis of H3 gene and its ability to resolve the deeper divergences in Pulmonata. However, the resolution at the deeper nodes is limited. There is a good resolution at the level of genera. In the future, inclusion of more number of taxa with increased sequence length of H3 marker may yield resolved topologies that may shed more insights into the phylogeny of Pulmonata.

Cretaceous amber fossils highlight the evolutionary history and morphological conservatism of land snails

Other than hard bones and shells, it is rare for soft tissues to fossilize, but occasionally they are well-preserved in amber. Here, we focus on both modern and fossilized species of the land snail superfamily Cyclophoroidea. Phylogenetic relationships within the Cyclophoroidea were previously studied using extant species, but timing of divergence within the group remains unclear. In addition, it is difficult to observe morphological traits such as the chitinous operculum and periostracum of fossil snails due to their poor preservation potential. Here we describe nine species including a new genus and five new species of well-preserved fossil cyclophoroideans from the mid-Cretaceous Burmese amber. These fossils include not only the shell, but also the chitinous operculum and periostracum, soft body, and excrements. We present the first estimation of divergence time among cyclophoroidean families using fossil records and molecular data, suggesting extreme morphological conservatism of the Cyclophoroidea for nearly 100 million years.

A new genus of air-breathing marine slugs from South-East Asia (Gastropoda, Pulmonata, Onchidiidae)

As part of an ongoing effort to revise the taxonomy of air-breathing, marine, onchidiid slugs, a new genus, Laspionchis Dayrat & Goulding, gen. nov., is described from the mangroves of South-East Asia. It includes two new species, Laspionchisboucheti Dayrat & Goulding, sp. nov., and Laspionchisbourkei Dayrat & Goulding, sp. nov., both distributed from the Malacca Strait to the Philippines and Australia. This study is based on extensive field work in South-East Asia, comparative anatomy, and both mitochondrial (COI and 16S) and nuclear (ITS2 and 28S) DNA sequences. The two new species are found in the same habitat (mud surface in mangrove forests) and are externally cryptic but are distinct anatomically. Both species are also strongly supported by DNA sequences. Three cryptic, least-inclusive, reciprocally-monophyletic units within Laspionchisbourkei are regarded as subspecies: L.bourkeibourkei Dayrat & Goulding, ssp. nov., L.bourkeilateriensis Dayrat & Goulding, ssp. nov., and L.bourkeimatangensis Dayrat & Goulding, ssp. nov. The present contribution shows again that species delineation is greatly enhanced by considering comparative anatomy and nuclear DNA sequences in addition to mitochondrial DNA sequences, and that thorough taxonomic revisions are the best and most efficient path to accurate biodiversity knowledge.

Phylogeography of freshwater planorbid snails reveals diversification patterns in Eurasian continental islands

Background

Islands have traditionally been the centre of evolutionary biological research, but the dynamics of immigration and differentiation at continental islands have not been well studied. Therefore, we focused on the Japanese archipelago, the continental islands located at the eastern end of the Eurasian continent. While the Japanese archipelago is characterised by high biodiversity and rich freshwater habitats, the origin and formation mechanisms of its freshwater organisms are not clear. In order to clarify the history of the planorbid gastropod fauna, we conducted phylogenetic analysis, divergence time estimation, ancestral state reconstruction, and lineage diversity estimations.

Results

Our analyses revealed the formation process of the planorbid fauna in the Japanese archipelago. Most lineages in the Japanese archipelago have closely related lineages on the continent, and the divergence within the Japanese lineages presumably occurred after the late Pliocene. In addition, each lineage is characterised by different phylogeographical patterns, suggesting that immigration routes from the continent to the Japanese archipelago differ among lineages. Furthermore, a regional lineage diversity plot showed that the present diversity in the Japanese archipelago potentially reflects the differentiation of lineages within the islands after the development of the Japanese archipelago.

Conclusions

Although additional taxon sampling and genetic analysis focused on each lineage are needed, our results suggest that immigration from multiple routes just prior to the development of the Japanese archipelago and subsequent diversification within the islands are major causes of the present-day diversity of the Japanese planorbid fauna.

Electronic supplementary material

The online version of this article (10.1186/s12862-018-1273-3) contains supplementary material, which is available to authorized users.

Integrative taxonomy of a new and highly-diverse genus of onchidiid slugs from the Coral Triangle (Gastropoda, Pulmonata, Onchidiidae)

A new genus of onchidiid slugs, Wallaconchis Goulding & Dayrat, gen. n., is described, including ten species. Five species were previously described but known only from the type material: Wallaconchisater (Lesson, 1830), W.graniferum (Semper, 1880), W.nangkauriense (Plate, 1893), W.buetschlii (Stantschinsky, 1907), and W.gracile (Stantschinsky, 1907), all of which were originally classified in Onchidium Buchannan, 1800. Many new records are provided for these five species, which greatly expand their known geographic distributions. Five species are new: Wallaconchisachleitneri Goulding, sp. n., W.comendadori Goulding & Dayrat, sp. n., W.melanesiensis Goulding & Dayrat, sp. n., W.sinanui Goulding & Dayrat, sp. n., and W.uncinus Goulding & Dayrat, sp. n. Nine of the ten Wallaconchis species are found in the Coral Triangle (eastern Indonesia and the Philippines). Sympatry is high, with up to six species found on the island of Bohol (Philippines) and eight species overlapping in northern Sulawesi (Indonesia). Wallaconchis is distinguished from other onchidiids by its bright dorsal colors (red, yellow, orange) but those are extremely variable and not useful for specific identification. Internally, the reproductive system can be used to identify all Wallaconchis species. The copulatory organs of Wallaconchis species are especially diverse compared to other onchidiid genera, and the possible role of reproductive incompatibility in species diversification is discussed. All specimens examined were freshly collected for the purpose of a worldwide revision of the Onchidiidae Rafinesque, 1815. The species are well delineated using DNA sequences and comparative anatomy. Mitochondrial DNA analysis yields thirteen molecular units separated by a large barcode gap, while nuclear DNA yields nine units. By integrating nuclear DNA and mitochondrial DNA with morphology, ten species are recognized. The natural history of each species (e.g., the microhabitat where they are found) is also documented. Nomenclature is addressed thoroughly (the types of all onchidiid species were examined, lectotypes were designated when needed, nomina dubia are discussed). Morphological characters, transitions to new microhabitats, and diversification processes are discussed in the context of a robust molecular phylogeny.

Complete mitochondrial and rDNA complex sequences of important vector species of Biomphalaria, obligatory hosts of the human-infecting blood fluke, Schistosoma mansoni

Using high throughput Illumina sequencing technology, we determined complete sequences for the mitochondrial genome (mitogenome) and nuclear ribosomal DNA (rDNA) complex for three African freshwater snail taxa within the genus Biomphalaria, B. pfeifferi, B. sudanica and B. choanomphala, and for two laboratory strains of B. glabrata originating from the Neotropics. Biomphalaria snails are obligate vectors of the blood fluke Schistosoma mansoni, a major etiologic agent of human intestinal schistosomiasis. Our data show that mitogenomes from African and Neotropical Biomphalaria are highly conserved. With respect to rDNA, the two internal transcribed spacers (ITS1 and 2) were found to be highly variable whereas the three ribosomal RNA genes (28S, 5.8S and 18S rRNA) exhibited no or very limited variation. Our analyses reveal that the two taxa inhabiting Lake Victoria, B. sudanica and B. choanomphala, are very similar to one another relative to the similarity either shows to B. pfeifferi or B. glabrata. This new sequence information may prove useful for developing new markers for snail identification, environmental detection/monitoring purposes or for tracking epidemiology and snail dependencies of S. mansoni in endemic areas. It also provides new information pertinent to still unresolved questions in Biomphalaria systematics and nomenclature.

A revision of Peronina Plate, 1893 (Gastropoda : Euthyneura : Onchidiidae) based on mitochondrial and nuclear DNA sequences, morphology and natural history

Peronina Plate, 1893 is a genus of onchidiids that live on the mud in mangrove forests. Peronina can be identified in the field by the lung opening at the margin between the ventral hyponotum and the dorsal notum, and by the distinctive scalloped notum edge. This genus was previously known only from the holotype of the type species, Peronina alta Plate, 1893, from eastern India. Onchidium tenerum Stoliczka, 1869 is moved to Peronina and applies to the same species as Peronina alta. Peronina species are described using an integrative approach (natural history, comparative anatomy and DNA sequences). Mitochondrial COI and 16S sequences and nuclear ITS2 and 28S sequences are used to independently test species boundaries. Mitochondrial sequences yielded three units separated by a large barcode gap, but nuclear sequences yielded two units. Because these two units are congruent with differences in the male copulatory apparatus, they are accepted as species. Explanations for highly divergent COI haplotypes within one species are discussed. Peronina tenera (Stoliczka, 1869) is distributed in the Bay of Bengal and the Strait of Malacca, while P. zulfigari Goulding & Dayrat, sp. nov. is endemic to the Strait of Malacca. The two species differ internally but are cryptic externally.

Transcriptome analysis of the threatened snail Ellobium chinense reveals candidate genes for adaptation and identifies SSRs for conservation genetics

Ellobium chinense (Pfeiffer, 1854) is a brackish pulmonate species that inhabits the bases of mangrove trees and is most commonly found in salt grass meadows. Threats to mangrove ecosystems due to habitat degradation and overexploitation have threatened the species with extinction. In South Korea, E. chinense has been assessed as vulnerable, but there are limited data on its population structure and distribution. The nucleotide and protein sequences for this species are not available in databases, which limits the understanding of adaptation-related traits. We sequenced an E. chinense cDNA library using the Illumina platform, and the subsequent bioinformatics analysis yielded 227,032 unigenes. Of these unigenes, 69,088 were annotated to matched protein and nucleotide sequences in databases, for an annotation rate of 30.42%. Among the predominant gene ontology terms, cellular and metabolic processes (under the biological process category), membrane and cell (under the cellular component category), and binding and catalytic activity (under the molecular function category) were noteworthy. In addition, 4850 unigenes were distributed to 15 Kyoto Encyclopaedia of Genes and Genomes based enrichment categories. Among the candidate genes related to adaptation, angiotensin I converting enzyme, adenylate cyclase activating polypeptide, and AMP-activated protein kinase were the most prominent. A total of 15,952 simple sequence repeats (SSRs) were identified in sequences of > 1 kb in length. The di- and trinucleotide repeat motifs were the most common. Among the repeat motif types, AG/CT, AC/GT, and AAC/GTT dominated. Our study provides the first comprehensive genomics dataset for E. chinense, which favors conservation programs for the restoration of the species and provides sufficient evidence for genetic variability among the wild populations.

Food availability and predation risk drive the distributional patterns of two pulmonate gastropods in a mangrove-saltmarsh transitional habitat

The pulmonate gastropods, Phallomedusa solida (Martens, 1878) and Ophicardelus ornatus (Férussac, 1821), exhibit characteristic distributional patterns at the upper intertidal zones in estuarine mangrove and saltmarsh habitats on the eastern Australian coast. Past studies suggested inundation condition, soil salinity, and percent of vegetation cover were responsible for these patterns. In this study, the role of environmental parameters, food availability, physical stress, and predation pressure in determining the distributional patterns of these gastropods was evaluated along transects spanning saltmarsh, mangrove, and the ecotone habitats. For both species, the maximum population abundance occurred in the upper saltmarsh and the ecotone between mangrove and saltmarsh at 361.0 and 358.0 ind.m^-2, respectively, which was four times that of the lower saltmarsh. Mangroves were evaluated as the optimal habitat for the pulmonates in terms of the environmental parameters moisture content and food availability. However, due to its longer inundation duration within each tidal cycle, use of the mangrove habitat by the pulmonates was impeded because of difficulties in oxygen acquisition under submerged conditions. Laboratory experiments revealed the oxygen intake of the pulmonates dropped abruptly to 4.3-9.0% of aerial rates when submerged. This result indicated that mangroves were not the optimal habitat for the pulmonates. Furthermore, the visiting frequency of predators (yellowfin bream Acanthopagrus australis and toadfishes, Tetraodontidae) was 1.3 times higher in the mangrove compared to those in the ecotone and upper saltmarsh habitats. Underwater video recording also suggested high mortality of these gastropods at 31.7-88.9% in mangrove and 0.80-0.98 times higher than that in saltmarsh, resulting from the predators preying in the mangrove habitat during high tides. Despite the abiotic factors facilitating the distribution of the pulmonates in the mangrove, the higher predation risk restricted the occurrence of P. solida and O. ornatus in the mangrove areas. More than verifying that the distributional pattern of macrobenthos is a complex outcome from environmental factors and interaction with predators, our study also indicated that the influencing strength of the biotic and abiotic factors on the pulmonates distribution might be spatially changeable within a geographically small-scale continuum.

Allocation of cytoplasm to macromeres in embryos of annelids and molluscs is positively correlated with egg size

Evolutionary transitions between feeding and nonfeeding larval development have occurred many times in marine invertebrates, but the developmental changes underlying these frequent and ecologically important transitions are poorly known, especially in spiralians. We use phylogenetic comparative methods to test the hypothesis that evolutionary changes in egg size and larval nutritional mode are associated with parallel changes in allocation of cytoplasm to macromere cell lineages in diverse annelids and molluscs. Our analyses show that embryos of species with large eggs and nonfeeding larvae tend to allocate relatively more embryonic cytoplasm to macromeres at 3rd cleavage than do embryos of species with small eggs and feeding larvae. The association between egg size and allocation to macromeres in these spiralians may be driven by constraints associated with mitotic spindle positioning and size, or may be a result of "adaptation in cleavage" to maintain rapid cell cycles in micromeres, position yolk in cell lineages where it can be most efficiently used, or adjust allocation to ectoderm to accommodate changes in embryonic surface area/volume ratio.

The complete mitogenome of Orcula dolium (Draparnaud, 1801); ultra-deep sequencing from a single long-range PCR using the Ion-Torrent PGM

BACKGROUND

With the increasing capacity of present-day next-generation sequencers the field of mitogenomics is rapidly changing. Enrichment of the mitochondrial fraction, is no longer necessary for obtaining mitogenomic data. Despite the benefits, shotgun sequencing approaches also have disadvantages. They do not guarantee obtaining the complete mitogenome, generally require larger amounts of input DNA and coverage is low compared to sequencing with enrichment strategies. If the mitogenome could be amplified in a single amplification, additional time and costs for sample preparation might outweigh these disadvantages.

RESULTS

A sequence of the complete mitochondrial genome of the pupilloid landsnail Orcula dolium is presented. The mitogenome was amplified in a single long-range (LR) PCR and sequenced on an Ion Torrent PGM (Life Technologies). The length is 14,063 nt and the average depth of coverage is 1112 X. This is the first published mitogenome for a member of the family Orculidae. It has the typical metazoan makeup of 13 protein coding genes (PCGs), 2 ribosomal RNAs (12S and 16S) and 22 transfer RNAs (tRNAs). Orcula is positioned between Pupilla and the Vertiginidae as the sister-group of Gastrocopta and Vertigo, together. An ancestral gene order reconstruction shows that Orthurethra in contrast to other Stylommatophora, have tRNA-H before tRNA-G and that the gene order in the 'non-achatinoid' clade is identical to that of closely related non-stylommatophoran taxa.

CONCLUSIONS

We show it is feasible to ultra-deep sequence a mitogenome from a single LR-PCR. This approach is particularly relevant to studies that have low concentrations of input DNA. It results in a more efficient use of NGS capacity (only the targeted fraction is sequenced) and is an effective selection against nuclear mitochondrial inserts (NUMTS). In contrast to previous studies based in particular on 28S, our results indicate that phylogeny reconstructions based on complete mitogenomes might be more suitable to resolve deep relationships within Stylommatophora. Ancestral gene order reconstructions reveal rearrangements that characterize systematic groups.

Phylogenetic analysis of the Lancinae (Gastropoda, Lymnaeidae) with a description of the U.S. federally endangered Banbury Springs lanx

We examined the patelliform snails of the subfamily Lancinae, endemic to northwestern North America, to test whether morphological variation correlated with genetic and anatomical differences. Molecular analyses using cox1, 16S, calmodulin intron, and 28S rDNA partial sequences and anatomical data supported recognition of four species in three genera. The relationships of lancines within Lymnaeidae are not yet well-resolved. The federally endangered Banbury Springs lanx is described as a new genus and species, Idaholanxfresti, confirming its distinctiveness and narrow endemicity.

The complete mitochondrial genomes of two freshwater snails provide new protein-coding gene rearrangement models and phylogenetic implications

BACKGROUND

Mitochondrial (mt) genome sequences are widely used for species identification and to study the phylogenetic relationships among Gastropoda. However, to date, limited data are available as taxon sampling is narrow. In this study we sequenced the complete mt genomes of the freshwater gastropods Radix swinhoei (Lymnaeidae) and Planorbarius corneus (Planorbidae). Based on these sequences, we investigated the gene rearrangement in these two species and the relationships with respect to the ancestral gene order and assessed their phylogenetic relationships.

METHODS

The complete mt genomes of R. swinhoei and P. corneus were sequenced using Illumina-based paired-end sequencing and annotated by comparing the sequence information with that of related gastropod species. Putative models of mitochondrial gene rearrangements were predicted for both R. swinhoei and P. corneus, using Reishia clavigera mtDNA structure as the ancestral gene order. The phylogenetic relationships were inferred using thirteen protein sequences based on Maximum likelihood and Bayesian inference analyses.

RESULTS

The complete circular mt genome sequences of R. swinhoei and P. corneus were 14,241 bp and 13,687 bp in length, respectively. Comparison of the gene order demonstrated complex rearrangement events in Gastropoda, both for tRNA genes and protein-coding genes. The phylogenetic analyses showed that the family Lymnaeidae was more closely related to the family Planorbidae, consistent with previous classification. Nevertheless, due to the position recovered for R. swinhoei, the family Lymnaeidae was not monophyletic.

CONCLUSION

This study provides the complete mt genomes of two freshwater snails, which will aid the development of useful molecular markers for epidemiological, ecological and phylogenetic studies. Additionally, the predicted models for mt gene rearrangement might provide novel insights into mt genome evolution in gastropods.

Origin and diversification of Lake Ohrid's endemic acroloxid limpets: the role of geography and ecology

BACKGROUND

Ancient Lake Ohrid, located on the Albania-Macedonia border, is the most biodiverse freshwater lake in Europe. However, the processes that gave rise to its extraordinary endemic biodiversity, particularly in the species-rich gastropods, are still poorly understood. A suitable model taxon to study speciation processes in Lake Ohrid is the pulmonate snail genus Acroloxus, which comprises two morphologically distinct and ecologically (vertically) separated endemic species. Using a multilocus phylogenetic framework of Acroloxus limpets from the Euro-Mediterranean subregion, together with molecular-clock and phylogeographic analyses of Ohrid taxa, we aimed to infer their geographic origin and the timing of colonization as well as the role of geography and ecology in intra-lacustrine diversification.

RESULTS

In contrast to most other endemic invertebrate groups in Lake Ohrid, the phylogenetic relationships of the endemic Ohrid Acroloxus species indicate that the Balkan region probably did not serve as their ancestral area. The inferred monophyly and estimated divergence times further suggest that these freshwater limpets colonized the lake only once and that the onset of intra-lacustrine diversification coincides with the time when the lake reached deep-water conditions ca 1.3 Mya. However, the difference in vertical distribution of these two ecologically distinct species is not reflected in the phylogeographic pattern observed. Instead, western and eastern populations are genetically more distinct, suggesting a horizontal structure.

CONCLUSIONS

We conclude that both geography and ecology have played a role in the intra-lacustrine speciation process. Given the distinct morphology (sculptured vs. smooth shell) and ecology (littoral vs. sublittoral), and the timing of intra-lacustrine diversification inferred, we propose that the onset of deep-water conditions initially triggered ecological speciation. Subsequent geographic processes then gave rise to the phylogeographic patterns observed today. However, the generally weak genetic differentiation observed suggests incipient speciation, which might be explained by the comparatively young age of the lake system and thus the relatively recent onset of intra-lacustrine diversification.