Microbiome Variability across the Native and Invasive Ranges of the Ascidian Clavelina oblonga.Ascidians are prolific colonizers of new environments and possess a range of well-studied features that contribute to their successful spread, but the role of their symbiotic microbial communities in their long-term establishment is mostly unknown.  In this study, we utilized next-generation amplicon sequencing to provide a comprehensive description of the microbiome in the colonial ascidian Clavelina oblonga and examined differences in the composition, diversity, and structure of symbiont communities in the host's native and invasive ranges.  To identify host haplotypes, we sequenced a fragment of the mitochondrial gene cytochrome c oxidase subunit I (COI).  C. oblonga harbored a diverse microbiome spanning 42 bacterial and three archaeal phyla.  Colonies in the invasive range hosted significantly less diverse symbiont communities and exhibited lower COI haplotype diversity than colonies in the native range.  Differences in microbiome structure were also detected across colonies in the native and invasive range, driven largely by novel bacteria representing symbiont lineages with putative roles in nitrogen cycling.  Variability in symbiont composition was also observed among sites within each range.  Together, these data suggest that C. oblonga hosts a dynamic microbiome resulting from  (i) reductions in symbiont diversity due to founder effects in host populations and  (ii) environmental selection of symbiont taxa in response to new habitats within a range.  Further investigation is required to document the mechanisms behind these changes and to determine how changes in microbiome structure relate to holobiont function and the successful establishment of C. oblonga worldwide.IMPORTANCE Nonnative species destabilize coastal ecosystems and microbial symbionts may facilitate their spread by enhancing host survival and fitness.  However, we know little of the microorganisms that live inside invasive species and whether they change as the host spreads to new areas.  In this study, we investigated the microbial communities of an introduced ascidian (Clavelina oblonga) and tracked symbiont changes across locations within the host's native and invasive ranges.  Ascidians in the invasive range had less-diverse microbiomes, as well as lower host haplotype diversity, suggesting that specific colonies reach new locations and carry select symbionts from native populations (i.e., founder effects).  Further, ascidians in the invasive range hosted a different composition of symbionts, including microbes with the potential to aid in processes related to invasion success (e.g., nutrient cycling).  We conclude that the putative functionality and observed flexibility of this introduced ascidian microbiome may represent an underappreciated factor in the successful establishment of nonnative species in new environments.
C