Genomes of most obligately endosymbiotic bacteria are characterized by fast evolutionary rates, extremely low GC contents, and small genome sizes. These shared features suggest that bacterial lifestyle severely affects patterns of molecular evolution. However, few comparative studies have addressed the processes that drive sequence and genome evolution in endosymbionts. The goal of the proposed study is to distinguish molecular and evolutionary mechanisms that shape endosymbiont genomes using phylogenetic and population genetic approaches. We propose to contrast rates and patterns of evolution across species of gamma-3 subdivision Proteobacteria, including free- living species of enterobacteria and related bacterial endosymbionts of carpenter ants, tsetse flies, and aphids.
The specific aims of this study are (1) to quantify the impact of mutational biases, drift, and selection on protein evolution of intracellular bacteria, through molecular evolutionary and population genetic analyses of several protein-coding loci, (2) to explore forces that shape synonymous variation in endosymbionts, by testing for adaptive codon usage and quantifying mutational biases in these species, (3) to examine patterns of genome reduction in endosymbionts by testing for reduced genome size and repair gene loss in the uncharacterized endosymbionts of carpenter ants, and exploring mutational biases toward deletions in these small bacterial genomes. The long-range objective of this study is to understand the molecular evolutionary consequences of endosymbiosis in bacteria. We will approach this issue by examining genomic changes that occur in the context of obligate, intracellular associations. Results of this project will inform our understanding of the modes of evolution of intracellular bacteria, including both pathogens and mutualists, and will shed light on forces that shift the balance between mutation and selection in bacterial evolution. This work will also contribute to the development of more realistic evolutionary models for DNA sequences with biased base compositions, including genomes of organelles, and biased gene regions of vertebrates and other taxa.

National Institute of Health (NIH)
National Institute of General Medical Sciences (NIGMS)
Research Project (R01)
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Genetics Study Section (GEN)
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Eckstrand, Irene A
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Marine Biological Laboratory
Woods Hole
United States
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Brown, Bryan P; Wernegreen, Jennifer J (2016) Deep divergence and rapid evolutionary rates in gut-associated Acetobacteraceae of ants. BMC Microbiol 16:140
Wernegreen, Jennifer J (2015) Endosymbiont evolution: predictions from theory and surprises from genomes. Ann N Y Acad Sci 1360:16-35
Williams, Laura E; Wernegreen, Jennifer J (2013) Sequence context of indel mutations and their effect on protein evolution in a bacterial endosymbiont. Genome Biol Evol 5:599-605
Wernegreen, Jennifer J (2013) First impressions in a glowing host-microbe partnership. Cell Host Microbe 14:121-3
Fan, Yongliang; Thompson, J Will; Dubois, Laura G et al. (2013) Proteomic analysis of an unculturable bacterial endosymbiont (Blochmannia) reveals high abundance of chaperonins and biosynthetic enzymes. J Proteome Res 12:704-18
Fan, Yongliang; Wernegreen, Jennifer J (2013) Can't take the heat: high temperature depletes bacterial endosymbionts of ants. Microb Ecol 66:727-33
Wernegreen, Jennifer J (2012) Strategies of genomic integration within insect-bacterial mutualisms. Biol Bull 223:112-22
Wernegreen, Jennifer J (2012) Mutualism meltdown in insects: bacteria constrain thermal adaptation. Curr Opin Microbiol 15:255-62
Williams, Laura E; Wernegreen, Jennifer J (2012) Purifying selection, sequence composition, and context-specific indel mutations shape intraspecific variation in a bacterial endosymbiont. Genome Biol Evol 4:44-51
Wernegreen, Jennifer J (2011) Reduced selective constraint in endosymbionts: elevation in radical amino acid replacements occurs genome-wide. PLoS One 6:e28905

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