BackgroundA number of shelled and shell-less gastropods are known to use multiple defensive mechanisms, including internally generated or externally obtained biochemically active compounds and structures. Within Nudipleura, nudibranchs within Cladobranchia possess such a special defense: the ability to sequester cnidarian nematocysts – small capsules that can inject venom into the tissues of other organisms. This ability is distributed across roughly 600 species within Cladobranchia, and many questions still remain in regard to the comparative morphology and evolution of the cnidosac – the structure that houses sequestered nematocysts (called kleptocnides). In this paper, we describe cnidosac morphology across the main groups of Cladobranchia in which it occurs, and place variation in its structure in a phylogenetic context to better understand the evolution of nematocyst sequestration.ResultsOverall, we find that the length, size and structure of the entrance to the cnidosac varies more than expected based on previous work, as does the structure of the exit, the musculature surrounding the cnidosac, and the position and orientation of the kleptocnides. The sequestration of nematocysts has originated at least twice within Cladobranchia based on the phylogeny presented here using 94 taxa and 409 genes.ConclusionsThe cnidosac is not homologous to cnidosac-like structures found in Hancockiidae. Additionally, the presence of a sac at the distal end of the digestive gland may have originated prior to the sequestration of nematocysts. This study provides a more complete picture of variation in, and evolution of, morphological characters associated with nematocyst sequestration in Cladobranchia.Electronic supplementary materialThe online version of this article (10.1186/s12983-018-0289-2) contains supplementary material, which is available to authorized users.
The application of mixed nucleotide ⁄ doublet substitution models has recently received attention in RNA-based phylogenetics. Within a Bayesian approach, it was shown that mixed models outperformed analyses relying on simple nucleotide models. We analysed an mt RNA data set of dragonflies representing all major lineages of Anisoptera plus outgroups, using a mixed model in a Bayesian and parsimony (MP) approach. We used a published mt 16S rRNA secondary consensus structure model and inferred consensus models for the mt 12S rRNA and tRNA valine. Secondary structure information was used to set data partitions for paired and unpaired sites on which doublet or nucleotide models were applied, respectively. Several different doublet models are currently available of which we chose the most appropriate one by a Bayes factor test. The MP reconstructions relied on recoded data for paired sites in order to account for character covariance and an application of the ratchet strategy to find most parsimonious trees. Bayesian and parsimony reconstructions are partly differently resolved, indicating sensitivity of the reconstructions to model specification. Our analyses depict a tree in which the damselfly family Lestidae is sister group to a monophyletic clade Epiophlebia + Anisoptera, contradicting recent morphological and molecular work. In Bayesian analyses, we found a deep split between Libelluloidea and a clade ÔAeshnoideaÕ within Anisoptera largely congruent with TillyardÕs early ideas of anisopteran evolution, which had been based on evidently plesiomorphic character states. However, parsimony analysis did not support a clade ÔAeshnoideaÕ, but instead, placed Gomphidae as sister taxon to Libelluloidea. Monophyly of Libelluloidea is only modestly supported, and many inter-family relationships within Libelluloidea do not receive substantial support in Bayesian and parsimony analyses. We checked whether high Bayesian node support was inflated owing to either: (i) wrong secondary consensus structures; (ii) under-sampling of the MCMC process, thereby missing other local maxima; or (iii) unrealistic prior assumptions on topologies or branch lengths. We found that different consensus structure models exert strong influence on the reconstruction, which demonstrates the importance of taxon-specific realistic secondary structure models in RNA phylogenetics.
The genus Phyllodesmium (Aeolidoidea, Gastropoda) comprises shell-less marine snails, whose defense strategies are not well investigated yet. Here we report results of the first chemical investigation of P. briareum, as well as a re-investigation of P. longicirrum and P. magnum. Briarane diterpenes were isolated from P. briareum, and their origin could be traced to its prey organism Briareum sp. (Octocorallia). Considerable enrichment of the soft coral secondary metabolites in the slug was shown. Re-investigation of P. magnum led to isolation of cembrane diterpenes, 2-phenylethylamide, and furano sesquiterpenes. Sequestration of chemicals seems to have influenced speciation and evolution of Phyllodesmium species. Structural similarity or dissimilarity of particular slug metabolites suggests a closer, or more distant relationship of the respective Phyllodesmium taxa.
Symbiosis with photoautotrophic organisms has evolved in various species and even whole animal lineages, which allowed them to directly benefit from photosynthesis. This so-called photosymbiosis is best studied in cnidarians, which primarily establish symbioses with dinoflagellates from the family Symbiodiniaceae. In most other animals the mechanisms of establishing photosymbiosis, the physiological basis, and the evolution of a photosymbiotic life history remain poorly understood. Sea slugs belonging to the Cladobranchia (Gastropoda, Nudibranchia) are no exception, and are a rather neglected animal lineage in the research field of photosymbiosis. Yet, studying these sea slugs holds great potential to establish a unique photosymbiosis model, as they are the only known taxon that has evolved two different strategies to acquire their symbiont: either from cnidarian prey (thus becoming a secondary host) or directly out of the water column. The mechanisms for photobiont uptake and maintenance are unknown for these sea slugs, but might be similar to those of cnidarians. However, in terms of the evolution of photosymbiosis, Cladobranchia seem to share many commonalities with more closely related sea slugs belonging to the Sacoglossa, which only maintain the chloroplasts of the algae they feed on. Hence, Cladobranchia have the potential to shed light on the evolution of photosymbiosis in taxonomically divergent animals that also harbor photobionts of different evolutionary lineages.
Berghia stephanieae (Nudibranchia, Cladobranchia) is a photosymbiotic sea slug that feeds exclusively on sea anemones from the genus Exaiptasia. It then specifically incorporates dinoflagellates belonging to the Symbiodiniaceae obtained from their prey. Here, we present the complete mitochondrial genome sequence of B. stephanieae combining Oxford Nanopore long read and Illumina short-read sequencing data. The mitochondrial genome has a total length of 14,786 bp, it contains the 13 proteinencoding genes, 23 tRNAs, and two rRNAs and is similar to other nudibranchs except for the presence of a duplicated tRNA-Ser 1.
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