Multidimensional variability of the microbiome of an invasive ascidian species

The past, the recent, and the ongoing evolutionary processes of the worldwide invasive ascidian Styela plicata

Invasive species are one of the main threats to global biodiversity and, within marine ecosystems, tunicates feature some prominent examples. Styela plicata is an ascidian species inhabiting harbours in all temperate oceans and seas, thus being considered a thriving invasive species. However, this species' adaptive mechanisms, introduction history, and population structure have never been completely elucidated. Here, by genotyping 87 S. plicata individuals from 18 localities worldwide with 2b-RADseq, we confirm the global presence of four chromosome inversions, demonstrate population structuring on this species, detect local adaptation signals, and infer historical demographic events. We show that North Carolina individuals constitute an unrelated population, Atlanto-Mediterranean and Pacific localities form their own genetic clusters with substructuring, being the most evident the split between northern and southern Atlantic localities. The locality of South Carolina presents an intermediate genetic position between North Carolina and the other two groups pointing to a hybrid origin with recurrent gene flow. We generate and test demographic models, providing evidence of two independent introduction events to the Atlantic and Pacific, and an admixture that originated the population of South Carolina. Finally, we identify candidate loci for adaptation, with functions involved with cell processes, metabolism, development, and ion transport, among others. Overall, this study highlights the complex historical processes of S. plicata, which have led this species to its current distribution, population structure, and local adaptation footprint in oceans worldwide.

Navigating spatio-temporal microbiome dynamics: Environmental factors and trace elements shape the symbiont community of an invasive marine species

The proliferation of marine invasive species is a mounting concern. While the role of microbial communities in invasive ascidian species is recognized, the role of seasonal shifts in microbiome composition remains largely unexplored. We sampled five individuals of the invasive ascidian Styela plicata quarterly from January 2020 to October 2021 in two harbours, examining gills, tunics, and surrounding water. By analysing Amplicon Sequence Variants (ASVs) and seawater trace elements, we found that compartment (seawater, tunic, or gills) was the primary differentiating factor, followed by harbour. Clear seasonal patterns were evident in seawater bacteria, less so in gills, and absent in tunics. We identified compartment-specific bacteria, as well as seasonal indicator ASVs and ASVs correlated with trace element concentrations. Among these bacteria, we found that Endozoicomonas, Hepatoplasma and Rhodobacteraceae species had reported functions which might be necessary for overcoming seasonality and trace element shifts. This study contributes to understanding microbiome dynamics in invasive holobiont systems, and the patterns found indicate a potential role in adaptation and invasiveness.

Effects of color variation and physiological state on ascidian microbiomes

Ascidians, known for their color variation, host species-specific microbial symbiont communities. Some ascidians can also transition into a nonfiltering (resting) physiological state. Recent studies suggest that the microbial symbiont communities may vary across different physiological states and color morphs of the host. The colonial ascidian, Polyclinum constellatum, which exhibits several color morphs in the Caribbean Sea, periodically ceases its filtering activity. To investigate if color variation in P. constellatum is indicative of sibling speciation, we sequenced fragments of the ribosomal 18S rRNA and the mitochondrial cytochrome oxidase subunit I genes. Additionally, we sequenced a fragment of the 16S rRNA gene to characterize the microbial communities of two common color morphs (red and green) in colonies that were either actively filtering (active) or nonfiltering (resting). Phylogenetic analyses of both ascidian genes resulted in well-supported monophyletic clades encompassing all color variants of P. constellatum. Interestingly, no significant differences were observed among the microbial communities of the green and red morphs, suggesting that color variation in this species is a result of intraspecific variation. However, the host's physiological state significantly influenced the microbial community structure. Nonfiltering (resting) colonies hosted higher relative abundances of Kiloniella (Alphaproteobacteria) and Fangia (Gammaproteobacteria), while filtering colonies hosted more Reugeria (Alphaproteobacteria) and Endozoicomonas (Gammaproteobacteria). This study demonstrates that microbial symbiont communities serve as reliable indicators of the taxonomic state of their host and are strongly influenced by the host's feeding condition.

Distinct microbial communities in an ascidian-crustacean symbiosis

Ascidians are marine invertebrates known to occasionally host symbiotic crustaceans. Although the microbiomes of both ascidians and free-living crustaceans have been characterized, there is no documentation of microbial communities in an ascidian-crustacean symbiosis. Samples of the solitary ascidian Ascidia sydneiensis and ambient seawater were collected in Belize. Four symbiotic amphipod crustaceans were retrieved from the branchial sac of the animal, and their microbiomes were compared with those from their ascidian host (tunic and branchial sac compartments) and seawater. Microbiome richness and diversity differed significantly between sample types, with amphipod microbiomes exhibiting significantly lower diversity than tunic and ambient seawater samples. Microbiome composition also differed significantly between sample types and among all pairwise comparisons, except for branchial sac and amphipod microbiomes. Differential operational taxonomic unit (OTU) analyses revealed that only 3 out of 2553 OTUs had significantly different relative abundances in amphipods compared with ascidian branchial sacs, whereas 72 OTUs differed between amphipod and tunic and 315 between amphipod and seawater samples. Thus, different body compartments of A. sydneiensis hosted distinct microbiomes, and symbiotic amphipods contained microbiomes resembling the region they inhabit (i.e., the branchial sac), suggesting that environmental filtering and co-evolutionary processes are determinants of microbiome composition within ascidian-crustacean symbioses.