Evolution and Specificity of a Recently Evolved Chemoautotrophic Ectosymbiosis

共生体对大多数生物体至关重要，是进化的主要驱动力。它们从共生体附着在宿主外表面的外共生体到共生体位于宿主内部的内共生体。外共生是最不完整的形式，提供了研究共生早期进化的模型系统;但与内共生相比，它们在很大程度上研究不足。在这里，我们提出了一个组合的基因组，生理和生化的方法来研究的进化和基础的化学自养外共生的主机特异性。这种片足类动物（Niphargus）和硫氧化细菌（Thiothrix）之间的共生关系最近在Frasassi洞穴（意大利）被发现。虽然化能自养共生体广泛存在于海洋环境中，但Niphargus共生体是淡水生态系统中唯一已知的情况。它可能是在不到一百万年前进化而来的，这使得它比几千万到几亿年前建立的海洋生物要年轻得多。它也是非常主机特定的：三个共同出现的Niphargus物种内Frasassi携带不同的Thiothrix外共生体。我们计划对外共生体的基因组进行测序，并将其与密切相关的非共生硫杆菌进行比较。这种比较基因组研究为内共生菌的进化提供了重要的见解，但从未对外共生菌进行过。我们将补充我们的基因组分析与培养为基础的和生物化学的方法，以检查在这种共生宿主特异性的基础。

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.