Evolutionary convergence and nitrogen metabolism in Blattabacterium strain Bge, primary endosymbiont of the cockroach Blattella germanica

Abstract

Bacterial endosymbionts of insects play a central role in upgrading the diet of their hosts. In certain cases, such as aphids and tsetse flies, endosymbionts complement the metabolic capacity of hosts living on nutrient-deficient diets, while the bacteria harbored by omnivorous carpenter ants are involved in nitrogen recycling. In this study, we describe the genome sequence and inferred metabolism of Blattabacterium strain Bge, the primary Flavobacteria endosymbiont of the omnivorous German cockroach Blattella germanica. Through comparative genomics with other insect endosymbionts and free-living Flavobacteria we reveal that Blattabacterium strain Bge shares the same distribution of functional gene categories only with Blochmannia strains, the primary Gamma-Proteobacteria endosymbiont of carpenter ants. This is a remarkable example of evolutionary convergence during the symbiotic process, involving very distant phylogenetic bacterial taxa within hosts feeding on similar diets. Despite this similarity, different nitrogen economy strategies have emerged in each case. Both bacterial endosymbionts code for urease but display different metabolic functions: Blochmannia strains produce ammonia from dietary urea and then use it as a source of nitrogen, whereas Blattabacterium strain Bge codes for the complete urea cycle that, in combination with urease, produces ammonia as an end product. Not only does the cockroach endosymbiont play an essential role in nutrient supply to the host, but also in the catabolic use of amino acids and nitrogen excretion, as strongly suggested by the stoichiometric analysis of the inferred metabolic network. Here, we explain the metabolic reasons underlying the enigmatic return of cockroaches to the ancestral ammonotelic state.

Figure 1. An integrated view of the predicted metabolism and transport in Blattabacterium strain Bge.

Figure 2. Absolute number of genes for the different COG categories.

Species abbreviations are as follows: BLB, Blattabacterium strain Bge; BFL, B. floridanus; BPE, B. pennsylvanicus; BCI, B. cicadellinicola (NC_007984); BAP, B. aphidicola Bap; BCC, B. aphidicola BCc; WGL, W. glossinidia (NC_003425); WRI, Wolbachia sp. wRi (NC_012416); SMU, S. muelleri; FPS, F. psychrophilum; FJO, F. johnsoniae (NC_009441); GFO, G. forsetii (NC_008571); SGL, Sodalis glossinidius (NC_007712).

Figure 3. Functional closeness and phylogenetic relationship of endosymbionts.

(A) Dendrogram obtained by a linkage clustering method from the matrix of Kulczynski distances between species for the observed distribution of COG categories (Figure 2, Table 2). In all cases, except one, the null hypothesis of getting by chance the corresponding cluster was rejected (bootstrap values were equal or higher than 90%). (B) 16S rDNA maximum likelihood phylogenetic tree, with bootstrap values (%) based on 1000 replicates, of the thirteen compared bacterial species. The methods used to derive the Kulczynski distance, the dendrogram and the phylogenetic tree are detailed in Materials and Methods. Species abbreviations as in Figure 2. 16S rDNA gene NCBI-GeneID: BLB, 99077774; BFL, 1499754; BPE, 3563224; BCI, 4056264; BAP, 7262504; BCC, 4441000; SMU, 5797390; WGL, 1257559; WRI, 7669911; FPS, 5300282; FJO, 5092512; GFO, 4652227; SGL, 3866283.

Figure 4. Central metabolic pathways of Blattabacterium strain Bge directly involved in the catabolism of amino acids.

The set of balanced reactions in this diagram constitutes the input file for the stoichiometric analysis using METATOOL. Broken double arrows (in red) indicate transamination reactions. Green arrows represent the oxidative deamination of Glu. Non-conventional abbreviations: Pyr, pyruvate; AcCoA, acetyl CoA; OAA, oxalacetate; Cit, citrate; IsoCit, isocitrate; OG, 2-oxoglutarate; SucCoA, succinyl CoA; Suc, succinate; Fum, fumarate; Mal, malate; CP, carbamoyl phosphate; Citru, citrulline; ArgSuc, argininosuccinate.