Differentiation of venoms of predatory marine gastropods: divergence of orthologous toxin genes of closely related Conus species with different dietary specializations

Prey specificity of predatory venoms

Venom represents a key adaptation of many venomous predators, allowing them to immobilise prey quickly through chemical rather than physical warfare. Evolutionary arms races between prey and a predator are believed to be the main factor influencing the potency and composition of predatory venoms. Predators with narrowly restricted diets are expected to evolve specifically potent venom towards their focal prey, with lower efficacy on alternative prey. Here, we evaluate hypotheses on the evolution of prey-specific venom, focusing on the effect of restricted diet, prey defences, and prey resistance. Prey specificity as a potential evolutionary dead end is also discussed. We then provide an overview of the current knowledge on venom prey specificity, with emphasis on snakes, cone snails, and spiders. As the current evidence for venom prey specificity is still quite limited, we also overview the best approaches and methods for its investigation and provide a brief summary of potential model groups. Finally, possible applications of prey-specific toxins are discussed.

Comparative transcriptomics of the venoms of continental and insular radiations of West African cones

The transcriptomes of the venom glands of 13 closely related species of vermivorous cones endemic to West Africa from genera Africonus and Varioconus were sequenced and venom repertoires compared within a phylogenetic framework using one Kalloconus species as outgroup. The total number of conotoxin precursors per species varied between 108 and 221. Individuals of the same species shared about one-fourth of the total conotoxin precursors. The number of common sequences was drastically reduced in the pairwise comparisons between closely related species, and the phylogenetical signal was totally eroded at the inter-generic level (no sequence was identified as shared derived), due to the intrinsic high variability of these secreted peptides. A common set of four conotoxin precursor superfamilies (T, O1, O2 and M) was expanded in all studied cone species, and thus, they are considered the basic venom toolkit for hunting and defense in the West African vermivorous cone snails. Maximum-likelihood ancestral character reconstructions inferred shared conotoxin precursors preferentially at internal nodes close to the tips of the phylogeny (between individuals and between closely related species) as well as in the common ancestor of Varioconus. Besides the common toolkit, the two genera showed significantly distinct catalogues of conotoxin precursors in terms of type of superfamilies present and the abundance of members per superfamily, but had similar relative expression levels indicating functional convergence. Differential expression comparisons between vermivorous and piscivorous cones highlighted the importance of the A and S superfamilies for fish hunting and defense.

Effects of Predator-Prey Interactions on Predator Traits: Differentiation of Diets and Venoms of a Marine Snail

Species interactions are fundamental ecological forces that can have significant impacts on the evolutionary trajectories of species. Nonetheless, the contribution of predator-prey interactions to genetic and phenotypic divergence remains largely unknown. Predatory marine snails of the family Conidae exhibit specializations for different prey items and intraspecific variation in prey utilization patterns at geographic scales. Because cone snails utilize venom to capture prey and venom peptides are direct gene products, it is feasible to examine the evolution of genes associated with changes in resource utilization. Here, we compared feeding ecologies and venom duct transcriptomes of individuals from three populations of Conus miliaris, a species that exhibits geographic variation in prey utilization and dietary breadth, in order to determine the extent to which dietary differences are correlated with differences in venom composition, and if expanded niche breadth is associated with increased variation in venom composition. While populations showed little to no overlap in resource utilization, taxonomic richness of prey was greatest at Easter Island. Changes in dietary breadth were associated with differences in expression patterns and increased genetic differentiation of toxin-related genes. The Easter Island population also exhibited greater diversity of toxin-related transcripts, but did not show increased variance in expression of these transcripts. These results imply that differences in dietary breadth contribute more to the structural and regulatory differentiation of venoms than differences in diet.

Targeted Sequencing of Venom Genes from Cone Snail Genomes Improves Understanding of Conotoxin Molecular Evolution

To expand our capacity to discover venom sequences from the genomes of venomous organisms, we applied targeted sequencing techniques to selectively recover venom gene superfamilies and nontoxin loci from the genomes of 32 cone snail species (family, Conidae), a diverse group of marine gastropods that capture their prey using a cocktail of neurotoxic peptides (conotoxins). We were able to successfully recover conotoxin gene superfamilies across all species with high confidence (> 100× coverage) and used these data to provide new insights into conotoxin evolution. First, we found that conotoxin gene superfamilies are composed of one to six exons and are typically short in length (mean = ∼85 bp). Second, we expanded our understanding of the following genetic features of conotoxin evolution: 1) positive selection, where exons coding the mature toxin region were often three times more divergent than their adjacent noncoding regions, 2) expression regulation, with comparisons to transcriptome data showing that cone snails only express a fraction of the genes available in their genome (24-63%), and 3) extensive gene turnover, where Conidae species varied from 120 to 859 conotoxin gene copies. Finally, using comparative phylogenetic methods, we found that while diet specificity did not predict patterns of conotoxin evolution, dietary breadth was positively correlated with total conotoxin gene diversity. Overall, the targeted sequencing technique demonstrated here has the potential to radically increase the pace at which venom gene families are sequenced and studied, reshaping our ability to understand the impact of genetic changes on ecologically relevant phenotypes and subsequent diversification.

Venomics-Accelerated Cone Snail Venom Peptide Discovery

Cone snail venoms are considered a treasure trove of bioactive peptides. Despite over 800 species of cone snails being known, each producing over 1000 venom peptides, only about 150 unique venom peptides are structurally and functionally characterized. To overcome the limitations of the traditional low-throughput bio-discovery approaches, multi-omics systems approaches have been introduced to accelerate venom peptide discovery and characterisation. This “venomic” approach is starting to unravel the full complexity of cone snail venoms and to provide new insights into their biology and evolution. The main challenge for venomics is the effective integration of transcriptomics, proteomics, and pharmacological data and the efficient analysis of big datasets. Novel database search tools and visualisation techniques are now being introduced that facilitate data exploration, with ongoing advances in related omics fields being expected to further enhance venomics studies. Despite these challenges and future opportunities, cone snail venomics has already exponentially expanded the number of novel venom peptide sequences identified from the species investigated, although most novel conotoxins remain to be pharmacologically characterised. Therefore, efficient high-throughput peptide production systems and/or banks of miniaturized discovery assays are required to overcome this bottleneck and thus enhance cone snail venom bioprospecting and accelerate the identification of novel drug leads.

δ-Conotoxin SuVIA suggests an evolutionary link between ancestral predator defence and the origin of fish-hunting behaviour in carnivorous cone snails

Some venomous cone snails feed on small fishes using an immobilizing combination of synergistic venom peptides that target Kv and Nav channels. As part of this envenomation strategy, δ-conotoxins are potent ichtyotoxins that enhance Nav channel function. δ-Conotoxins belong to an ancient and widely distributed gene superfamily, but any evolutionary link from ancestral worm-eating cone snails to modern piscivorous species has not been elucidated. Here, we report the discovery of SuVIA, a potent vertebrate-active δ-conotoxin characterized from a vermivorous cone snail (Conus suturatus). SuVIA is equipotent at hNaV1.3, hNaV1.4 and hNaV1.6 with EC50s in the low nanomolar range. SuVIA also increased peak hNaV1.7 current by approximately 75% and shifted the voltage-dependence of activation to more hyperpolarized potentials from -15 mV to -25 mV, with little effect on the voltage-dependence of inactivation. Interestingly, the proximal venom gland expression and pain-inducing effect of SuVIA in mammals suggest that δ-conotoxins in vermivorous cone snails play a defensive role against higher order vertebrates. We propose that δ-conotoxins originally evolved in ancestral vermivorous cones to defend against larger predators including fishes have been repurposed to facilitate a shift to piscivorous behaviour, suggesting an unexpected underlying mechanism for this remarkable evolutionary transition.

Age-related association of venom gene expression and diet of predatory gastropods

Background

Venomous organisms serve as wonderful systems to study the evolution and expression of genes that are directly associated with prey capture. To evaluate the relationship between venom gene expression and prey utilization, we examined these features among individuals of different ages of the venomous, worm-eating marine snail Conus ebraeus. We determined expression levels of six genes that encode venom components, used a DNA-based approach to evaluate the identity of prey items, and compared patterns of venom gene expression and dietary specialization.

Results

C. ebraeus exhibits two major shifts in diet with age—an initial transition from a relatively broad dietary breadth to a narrower one and then a return to a broader diet. Venom gene expression patterns also change with growth. All six venom genes are up-regulated in small individuals, down-regulated in medium-sized individuals, and then either up-regulated or continued to be down-regulated in members of the largest size class. Venom gene expression is not significantly different among individuals consuming different types of prey, but instead is coupled and slightly delayed with shifts in prey diversity.

Conclusion

These results imply that changes in gene expression contribute to intraspecific variation of venom composition and that gene expression patterns respond to changes in the diversity of food resources during different growth stages.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-016-0592-5) contains supplementary material, which is available to authorized users.

Structural features of conopeptide genes inferred from partial sequences of the Conus tribblei genome

The evolvability of venom components (in particular, the gene-encoded peptide toxins) in venomous species serves as an adaptive strategy allowing them to target new prey types or respond to changes in the prey field. The structure, organization, and expression of the venom peptide genes may provide insights into the molecular mechanisms that drive the evolution of such genes. Conus is a particularly interesting group given the high chemical diversity of their venom peptides, and the rapid evolution of the conopeptide-encoding genes. Conus genomes, however, are large and characterized by a high proportion of repetitive sequences. As a result, the structure and organization of conopeptide genes have remained poorly known. In this study, a survey of the genome of Conus tribblei was undertaken to address this gap. A partial assembly of C. tribblei genome was generated; the assembly, though consisting of a large number of fragments, accounted for 2160.5 Mb of sequence. A large number of repetitive genomic elements consisting of 642.6 Mb of retrotransposable elements, simple repeats, and novel interspersed repeats were observed. We characterized the structural organization and distribution of conotoxin genes in the genome. A significant number of conopeptide genes (estimated to be between 148 and 193) belonging to different superfamilies with complete or nearly complete exon regions were observed, ~60 % of which were expressed. The unexpressed conopeptide genes represent hidden but significant conotoxin diversity. The conotoxin genes also differed in the frequency and length of the introns. The interruption of exons by long introns in the conopeptide genes and the presence of repeats in the introns may indicate the importance of introns in facilitating recombination, evolution and diversification of conotoxins. These findings advance our understanding of the structural framework that promotes the gene-level molecular evolution of venom peptides.

Comparison of the Venom Peptides and Their Expression in Closely Related Conus Species: Insights into Adaptive Post-speciation Evolution of Conus Exogenomes

Genes that encode products with exogenous targets, which comprise an organism's "exogenome," typically exhibit high rates of evolution. The genes encoding the venom peptides (conotoxins or conopeptides) in Conus sensu lato exemplify this class of genes. Their rapid diversification has been established and is believed to be linked to the high speciation rate in this genus. However, the molecular mechanisms that underlie venom peptide diversification and ultimately emergence of new species remain poorly understood. In this study, the sequences and expression levels of conotoxins from several specimens of two closely related worm-hunting species, Conus tribblei and Conus lenavati, were compared through transcriptome analysis. Majority of the identified putative conopeptides were novel, and their diversity, even in each specimen, was remarkably high suggesting a wide range of prey targets for these species. Comparison of the interspecific expression patterns of conopeptides at the superfamily level resulted in the discovery of both conserved as well as species-specific expression patterns, indicating divergence in the regulatory network affecting conotoxin gene expression. Comparison of the transcriptomes of the individual snails revealed that each specimen produces a distinct set of highly expressed conopeptides, reflecting the capability of individual snails to fine-tune the composition of their venoms. These observations reflect the role of sequence divergence and divergence in the control of expression for specific conopeptides in the evolution of the exogenome and hence venom composition in Conus.

Not as docile as it looks? Loxosceles venom variation and loxoscelism in the Mediterranean Basin and the Canary Islands

The medical importance of Loxosceles spiders has promoted extensive research on different aspects of their venoms. Most of the reported cases of loxoscelism have occurred in the Americas, and thus, much work has focused on North and South American Loxosceles species. Interestingly, loxoscelism cases are rare in the Mediterranean Basin although Loxosceles rufescens, endemic to the Mediterranean, is an abundant spider even in human-altered areas. Thus, it has been suggested that the venom of L. rufescens could be of less medical relevance than that of its congeners. In this study, we challenge this hypothesis by using multiple approaches to study venom variation in selected species and lineages from the Mediterranean Basin and the Canary Islands. We found that SMase D activity, the key bioactive component of Loxosceles venom, is comparable to American species that are confirmed to have medically relevant bites. The venom protein composition using SDS-PAGE presents some differences among regional Loxosceles taxa in banding pattern and intensity, mostly between the Canarian and L. rufescens lineages. Differences between these species also exist in the expression of different paralogs of the SicTox gene family, with the Canarian species being less diverse. In conclusion, our results do not support the challenged hypothesis, and suggest that venom of these species may indeed be as potent as other Loxosceles species. Pending confirmation of loxoscelism with direct evidence of Loxosceles bites with species identification by professionals, Loxosceles in the Mediterranean region should conservatively be considered medically relevant taxa.

When everything converges: integrative taxonomy with shell, DNA and venomic data reveals Conus conco, a new species of cone snails (Gastropoda: Conoidea)

Cone snails have long been studied both by taxonomists for the diversity of their shells and by biochemists for the potential therapeutic applications of their toxins. Phylogenetic approaches have revealed that different lineages of Conus evolved divergent venoms, a property that is exploited to enhance the discovery of new conotoxins, but is rarely used in taxonomy. Specimens belonging to the Indo-West Pacific Conus lividus species complex were analyzed using phenetic and phylogenetic methods based on shell morphology, COI and 28S rRNA gene sequences and venom mRNA expression and protein composition. All methods converged to reveal a new species, C. conco n. sp. (described in Supplementary data), restricted to the Marquesas Islands, where it diverged recently (∼3mya) from C. lividus. The geographical distribution of C. conco and C. lividus and their phylogenetic relationships suggest that the two species diverged in allopatry. Furthermore, the diversity of the transcript sequences and toxin molecular masses suggest that C. conco evolved unique toxins, presumably in response to new selective pressure, such as the availability of new preys and ecological niches. Furthermore, this new species evolved new transcripts giving rise to original toxin structures, probably each carrying specific biological activity.

Venom variation during prey capture by the cone snail, Conus textile

Observations of the mollusc-hunting cone snail Conus textile during feeding reveal that prey are often stung multiple times in succession. While studies on the venom peptides injected by fish-hunting cone snails have become common, these approaches have not been widely applied to the analysis of the injected venoms from mollusc-hunters. We have successfully obtained multiple injected venom samples from C. textile individuals, allowing us to investigate venom compositional variation during prey capture. Our studies indicate that C. textile individuals alter the composition of prey-injected venom peptides during single feeding events. The qualitative results obtained by MALDI-ToF mass spectrometry are mirrored by quantitative changes in venom composition observed by reverse-phase high performance liquid chromatography. While it is unclear why mollusc-hunting cone snails inject prey multiple times prior to engulfment, our study establishes for the first time a link between this behavior and compositional changes of the venom during prey capture. Changes in venom composition during hunting may represent a multi-step strategy utilized by these venomous animals to slow and incapacitate prey prior to engulfment.

Evolution of separate predation- and defence-evoked venoms in carnivorous cone snails

Venomous animals are thought to inject the same combination of toxins for both predation and defence, presumably exploiting conserved target pharmacology across prey and predators. Remarkably, cone snails can rapidly switch between distinct venoms in response to predatory or defensive stimuli. Here, we show that the defence-evoked venom of Conus geographus contains high levels of paralytic toxins that potently block neuromuscular receptors, consistent with its lethal effects on humans. In contrast, C. geographus predation-evoked venom contains prey-specific toxins mostly inactive at human targets. Predation- and defence-evoked venoms originate from the distal and proximal regions of the venom duct, respectively, explaining how different stimuli can generate two distinct venoms. A specialized defensive envenomation strategy is widely evolved across worm, mollusk and fish-hunting cone snails. We propose that defensive toxins, originally evolved in ancestral worm-hunting cone snails to protect against cephalopod and fish predation, have been repurposed in predatory venoms to facilitate diversification to fish and mollusk diets.

Molecular phylogeny, classification and evolution of conopeptides

Conopeptides are toxins expressed in the venom duct of cone snails (Conoidea, Conus). These are mostly well-structured peptides and mini-proteins with high potency and selectivity for a broad range of cellular targets. In view of these properties, they are widely used as pharmacological tools and many are candidates for innovative drugs. The conopeptides are primarily classified into superfamilies according to their peptide signal sequence, a classification that is thought to reflect the evolution of the multigenic system. However, this hypothesis has never been thoroughly tested. Here we present a phylogenetic analysis of 1,364 conopeptide signal sequences extracted from GenBank. The results validate the current conopeptide superfamily classification, but also reveal several important new features. The so-called "cysteine-poor" conopeptides are revealed to be closely related to "cysteine-rich" conopeptides; with some of them sharing very similar signal sequences, suggesting that a distinction based on cysteine content and configuration is not phylogenetically relevant and does not reflect the evolutionary history of conopeptides. A given cysteine pattern or pharmacological activity can be found across different superfamilies. Furthermore, a few conopeptides from GenBank do not cluster in any of the known superfamilies, and could represent yet-undefined superfamilies. A clear phylogenetically based classification should help to disentangle the diversity of conopeptides, and could also serve as a rationale to understand the evolution of the toxins in the numerous other species of conoideans and venomous animals at large.

Macroevolution of venom apparatus innovations in auger snails (Gastropoda; Conoidea; Terebridae)

The Terebridae are a diverse family of tropical and subtropical marine gastropods that use a complex and modular venom apparatus to produce toxins that capture polychaete and enteropneust preys. The complexity of the terebrid venom apparatus suggests that venom apparatus development in the Terebridae could be linked to the diversification of the group and can be analyzed within a molecular phylogenetic scaffold to better understand terebrid evolution. Presented here is a molecular phylogeny of 89 terebrid species belonging to 12 of the 15 currently accepted genera, based on Bayesian inference and Maximum Likelihood analyses of amplicons of 3 mitochondrial (COI, 16S and 12S) and one nuclear (28S) genes. The evolution of the anatomy of the terebrid venom apparatus was assessed by mapping traits of six related characters: proboscis, venom gland, odontophore, accessory proboscis structure, radula, and salivary glands. A novel result concerning terebrid phylogeny was the discovery of a previously unrecognized lineage, which includes species of Euterebra and Duplicaria. The non-monophyly of most terebrid genera analyzed indicates that the current genus-level classification of the group is plagued with homoplasy and requires further taxonomic investigations. Foregut anatomy in the family Terebridae reveals an inordinate diversity of features that covers the range of variability within the entire superfamily Conoidea, and that hypodermic radulae have likely evolved independently on at least three occasions. These findings illustrate that terebrid venom apparatus evolution is not perfunctory, and involves independent and numerous changes of central features in the foregut anatomy. The multiple emergence of hypodermic marginal radular teeth in terebrids are presumably associated with variable functionalities, suggesting that terebrids have adapted to dietary changes that may have resulted from predator-prey relationships. The anatomical and phylogenetic results presented serve as a starting point to advance investigations about the role of predator-prey interactions in the diversification of the Terebridae and the impact on their peptide toxins, which are promising bioactive compounds for biomedical research and therapeutic drug development.

Large-scale species delimitation method for hyperdiverse groups

Accelerating the description of biodiversity is a major challenge as extinction rates increase. Integrative taxonomy combining molecular, morphological, ecological and geographical data is seen as the best route to reliably identify species. Classic molluscan taxonomic methodology proposes primary species hypotheses (PSHs) based on shell morphology. However, in hyperdiverse groups, such as the molluscan family Turridae, where most of the species remain unknown and for which homoplasy and plasticity of morphological characters is common, shell-based PSHs can be arduous. A four-pronged approach was employed to generate robust species hypotheses of a 1000 specimen South-West Pacific Turridae data set in which: (i) analysis of COI DNA Barcode gene is coupled with (ii) species delimitation tools GMYC (General Mixed Yule Coalescence Method) and ABGD (Automatic Barcode Gap Discovery) to propose PSHs that are then (iii) visualized using Klee diagrams and (iv) evaluated with additional evidence, such as nuclear gene rRNA 28S, morphological characters, geographical and bathymetrical distribution to determine conclusive secondary species hypotheses (SSHs). The integrative taxonomy approach applied identified 87 Turridae species, more than doubling the amount previously known in the Gemmula genus. In contrast to a predominantly shell-based morphological approach, which over the last 30 years proposed only 13 new species names for the Turridae genus Gemmula, the integrative approach described here identified 27 novel species hypotheses not linked to available species names in the literature. The formalized strategy applied here outlines an effective and reproducible protocol for large-scale species delimitation of hyperdiverse groups.

A diverse family of novel peptide toxins from an unusual cone snail, Conus californicus

Diversity among Conus toxins mirrors the high species diversity in the Indo-Pacific region, and evolution of both is thought to stem from feeding-niche specialization derived from intra-generic competition. This study focuses on Conus californicus, a phylogenetic outlier endemic to the temperate northeast Pacific. Essentially free of congeneric competitors, it preys on a wider variety of organisms than any other cone snail. Using molecular cloning of cDNAs and mass spectrometry, we examined peptides isolated from venom ducts to elucidate the sequences and post-translational modifications of two eight-cysteine toxins (cal12a and cal12b of type 12 framework) that block voltage-gated Na(+) channels. Based on homology of leader sequence and mode of action, these toxins are related to the O-superfamily, but differ significantly from other members of that group. Six of the eight cysteine residues constitute the canonical framework of O-members, but two additional cysteine residues in the N-terminal region define an O+2 classification within the O-superfamily. Fifteen putative variants of Cal12.1 toxins have been identified by mRNAs that differ primarily in two short hypervariable regions and have been grouped into three subtypes (Cal12.1.1-3). This unique modular variation has not been described for other Conus toxins and suggests recombination as a diversity-generating mechanism. We propose that these toxin isoforms show specificity for similar molecular targets (Na(+) channels) in the many species preyed on by C. californicus and that individualistic utilization of specific toxin isoforms may involve control of gene expression.

Venom kinematics during prey capture in Conus: the biomechanics of a rapid injection system

Cone snails use an extensile, tubular proboscis as a conduit to deliver a potent cocktail of bioactive venom peptides into their prey. Previous studies have focused mainly on understanding the venom's role in prey capture but successful prey capture requires both rapid physiological and biomechanical mechanisms. Conus catus, a fish-hunting species, uses a high-speed hydraulic mechanism to inject its hollow, spear-like radular tooth into prey. We take an integrated approach to investigating the biomechanics of this process by coupling kinematic studies with morphological analyses. Taking advantage of the opaque venom and translucent proboscis of a mollusc-hunting juvenile cone snail, Conus pennaceus, we have determined that a high-speed prey capture mechanism is not unique to cone species that hunt fish prey. Two morphological structures were found to play crucial roles in this process. A constriction of the lumen near the tip of the proboscis, composed of tall epithelial cells densely packed with microfilaments, impedes forward movement of the radular tooth prior to its propulsion. Proximal to the constriction, a muscular sphincter was found to regulate venom flow and pressurization in the proboscis. In C. pennaceus, the rapid appearance and flushing of venom within the proboscis during prey capture suggests a mechanism involving the delivery of a discrete quantity of venom. The interplay between these elements provides a unique and effective biomechanical injection system for the fast-acting cone snail venom peptides.

Evolution of Conus peptide toxins: analysis of Conus californicus Reeve, 1844

Conus species are characterized by their hyperdiverse toxins, encoded by a few gene superfamilies. Our phylogenies of the genus, based on mitochondrial genes, confirm previous results that C. californicus is highly divergent from all other species. Genetic and biochemical analysis of their venom peptides comprise the fifteen most abundant conopeptides and over 50 mature cDNA transcripts from the venom duct. Although C. californicus venom retains many of the general properties of other Conus species, they share only half of the toxin gene superfamilies found in other Conus species. Thus, in these two lineages, approximately half of the rapidly diversifying gene superfamilies originated after an early Tertiary split. Such results demonstrate that, unlike endogenously acting gene families, these genes are likely to be significantly more restricted in their phylogenetic distribution. In concordance with the evolutionary distance of C. californicus from other species, there are aspects of prey-capture behavior and prey preferences of this species that diverges significantly from all other Conus.

Dramatic intraspecimen variations within the injected venom of Conus consors: an unsuspected contribution to venom diversity

With the advent of highly sensitive mass spectrometry techniques, the minute amount of various secretions produced by living animals can be studied to a level of details never attained before. In this study, we used LC-ESI-MS to analyse the injected venom of an indo-pacific piscivorous cone snail, Conus consors. While long-term follow up of several captive specimens have revealed a typical "venom fingerprint" for this species, dramatic variations were also observed. In the most extreme case, a single cone snail unexpectedly produced two very distinct venom profiles containing completely different sets of peptides with no overlap of detected masses. Surprisingly, there was no correlation between the peptides produced in the venom duct and those obtained after milking live cone snails, implying yet unknown mechanisms of selection and regulation. Our study defines the notion of intraspecimen variation and demonstrates how this phenomenon contributes to the overall venom diversity.

Evolution of Conus peptide genes: duplication and positive selection in the A-superfamily

A remarkable diversity of venom peptides is expressed in the genus Conus (known as "conotoxins" or "conopeptides"). Between 50 and 200 different venom peptides can be found in a single Conus species, each having its own complement of peptides. Conopeptides are encoded by a few gene superfamilies; here we analyze the evolution of the A-superfamily in a fish-hunting species clade, Pionoconus. More than 90 conopeptide sequences from 11 different Conus species were used to build a phylogenetic tree. Comparison with a species tree based on standard genes reveals multiple gene duplication events, some of which took place before the Pionoconus radiation. By analysing several A-conopeptides from other Conus species recorded in GenBank, we date the major duplication events after the divergence between fish-hunting and non-fish-hunting species. Furthermore, likelihood approaches revealed strong positive selection; the magnitude depends on which A-conopeptide lineage and amino-acid locus is analyzed. The four major A-conopeptide clades defined are consistent with the current division of the superfamily into families and subfamilies based on the Cys pattern. The function of three of these clades (the κA-family, the α4/7-subfamily, and α3/5-subfamily) has previously been characterized. The function of the remaining clade, corresponding to the α4/4-subfamily, has not been elucidated. This subfamily is also found in several other fish-hunting species clades within Conus. The analysis revealed a surprisingly diverse origin of α4/4 conopeptides from a single species, Conus bullatus. This phylogenetic approach that defines different genetic lineages of Conus venom peptides provides a guidepost for identifying conopeptides with potentially novel functions.

Geographic variation in venom allelic composition and diets of the widespread predatory marine gastropod Conus ebraeus

Background

Members of the predatory gastropod genus Conus use a venom comprised of a cocktail of peptide neurotoxins, termed conotoxins or conopeptides, to paralyze prey and conotoxin gene family members diversify via strong positive selection. Because Conus venoms are used primarily to subdue prey, the evolution of venoms is likely affected by predator-prey interactions.

Methodology/Principal Findings

To identify the selective forces that drive the differentiation of venoms within species of Conus, we examined the distribution of alleles of a polymorphic O-superfamily conotoxin locus of Conus ebraeus at Okinawa, Guam and Hawaii. Previous analyses of mitochondrial cytochrome oxidase I gene sequences suggest that populations of C. ebraeus, a worm-eating Conus, are not structured genetically in the western and central Pacific. Nonetheless, because the sample size from Guam was relatively low, we obtained additional data from this location and reexamined patterns of genetic variation at the mitochondrial gene at Okinawa, Guam and Hawaii. We also utilized a DNA-based approach to identify prey items of individuals of C. ebraeus from Guam and compared this information to published data on diets at Okinawa and Hawaii. Our results show that conotoxin allelic frequencies differ significantly among all three locations, with strongest differentiation at Hawaii. We also confirm previous inferences that C. ebraeus exhibits no genetic differentiation between Okinawa, Guam and Hawaii at the mitochondrial locus. Finally, DNA-based analyses show that eunicid polychaetes comprise the majority of the prey items of C. ebraeus at Guam; while this results compares well with observed diet of this species at Okinawa, C. ebraeus preys predominantly on nereid polychaetes at Hawaii.

Conclusions/Significance

These results imply that strong selection pressures affect conotoxin allelic frequencies. Based on the dietary information, the selection may derive from geographic variation in dietary specialization and local coevolutionary arms races between Conus and their prey.