Variation and evolution of toxin gene expression patterns of six closely related venomous marine snails.

Venoms of predatory marine gastropods of the genus Conus show amazing levels of interspecific diversity and are comprised of a cocktail of peptide neurotoxins, termed conotoxins, that are encoded by large gene families. Conotoxin gene family evolution is characterized by gene duplications and high rates of nonsynonymous substitution among paralogues; yet, what controls the differentiation of venoms among species is not clear. We compared four-loop conotoxin transcripts of six closely related Conus species to examine conotoxin expression patterns among species. The species examined appear to express different numbers of four-loop conotoxin loci and similarity in expression patterns does not seem to correspond with phylogenetic affinity. Moreover, several loci appear to have been independently silenced while others appear to have been revived from previously silenced states. Some loci also appear to exhibit coordinated expression patterns. These results suggest that the evolution of conotoxin expression patterns is incredibly dynamic and the differentiation of venoms of Conus is controlled in part by the evolution of unique conotoxin expression patterns.

Evolution of ecological specialization and venom of a predatory marine gastropod.

Understanding the evolution of ecological specialization is important for making inferences about the origins of biodiversity. Members of the predatory, marine gastropod genus Conus exhibit a variety of diets and the ability to capture prey is linked to a venom comprised of peptide neurotoxins, termed conotoxins. We identified conotoxin transcripts from Conus leopardus, a species of Conus that uniquely preys exclusively on hemichordates, and compared its venom duct transcriptome to that of four other Conus species to determine whether a shift to a specialized diet is associated with changes in the venom composition of this species. We also examined the secondary structure of predicted amino acid sequences of conotoxin transcripts of C. leopardus to identify substitutions that may be linked to specialization on hemichordates. We identified seven distinct conotoxin sequences from C. leopardus that appear to represent transcripts of seven distinct loci. Expression levels and the diversity of conotoxins expressed by C. leopardus are considerably less than those of other Conus. Moreover, gene products of two transcripts exhibited unique secondary structures that have not been previously observed from other Conus. These results suggest that transition to a specialist diet is associated with reduction in the number of components expressed in venoms of Conus and that diverse venoms of Conus are maintained in species with a broad dietary width.

Molecular phylogenetic relationships in the aquatic snail genus Lymnaea, the intermediate host of the causative agent of fascioliasis: insights from broader taxon sampling.

Phylogenetic analyses were performed on partial mitochondrial (16S) gene sequences that included new 16S data for 23 exemplars of the freshwater pulmonate snail genus Lymnaea sensu lato and putative outgroup species. This procedure yielded relatively congruent patterns of evolutionary divergence and phylogenetic affinities, and greater resolution and support for many lineages at different levels of divergence than from a previous work based on fewer samples. It has also clarified the relationships between key taxa. Molecular differentiation was evident among genera. Lymnaeids with n=16 chromosomes are a distinct, well-supported monophyletic group of recent origin. The genus Radix appears to be paraphyletic. Among the indigenous North American lymnaeids, a closer alliance was found between Fossaria spp. and Stagnicola caperata, a member of the subgenus Hinkleyia, than between the latter species and members of the subgenus Stagnicola s.str. The North American population of Lymnaea stagnalis is likely to be of European origin as it clustered with its European counterparts. The relevance of the molecular findings to efforts aimed at controlling snail-transmitted trematode diseases is discussed.

Affinities among anostracan (Crustacea: Branchiopoda) families inferred from phylogenetic analyses of multiple gene sequences.

To gain insights into the relationships among anostracan families, molecular phylogenetic analyses were performed on nuclear (28S D1-D3 ribosomal DNA) and mitochondrial (16S rDNA, COI) gene regions for representatives of seven families and an outgroup. Data matrices used in the analyses included 951 base pairs (bp) of aligned sequences for 28S, 465 bp for 16S, and 658 bp (219 amino acids) for COI. Maximum-parsimony and maximum-likelihood methods were used to construct phylogenetic trees, enabling the evaluation of both previous hypotheses of taxonomic relationships among families based on morphology, and of the relative merits of independent versus simultaneous analyses of multiple data sets for phylogeny construction. Data from various combinations of the gene regions produced relatively congruent patterns of phylogenetic affinity. In most analyses, two monophyletic groups were resolved: one cluster included the families Polyartemiidae, Chirocephalidae, Branchinectidae, Streptocephalidae, and Thamnocephalidae, while the other contained the Artemiidae and Branchipodidae. Comparative analyses showed that combining gene regions in a single matrix generally resulted in increased resolution and support for each cluster relative to those obtained from single-gene analyses. Statistical tests demonstrated that morphology-based hypotheses of relationships among families had poorer support than those determined from molecular data, reflecting the homoplasy in characters used to differentiate families.