Evolution and Specificity of a Recently Evolved Chemoautotrophic Ectosymbiosis

共生对大多数生物体至关重要，是进化的主要推动力。它们从共生体附着在宿主外表面的胞外共生体到共生体位于宿主内部的内共生体都有不同。胞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；然而，与内共生相比，它们在很大程度上被忽视了。在这里，我们提出了一种基因组、生理和生化相结合的方法来研究化学自养性外生共生中宿主专一性的进化和基础。这种两足类(Niphargus)和硫氧化细菌(Thiothrix)之间的共生关系最近在意大利的Frasassi洞穴中被发现。尽管化学自养共生在海洋环境中广泛存在，但Niphargus共生是淡水生态系统中唯一已知的病例。它可能是在不到100万年前进化出来的，这使得它比数千万年前建立的海洋同行要年轻得多。它还具有极强的寄主特异性：在Frasassi内共存的三种Niphargus携带不同的硫杆菌外共生体。我们计划对胞外共生菌的基因组进行测序，并将它们与密切相关的非共生硫藻的基因组进行比较。这种比较基因组学研究为内共生体的进化提供了重要的见解，但从未对外共生体进行过。我们将用基于培养的和生化的方法来补充我们的基因组分析，以检查这种共生中宿主专一性的基础。

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.