title: Winding paths to simplicity: genome evolution in facultative insect symbionts.
abstract: Symbiosis between organisms is an important driving force in evolution. Among the diverse relationships described, extensive progress has been made in insect–bacteria symbiosis, which improved our understanding of the genome evolution in host-associated bacteria. Particularly, investigations on several obligate mutualists have pushed the limits of what we know about the minimal genomes for sustaining cellular life. To bridge the gap between those obligate symbionts with extremely reduced genomes and their non-host-restricted ancestors, this review focuses on the recent progress in genome characterization of facultative insect symbionts. Notable cases representing various types and stages of host associations, including those from multiple genera in the family Enterobacteriaceae (class Gammaproteobacteria), Wolbachia (Alphaproteobacteria) and Spiroplasma (Mollicutes), are discussed. Although several general patterns of genome reduction associated with the adoption of symbiotic relationships could be identified, extensive variation was found among these facultative symbionts. These findings are incorporated into the established conceptual frameworks to develop a more detailed evolutionary model for the discussion of possible trajectories. In summary, transitions from facultative to obligate symbiosis do not appear to be a universal one-way street; switches between hosts and lifestyles (e.g. commensalism, parasitism or mutualism) occur frequently and could be facilitated by horizontal gene transfer.
authors.affiliation: Institute of Plant and Microbial Biology, Academia Sinica, Taipei, Taiwan.|Molecular and Biological Agricultural Sciences Program, Taiwan International Graduate Program, National Chung Hsing University and Academia Sinica, Taipei 11529, Taiwan.|Graduate Institute of Biotechnology, National Chung Hsing University, Taichung, Taiwan.
authors.forename: Wen-Sui
authors.initials: WS
authors.lastname: Lo
authors.affiliation: Institute of Plant and Microbial Biology, Academia Sinica, Taipei, Taiwan.
authors.forename: Ya-Yi
authors.initials: YY
authors.lastname: Huang
authors.affiliation: Institute of Plant and Microbial Biology, Academia Sinica, Taipei, Taiwan.|Molecular and Biological Agricultural Sciences Program, Taiwan International Graduate Program, National Chung Hsing University and Academia Sinica, Taipei 11529, Taiwan.|Graduate Institute of Biotechnology, National Chung Hsing University, Taichung, Taiwan.
authors.forename: Chih-Horng
authors.initials: CH
authors.lastname: Kuo
mesh_terms: D000818:Animals; D004755:Enterobacteriaceae / Q000235:genetics; D019143:Evolution, Molecular; D022761:Gene Transfer, Horizontal / Q000235:genetics; D016680:Genome, Bacterial / Q000235:genetics*; D007313:Insecta / Q000235:genetics* / Q000382:microbiology*; D020560:Proteobacteria / Q000235:genetics; D013149:Spiroplasma / Q000235:genetics; D013559:Symbiosis / Q000235:genetics*
keywords: genome degradation; genome size; coding density; horizontal gene transfer; deletional bias; pseudogene; GC content; symbiosis; symbiotic bacteria; Arsenophonus; Hamiltonella; Regiella; Pantoea; Serratia; Sodalis; Spiroplasma; Wolbachia