ena-STUDY-UMR5558 CNRS-23-06-2014-17:00:54:452-426
Two host clades, two bacterial arsenals: evolution through gene losses in facultative endosymbionts
Bacterial endosymbiosis is an important evolutionary process in insects, which harbor both obligate and facultative symbionts. The first are clearly mutualist, while the persistence of the latter relies on different strategies including reproductive manipulation, protection against enemies or nutriment supplying. While genome differences between obligate and facultative symbionts are well documented, it is less clear how specificities in the relationship between the host and facultative symbionts can affect the evolution of the bacterial genomes. Hamiltonella defensa is an endosymbiont found in two clades of insects, aphids and whiteflies, with which it has established two distinct symbioses: (i) in aphids, it's a protective endosymbiont with intermediary prevalence and a large tissue tropism, which frequently transfer between host species; (ii) in whiteflies, it's a putative nutriment supplying symbiont sequestered in symbiotic cells (bacteriocytes), which is near fixation in populations, and only found in two species. In this work, we have sequenced the genome of H. defensa symbiont of the whitefly Bemisia tabaci MEAM1. We compared it with the H. defensa strain infecting B. tabaci MED-Q1 and the strain infecting aphid Acyrthosiphon pisum. We reconstructed the ancestral gene content of their ancestor, and found that the symbionts have a genomic evolution biased towards gene losses. The lost and inactivated functions reflect the specialization of the bacteria in two distinct host clades as well as the different phenotypes and life conditions. The aphids' symbionts would need some highly virulent factors, like type 3 secretion system, and different environmental receptors, in order to invade host tissues, transfer to the next generation and protect the host against parasitoids. These systems would be obsolete in the bacteriocyte-associated whiteflies' symbionts and have logically been lost. Nevertheless, these last retained others virulence factors which could have an important role for the communication of the bacteria with the host. Globally, while both lineages have mutualistic interactions with their hosts, the evolution of their genomes shows different trends that could have important consequences on the evolvabilty of these organisms.
Bacterial endosymbiosis is an important evolutionary process in insects, which harbor both obligate and facultative symbionts. The first are clearly mutualist, while the persistence of the latter relies on different strategies including reproductive manipulation, protection against enemies or nutriment supplying. While genome differences between obligate and facultative symbionts are well documented, it is less clear how specificities in the relationship between the host and facultative symbionts can affect the evolution of the bacterial genomes. Hamiltonella defensa is an endosymbiont found in two clades of insects, aphids and whiteflies, with which it has established two distinct symbioses: (i) in aphids, it's a protective endosymbiont with intermediary prevalence and a large tissue tropism, which frequently transfer between host species; (ii) in whiteflies, it's a putative nutriment supplying symbiont sequestered in symbiotic cells (bacteriocytes), which is near fixation in populations, and only found in two species. In this work, we have sequenced the genome of H. defensa symbiont of the whitefly Bemisia tabaci MEAM1. We compared it with the H. defensa strain infecting B. tabaci MED-Q1 and the strain infecting aphid Acyrthosiphon pisum. We reconstructed the ancestral gene content of their ancestor, and found that the symbionts have a genomic evolution biased towards gene losses. The lost and inactivated functions reflect the specialization of the bacteria in two distinct host clades as well as the different phenotypes and life conditions. The aphids' symbionts would need some highly virulent factors, like type 3 secretion system, and different environmental receptors, in order to invade host tissues, transfer to the next generation and protect the host against parasitoids. These systems would be obsolete in the bacteriocyte-associated whiteflies' symbionts and have logically been lost. Nevertheless, these last retained others virulence factors which could have an important role for the communication of the bacteria with the host. Globally, while both lineages have mutualistic interactions with their hosts, the evolution of their genomes shows different trends that could have important consequences on the evolvabilty of these organisms.