Research Progress in Area 3 方向 ( 三 ) 課題進展 163 Abstract Estimating a co-evolutionary timeline of deepsea animals and their bacterial symbionts has been a very challenging task. This is mainly because bacteria have rarely left fossil records to calibrate their evolution. Dr. Luo’s team plans to develop computational approaches to tackle this difficult issue. They propose a new strategy based on the mitochondrial endosymbiosis that eukaryotic mitochondrial lineages originated from an alphaproteobacterial lineage. This will allow i) taking the advantages of abundant eukaryotic fossils to date bacterial evolution, ii) co-estimating the ages of marine symbionts and their hosts in the same phylogeny. This approach is very promising to study the host-symbiont coevolution in deep-sea ecosystem because of the phylogenetic affinity between α-proteobacteria, where mitochondrial endosymbiosis occurred, and γ-proteobacteria, where most deep-sea bacterial symbionts are distributed. They further plan to leverage the information of well-studied host-symbionts relationship (e.g., legumerhizobia) to better inform the evolutionary history of hosts and symbionts living in the deep sea, where much less is known about their biology. Research Activities and Progress • Reconstructed a comprehensive phylogeny of Proteobacteria and mitochondria, where major hosts and symbionts in deep-sea ecosystems are included; • Developed the mito-endosymbiosis-based molecular dating approach to divergence time estimation; • Established an accurate co-evolutionary timeline of diverse (deep-sea) animals and their symbionts in α-proteobacteria. Key Findings • The mitochondrial lineages branched out earlier than the last common ancestor of all modern α-proteobacteria in the phylogeny; • The last common ancestor of α-proteobacteria originated in early Proterozoic, followed by rapid divergence of its major lineages; • The evolutionary emergence of many deep-sea symbionts are more ancient than that of their hosts, suggesting that they have an ancestor that adapted to free-living lifestyle or that are associated with deep-sea animals different from their present-day hosts. Research Output Publication 3 Trained personnel 1 Establishing the Co-Evolutionary History of Deep-sea Animals and Their Bacterial Symbionts Prof. Haiwei Luo The Chinese University of Hong Kong Fig 1. A co-evolutionary timeline of eukaryotes, including deep-sea animals, and their alphaproteobacterial symbionts based on the mito-endosymbiosis dating approach.
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