| Description |
Intracellular mutualistic endosymbionts are widespread among insect species and perform many different functions for their hosts. While the role of each symbiont varies among hosts, the evolution of symbiont genomes follows a similar degenerative trajectory. Genome degeneration is a consequence of relaxed selection on gene functions no longer required in the symbiotic relationship. Previous efforts to understand the forces shaping symbiont genomes have involved comparing long established symbionts with distantly related free-living bacteria. In order to characterize the mechanisms driving symbiont genome degeneration, recently established symbionts must be compared with closely related free-living relatives. This work describes the discovery of a non host-associated member of the Sodalis-allied clade of insect symbionts, Sodalis praecaptivus str. HS1. Whole genome comparisons between S. praecaptivus and the recently established symbionts, Sodalis glossinidius and Candidatus Sodalis pierantonius show that these symbionts evolved independently from an S. praecaptivus-like ancestor. Detailed genomic comparisons between S. praecaptivus and Ca. S. pierantonius reveal that insertions and deletions resulting from replication slippage errors associated with G+C-rich repeat sequences are an important driver of gene inactivation and deletion in the early stages of symbiosis. This slippage-prone phenotype is mechanistically associated with the loss of certain components of the bacterial DNA recombination machinery at an early stage in symbiotic life and is expected to facilitate rapid adaptation to the novel host environment. This work also includes a comparative analysis of the PhoP-PhoQ two-component systems of S. praecaptivus and S. glossinidius, which is involved in mediating resistance to antimicrobial peptides produced by the host immune system. Comparative transcriptomics identifies significant differences in the PhoP regulatory networks of these closely related organisms. This work also describes the discovery of a novel transcriptional regulator involved in resistance to antimicrobial peptides. The role of genome degeneration as it relates to bacterial adaptation in novel environments is also explored. Using a library of transposon Tn 5 mutants of S. praecaptivus this work shows that the inactivation of various genes, many of which are global transcriptional regulators, can provide significant fitness advantages in novel environments, including an insect host. This rapid adaptation is thought to occur by disrupting established regulatory networks allowing existing genes and metabolic pathways to be reconfigured in ways that are better suited to the novel environments encountered by the cell. |
