Abstract
Symbiosis is a widespread phenomenon in nature, in which insects show a great number of these associations. Buchnera aphidicola, the obligate endosymbiont of aphids, coexists in some species with another intracellular bacterium, Serratia symbiotica. Of particular interest is the case of the cedar aphid Cinara cedri, where B. aphidicola BCc and S. symbiotica SCc need each other to fulfil their symbiotic role with the insect. Moreover, various features seem to indicate that S. symbiotica SCc is closer to an obligate endosymbiont than to other facultative S. symbiotica, such as the one described for the aphid Acirthosyphon pisum (S. symbiotica SAp). This work is based on the comparative genomics of five strains of Serratia, three free-living and two endosymbiotic ones (one facultative and one obligate) which should allow us to dissect the genome reduction taking place in the adaptive process to an intracellular life-style. Using a pan-genome approach, we have identified shared and strain-specific genes from both endosymbiotic strains and gained insight into the different genetic reduction both S. symbiotica have undergone. We have identified both retained and reduced functional categories in S. symbiotica compared to the Free-Living Serratia (FLS) that seem to be related with its endosymbiotic role in their specific host-symbiont systems. By means of a phylogenomic reconstruction we have solved the position of both endosymbionts with confidence, established the probable insect-pathogen origin of the symbiotic clade as well as the high amino-acid substitution rate in S. symbiotica SCc. Finally, we were able to quantify the minimal number of rearrangements suffered in the endosymbiotic lineages and reconstruct a minimal rearrangement phylogeny. All these findings provide important evidence for the existence of at least two distinctive S. symbiotica lineages that are characterized by different rearrangements, gene content, genome size and branch lengths.
Highlights
Symbiosis is a widespread phenomenon among all branches of life
We found a massive level of genomic reduction in S. symbiotica SCc, even when compared to S. symbiotica SAp in which a great number of accessory genes are still retained in its genome
If we take into account the genes shared by the three Free-Living Serratia (FLS) plus the core (3, 452), the percentage is greatly increased (51.05%) due to the presence of the endosymbiotic genomes, mainly the genome from S. symbiotica SCc, which displays an extensive genomic reduction due to its adaptation to an intra-cellular lifestyle [13]
Summary
Insects show a tight relationship with a variety of these organisms [1] mostly having a metabolic foundation, as bacteria provide the insects with the nutrients lacking in their diet. This is the case for many aphids that maintain a close association with the ancient obligate bacterium B. aphidicola. These facultative bacteria undergo occasional horizontal transfer [14,15,16,17] These three bacteria have been shown to benefit the host, providing defense against fungal pathogens, parasitoid wasps or even increasing survival after environmental heat stress (revised in [17]). An interesting genomic feature from these young associations, contrary to more ancient ones, is the massive presence of mobile genetic elements in their genomes [18,19,20,21], which would cause their genomes to undergo a number of rearrangements as compared to their freeliving relatives
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