Solving a bloody mess: B-vitamin independentmetabolic convergence among gammaproteobacterial obligate endosymbionts from blood-feeding arthropods and the Leech haementeria officinalis

Abstract

Endosymbiosis is a common phenomenon in nature, especially between bacteria and insects, whose typically unbalanced diets are usuallycomplementedby theirobligateendosymbionts. Whilemuchinterestandfocus hasbeendirected toward phloem-feeders like aphids andmealybugs, blood-feeders such as the Lone star tick (Amblyomma americanum),Glossina flies, and the human body louse (Pediculus humanus corporis) depend on obligate endosymbionts which complement their B-vitamin-deficient diets, and thus are required for growth and survival.Glossiphoniid leeches have also been found to harbor distinct endosymbionts housed in specialized organs. Here, we present the genome of the bacterial endosymbiont from Haementeria officinalis, first of a glossiphoniid leech. This as-yet-unnamedendosymbiont belongs to theGammaproteobacteria,has apleomorphic shape and is restricted to bacteriocytes. For this bacterialendosymbiont,wepropose thenameCandidatus Providencia siddallii. This symbiont possessesa highly reducedgenome with high A+T content and a reduced set of metabolic capabilities, all of which are common characteristics of ancient obligate endosymbionts of arthropods. Its genome has retainedmany pathways related to the biosynthesis of B-vitamins, pointing toward a role in supplementing the blood-restricted diet of its host. Through comparative genomics against the endosymbionts of A. americanum, Glossina flies, and P. humanus corporis,wewere able to detect a high degree ofmetabolic convergence among these four very distantly related endosymbiotic bacteria.