PANSYMBIOSIS · Molecular divergence in a marine animal-microbial symbiosis since the closure of the Isthmus of Panamá

Molecular divergence in a marine animal-microbial symbiosis since the closure of the Isthmus of Panamá

As oceans change due to human activities, understanding marine adaptation is crucial. Studying past geological events like the formation of the Isthmus of Panamá, which divided marine life into the Pacific Ocean and the Caribbean Sea, provides valuable insights. This separation led to different habitats, making the Isthmus ideal for exploring speciation, diversification, and adaptation through convergent evolution. Microorganisms (archaea, bacteria, viruses, fungi) significantly impact host health and fitness, influencing responses to environmental changes. This project examined the convergent evolution of closely related hosts and their associated bacteria on both sides of the Isthmus. We focused on lucinid clams, which rely on bacterial symbionts in their gills for nutrition, comparing these symbionts at the genomic and transcriptomic levels across the Isthmus. Documenting Bacterial Symbiont Adaptation: ● Estimating the molecular evolution and divergence in host-associated bacterial genomes. ● Inferring signals of positive selection and increased recombination rates in orthologous genes. ● Performing a comparative genomic analysis with other closely related lucinid symbionts in the same habitat. Genotype to Phenotype Focus: ● Comparing gene expression among symbiont populations in geminate host species pairs divided by the Isthmus of Panama. ● Revealing whether genes identified as evolutionarily relevant are also expressed and identifying consistently differentially expressed genes across the Isthmus. Investigating Co-speciation and Diversification: ● Comparing molecular divergence rates between symbiont genomes, host mitochondrial genomes, and exome-wide host genomes. ● Applying a recently developed pipeline to infer non-model host population genetic structure and differentiation by integrating metagenomic and metatranscriptomic data. ● By focusing on bivalve species pairs that survived on both sides of the Isthmus, we aimed to understand how their associated bacterial symbionts enabled them to adapt to this massive environmental change. Although both Caribbean and Tropical Eastern Pacific (TEP) bivalves host Candidatus Thiodiazotropha symbionts, only those on the Caribbean side are capable of nitrogen fixation. This capability does not align with symbiont evolutionary history, indicating convergent evolution due to similar environmental pressures. Exploring the genetic history of lucinid symbionts globally revealed that the ancestor of Ca. Thiodiazotropha lacked nitrogen fixation genes. Populations in nutrient-poor habitats acquired this capability multiple times through horizontal gene transfer (HGT). Our research underscores the role of HGT in bacterial adaptation and highlights the impact of nitrogen availability on symbiont ecological diversification. It demonstrates how bacterial symbionts can aid marine organisms in adapting to environmental change.

Data: CORDIS, © European Union