Serial horizontal transfer of vitamin-biosynthetic genes enables the establishment of new nutritional symbionts in aphids' di-symbiotic systems

Abstract

Many insects with a nutrient-restricted diet depend on obligate mutualistic bacteria for the provisioning of essential nutrients lacking from their food source, namely essential amino acids and B vitamins. Most aphids (Hemiptera: Aphididae), whose diet consists of phloem, rely on the bacterial endosymbiont Buchnera for the supply of the aforementioned compounds. However, in some aphid lineages Buchnera have lost the capability of producing these nutrients and thus the symbiotic consortium has accommodated an extra bacterial partner to supplement Buchneras deficiencies. In this work, we explore the di-symbiotic nutritional endosymbiosis of a group of Cinara aphids which has been found to harbour both Buchnera and an Erwinia-related symbiont. Using fluorescence in situ hybridisation, we have located this symbiont to the bacteriome where it inhabits its own bacteriocytes. Through whole-genome sequencing of the endosymbionts of 9 species of Erwinia-associated Cinara aphids, we have found that Ewrinia genomes are highly syntenic and all show significant genome reduction. Additionally, Erwinia symbionts display phylogenetic congruency with Buchnera, suggesting long-term co-divergence. Most significantly, we found that not only is Erwinia capable of complementing Buchneras auxotrophies, but that the genes involved in the biosynthesis of two B vitamins have actually been horizontally acquired from a Sodalis-related bacterium. Finally, this B-vitamin biosynthetic genes have been further transferred to a new Hamiltonella co-obligate symbiont in a specific Cinara lineage, thus displaying a tri-symbiotic system. These results highlight the important role horizontal gene transfer plays in the establishment of new obligate nutritional symbionts.