Arsenophonus and Sodalis Symbionts in Louse Flies: an Analogy to the Wigglesworthia and Sodalis System in Tsetse Flies

Symbiosis between insects and bacteria result in a variety of arrangements, genomic modifications, and metabolic interconnections. Here, we present genomic, phylogenetic, and morphological characteristics of a symbiotic system associated withMelophagus ovinus , a member of the blood-feeding family Hippoboscidae. The system comprises four unrelated bacteria representing different stages in symbiosis evolution, from typical obligate mutualists inhabiting bacteriomes to freely associated commensals and parasites. Interestingly, the whole system provides a remarkable analogy to the association betweenGlossina and its symbiotic bacteria. In both, the symbiotic systems are composed of an obligate symbiont and two facultative intracellular associates,Sodalis andWolbachia . In addition, extracellularBartonella resides in the gut ofMelophagus . However, the phylogenetic origins of the two obligate mutualist symbionts differ. InGlossina , the mutualisticWigglesworthia appears to be a relatively isolated symbiotic lineage, whereas inMelophagus , the obligate symbiont originated within the widely distributedArsenophonus cluster. Although phylogenetically distant, the two obligate symbionts display several remarkably similar traits (e.g., transmission via the host's “milk glands” or similar pattern of genome reduction). To obtain better insight into the biology and possible role of theM. ovinus obligate symbiont, “Candidatus Arsenophonus melophagi,” we performed several comparisons of its gene content based on assignments of the Cluster of Orthologous Genes (COG). Using this criterion, we show that within a set of 44 primary and secondary symbionts, “Ca . Arsenophonus melophagi” is most similar toWigglesworthia . On the other hand, these two bacteria also display interesting differences, such as absence of flagellar genes inArsenophonus and their presence inWigglesworthia . This finding implies that a flagellum is not essential for bacterial transmission via milk glands.