Insect beneficial symbiotic island within Burkholderiaceae localized within Burkholderia genus

Microbial symbioses are important for the survival of many organisms from plants to animals. The microbial genes that enable these relationships are often located in horizontally transferred genomic islands, termed “symbiosis islands,” with rhizobial symbiosis islands being the best-studied system. Recently a new 33-49 kb symbiotic island was found that appears to regulate widespread symbioses between hundreds of species of true bugs (Heteroptera) and numerous bacterial strains in the genus Burkholderia. The island was proposed based on genomic analysis of five species of Burkholderia, leaving its prevalence, origin, and functional effects across this large bacterial clade relatively unexplored. We performed a broad comparative analysis of 229 Burkholderiaceae genomes, including eight new genomes of insect-associated Burkholderia sequenced for this study. We detected the island in 23% of the analyzed genomes located solely within two Burkholderia clades. The symbiotic island arose at the origin of the Burkholderia sensu lato and has experienced several independent loss events. In contrast to the rhizobial symbiosis island, there have been few instances of horizontal transfer within the Burkholderia. Finally, to begin to deduce the island’s function, we combined our data with information on several strains’ abilities to colonize the insect symbiotic organ. We found that the island is associated with improved symbiont colonization of the host. These findings advance our understanding of a widespread insect-bacterial symbiosis and provide a new comparative system for the evolutionary dynamics of symbiotic islands.