Periodic Reporting for period 1 - SYMOBLIGA (Comparative genomics of host-symbiont dependency) Reporting period: 2018-09-01 to 2020-08-31 Summary of the context and overall objectives of the project Insects are frequently infected by bacteria. Some of them are obligate symbionts, providing functions essential to the survival and fitness of their hosts, while other are facultative. Aphids ancestrally harbour one obligate symbiont: Buchnera aphidicola. While this remain the most common situation, some clades of aphids transitioned to a state of co-obligate symbiosis, where they depend both on Buchnera and on an additional symbiont: Serratia symbiotica. The goal of the project is to understand how such evolutionary transitions occurred. Dependence on Serratia was previously reported only within an aphid clade known as the Lachninae subfamily. Our project focus on other aphid clades that independently evolved the same dependence on Serratia. The overall objectives were to identify the genomic and physiological bases of dependence to Serratia symbiotica in aphids. Taking advantage of the multiple independent origins of the same association (aphid-Buchnera-Serratia), we aimed at identifying the evolutionary processes that are shaping the evolution of symbioses. In particular: are they stochastic processes or rather deterministic, repeatable, ones? Do aphids evolve similar structures to host their newly obligate symbionts? Is Serratia always providing the same benefits to its host? As aphids are important agricultural pests, a better understanding of their biology is crucial to regulate them in efficient and sustainable ways. Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far Good quality genomic DNA was obtained from 5 aphid species dependent upon both Buchnera aphidicola and Serratia symbiotica. Genome assemblies of the symbionts (Serratia symbiotica and Buchnera aphidicola) were performed in 5 species, allowing to assess their metabolic abilities. Fluorescent In Situ Hybridization was performed to localize both symbionts within the aphid host.We found that dependence of aphids on Serratia symbiotica usually evolves through losses of function in the aphid symbiont Buchnera aphidicola. Functions typically lost by Buchnera aphidicola and taken over by Serratia symbiotica include riboflavin and tryptophan synthesis. We further showed that the age of the dependence was positively correlated to the density of Serratia symbiotica in host tissues.The results of this study were published in May 2020 (Monnin, D., Jackson, R., Kiers, E. T., Bunker, M., Ellers, J., & Henry, L. M. (2020). Parallel evolution in the integration of a co-obligate aphid symbiosis. Current Biology).The results were further communicated at the Animal-Microbe Gordon Research Seminar and Conference (2019) and at the Symbiosis zoom seminars (2020). Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) We provided evidence for convergent evolution on Serratia symbiotica in aphids. The project results will be of wide interest within the symbiosis community. The article resulting from the fellowship, published in May 2020, was the object of a dispatch article in Current Biology (Kost 2020) and was already cited in 3 additional peer-reviewed articles bearing on the role of compartmentalization in the evolution of symbiotic cooperation, symbiotic provision of B-vitamin in aphids, and the role of stress in symbiosis, respectively.