Evolution and Specificity of a Recently Evolved Chemoautotrophic Ectosymbiosis

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Symbioses are vital for most organisms and are a major driving force in evolution. They vary from ectosymbioses, where symbionts are attached to the host’s external surface, to endosymbioses, where symbionts are located inside their hosts. Ectosymbioses, being the least integrated form, offer model systems to study the early evolution of symbioses; yet they are largely understudied compared to endosymbioses. Here we propose a combined genomic, physiological and biochemical approach to study the evolution and basis for host specificity in a chemoautotrophic ectosymbiosis. This symbiosis between amphipods (Niphargus) and sulfur-oxidizing bacteria (Thiothrix) was recently discovered in the Frasassi caves (Italy). Although chemoautotrophic symbioses are widespread in marine environments, the Niphargus symbiosis is the only known case from a freshwater ecosystem. It has likely evolved less than one million years ago, making it significantly younger than its marine counterparts that were established tens to hundreds of million years ago. It is also extremely host specific: three co-occuring Niphargus species within Frasassi carry distinct Thiothrix ectosymbionts. We plan to sequence the ectosymbiont genomes and compare them with those of closely related, non-symbiotic Thiothrix. Such comparative genomic studies have provided important insights into the evolution of endosymbioses, but have never been carried out for ectosymbionts. We will complement our genomic analyses with cultivation-based and biochemical methods to examine the basis for host specificity in this symbiosis.