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Background: The whitefly Bemisia tabaci is an important agricultural pest with global distribution. This phloem-sap feeder harbors a primary symbiont, "Candidatus Portiera aleyrodidarum", which compensates for the deficient nutritional composition of its food sources, and a variety of secondary symbionts. Interestingly, all of these secondary symbionts are found in co-localization with the primary symbiont within the same bacteriocytes, which should favor the evolution of strong interactions between symbionts. Results: In this paper, we analyzed the genome sequences ofthe primary symbiont Portiera and ofthe secondary symbiont Hamiltonella in the B. tabaci Mediterranean (MED) species in order to gain insight into the metabolic role of each symbiont in the biology of their host. The genome sequences of the uncultured symbionts Portiera and Hamiltonella were obtained from one single bacteriocyte of MED B. tabaci. As already reported, the genome of Portiera is highly reduced (357 kb), but has kept a number of genes encoding most essential amino-acids and carotenoids. On the other hand, Portiera lacks almost all the genes involved in the synthesis of vitamins and cofactors. Moreover, some pathways are incomplete, notably those involved in the synthesis of some essential amino-acids. Interestingly, the genome of Hamiltonella revealed that this secondary symbiont can not only provide vitamins and cofactors, but also complete the missing steps of some of the pathways of Portiera. In addition, some critical amino-acid biosynthetic genes are missing in the two symbiotic genomes, but analysis of whitefly transcriptome suggests that the missing steps may be performed by the whitefly itself or its microbiota. Conclusions: These data suggest that Portiera and Hamiltonella are not only complementary but could also be mutually dependent to provide a full complement of nutrients to their host. Altogether, these results illustrate how functional redundancies can lead to gene losses in the genomes ofthe different symbiotic partners, reinforcing their inter-dependency. [ABSTRACT FROM AUTHOR]