Degnan Patrick H.

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Degnan
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Patrick H.
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Host-symbiont stability and fast evolutionary rates in an ant-bacterium association : cospeciation of Camponotus species and their endosymbionts, Candidatus Blochmannia

2004-02 , Degnan, Patrick H. , Lazarus, Adam B. , Brock, Chad D. , Wernegreen, Jennifer J.

Bacterial endosymbionts are widespread across several insect orders and are involved in interactions ranging from obligate mutualism to reproductive parasitism. Candidatus Blochmannia gen. nov. (Blochmannia) is an obligate bacterial associate of Camponotus and related ant genera (Hymenoptera: Formicidae). The occurrence of Blochmannia in all Camponotus species sampled from field populations and its maternal transmission to host offspring suggest that this bacterium is engaged in a long-term, stable association with its ant hosts. However, evidence for cospeciation in this system is equivocal because previous phylogenetic studies were based on limited gene sampling, lacked statistical analysis of congruence, and have even suggested host switching. We compared phylogenies of host genes (the nuclear EF-1alphaF2 and mitochondrial COI/II) and Blochmannia genes (16S ribosomal DNA [rDNA], groEL, gidA, and rpsB), totaling more than 7 kilobases for each of 16 Camponotus species. Each data set was analyzed using maximum likelihood and Bayesian phylogenetic reconstruction methods. We found minimal conflict among host and symbiont phylogenies, and the few areas of discordance occurred at deep nodes that were poorly supported by individual data sets. Concatenated protein-coding genes produced a very well-resolved tree that, based on the Shimodaira-Hasegawa test, did not conflict with any host or symbiont data set. Correlated rates of synonymous substitution (dS) along corresponding branches of host and symbiont phylogenies further supported the hypothesis of cospeciation. These findings indicate that Blochmannia-Camponotus symbiosis has been evolutionarily stable throughout tens of millions of years. Based on inferred divergence times among the ant hosts, we estimated rates of sequence evolution of Blochmannia to be sim0.0024 substitutions per site per million years (s/s/MY) for the 16S rDNA gene and sim0.1094 s/s/MY at synonymous positions of the genes sampled. These rates are several-fold higher than those for related bacteria Buchnera aphidicola and Escherichia coli. Phylogenetic congruence among Blochmannia genes indicates genome stability that typifies primary endosymbionts of insects.

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Small genome of Candidatus Blochmannia, the bacterial endosymbiont of Camponotus, implies irreversible specialization to an intracellular lifestyle

2002 , Wernegreen, Jennifer J. , Lazarus, Adam B. , Degnan, Patrick H.

Blochmannia (Candidatus Blochmannia gen. nov.) is the primary bacterial endosymbiont of the ant genus Camponotus. Like other obligate endosymbionts of insects, Blochmannia occurs exclusively within eukaryotic cells and has experienced long-term vertical transmission through host lineages. In this study, PFGE was used to estimate the genome size of Blochmannia as approximately 800 kb, which is significantly smaller than its free-living relatives in the enterobacteria. This small genome implies that Blochmannia has deleted most of the genetic machinery of related free-living bacteria. Due to restricted gene exchange in obligate endosymbionts, the substantial gene loss in Blochmannia and other insect mutualists may reflect irreversible specialization to a host cellular environment.