The genome sequence of Blochmannia floridanus : Comparative analysis of reduced genomes

Abstract

Bacterial symbioses are widespread among insects, probably being one of the key factors of their evolutionary success. We present the complete genome sequence of Blochmannia floridanus, the primary endosymbiont of carpenter ants. Although these ants feed on a complex diet, this symbiosis very likely has a nutritional basis: Blochmannia is able to supply nitrogen and sulfur compounds to the host while it takes advantage of the host metabolic machinery. Remarkably, these bacteria lack all known genes involved in replication initiation (dnaA, priA, and recA). The phylogenetic analysis of a set of conserved protein-coding genes shows that Bl. floridanus is phylogenetically related to Buchnera aphidicola and Wigglesworthia glossinidia, the other endosymbiotic bacteria whose complete genomes have been sequenced so far. Comparative analysis of the five known genomes from insect endosymbiotic bacteria reveals they share only 313 genes, a number that may be close to the minimum gene set necessary to sustain endosymbiotic life.

Gene expression levels influence amino acid usage and evolutionary rates in endosymbiotic bacteria

“…Specific nutritional needs of the insect host have clearly affected gene content of the bacterial partner. Bfl and Buchnera provide their hosts with essential amino acids, whereas Wgl provisions vitamins and cofactors (Shigenobu et al, 2000;Akman et al, 2002;Gil et al, 2003). As expected, these small endosymbionts genomes retain the specific biosynthetic functions that the host requires.…”

“…High and low Faromatic_ and Fhydrophobic_ genes were identified by selecting genes in the lower and upper quartiles (25%) of their respective score. In addition, we also analyzed other discrete categories: (a) high (HEG) and low expression genes (LEG), (b) putative high (PHEG) and putative low expression genes (PLEG), and (c) genes of different metabolic pathways (META), particularly genes involved in synthesis of essential amino acids and genes involved in the synthesis of cofactors (Gil et al, 2003, (Wilcox et al, 2003). We used expression levels of 530 protein coding genes of the reference sample (nonheat stressed) (GEO sample accession numbers GSM2470-2473).…”

“…To more clearly distinguish the effects of conservation and selection on particular amino acids, we extended the previous analyses to include additional loci, other endosymbiont genomes, and amino acids that are AT-rich in EK12 as well and for HEG and LEG. We used the EK12 genome as proxy for ancestral sequences, based on its close phylogenetic position to endosymbionts and its relatively slow evolutionary rate (Tamas et al, 2002;Gil et al, 2003).…”

“…The recent availability of the Bfl (Gil et al, 2003) and Wgl (Akman et al, 2002) genomes allows a broader comparison of codon usage and amino acid usage across endosymbionts of divergent insect host groups that have distinct nutritional requirements. While aphids (the host of Buchnera) feed on plant sap, tsetse flies (host of Wgl) feed on mammalian blood, and Camponotus and related ant genera (host of Bfl) are thought to have a more complex diet.…”

“…While aphids (the host of Buchnera) feed on plant sap, tsetse flies (host of Wgl) feed on mammalian blood, and Camponotus and related ant genera (host of Bfl) are thought to have a more complex diet. Distinct selective pressures relating to host diet are reflected in the gene contents of endosymbionts, which retain abilities to biosynthesize specific nutrients that the host requires (Gil et al, 2003). Whereas Buchnera and Bfl provide their hosts primarily with essential amino acids that lack from the host's diet, Wgl provides cofactors to the host to enrich the host's diet with vitamins.…”