Planococcus citri strain:London, UK Genome sequencing. Planococcus citri strain:London, UK

The smallest bacterial genome yet reported is from Tremblaya princeps, a symbiont of the mealybug Planococcus citri (PCIT). Tremblaya PCIT is unusual not only for having a 139 kb genome, but for possessing a bacterial endosymbiont of its own, Moranella endobia. Here, using a combination of genome and transcriptome sequencing, we show that the extreme genomic degeneracy of Tremblaya PCIT is likely due to the presence of Moranella as an endosymbiont, and that horizontal gene transfer from Tremblaya PCIT to the insect host has not been a factor enabling this reductive process. We find at least 22 expressed horizontally transferred genes from other bacterial lineages on the Pl. citri genome that appear to complement missing symbiont genes. Our data show that the functioning of this complex symbiosis is dependent on genes from at least six distinct groups of organisms, including several horizontally transferred genes from a diverse suite of bacteria to the host insect genome.