Journal Article

Functional Convergence in Reduced Genomes of Bacterial Symbionts Spanning 200 My of Evolution

John P. McCutcheon and Nancy A. Moran

in Genome Biology and Evolution

Published on behalf of Society for Molecular Biology and Evolution

Volume 2, issue , pages 708-718
Published in print January 2010 |
Published online September 2010 | e-ISSN: 1759-6653 | DOI: http://dx.doi.org/10.1093/gbe/evq055

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The main genomic changes in the evolution of host-restricted microbial symbionts are ongoing inactivation and loss of genes combined with rapid sequence evolution and extreme structural stability; these changes reflect high levels of genetic drift due to small population sizes and strict clonality. This genomic erosion includes irreversible loss of genes in many functional categories and can include genes that underlie the nutritional contributions to hosts that are the basis of the symbiotic association. Candidatus Sulcia muelleri is an ancient symbiont of sap-feeding insects and is typically coresident with another bacterial symbiont that varies among host subclades. Previously sequenced Sulcia genomes retain pathways for the same eight essential amino acids, whereas coresident symbionts synthesize the remaining two. Here, we describe a dual symbiotic system consisting of Sulcia and a novel species of Betaproteobacteria, Candidatus Zinderia insecticola, both living in the spittlebug Clastoptera arizonana. This Sulcia has completely lost the pathway for the biosynthesis of tryptophan and, therefore, retains the ability to make only 7 of the 10 essential amino acids. Zinderia has a tiny genome (208 kb) and the most extreme nucleotide base composition (13.5% G + C) reported to date, yet retains the ability to make the remaining three essential amino acids, perfectly complementing capabilities of the coresident Sulcia. Combined with the results from related symbiotic systems with complete genomes, these data demonstrate the critical role that bacterial symbionts play in the host insect’s biology and reveal one outcome following the loss of a critical metabolic activity through genome reduction.

Keywords: genome reduction; minimal genome; genome sequencing; Bacteroidetes; nutritional symbioses

Journal Article.  5128 words.  Illustrated.

Subjects: Bioinformatics and Computational Biology ; Evolutionary Biology ; Genetics and Genomics

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