Characterizing a Novel Symbiopectobacterium purcellii MEX Strain at the Early Stages of Establishing a Symbiotic Relationship.

Insects ally with microbial symbionts for a diversity of services. The range of these interactions is wide, spanning from beneficial to pathogenic and facultative to obligate. In many cases, such insect-microbial interactions veer towards mutual dependency with integrated physiologies. This evolutionary outcome is relatively common in insects that depend on microbes to fill gaps in their nutritional ecologies (e.g. plant-sap feeding). However, the initiation and transition towards such dependent symbiotic interactions are difficult to observe in nature. Identifying these events can provide key insights into the origins and evolutionary processes that shape symbiotic interactions. Here, we report on a novel interaction between a leafhopper (Typhlocybinae: Empoasca mexicana) and a bacterium, Symbiopectobacterium purcellii MEX strain (S-MEX). To characterize this symbiont, we assembled and annotated its complete genome. We compared its content and structure to the genomes of other Symbiopectobacterium. The S-MEX genome is unique among members of this genus. It is the largest yet sequenced at 5.3 Mb, encoding 6,838 genes (∼25% more than other strains). S-MEX's genome has significantly expanded due to the proliferation of insertion sequences and 2,723 identifiable pseudogenes-processes generally seen as accelerators of genome reduction and emerging host dependence. S-MEX and other Symbiopectobacterium strains have a core set of 818 genes shared in >90% of strains, of which S-MEX has uniquely lost 36 genes. Taken together, we hypothesize that due to expansion of IS elements, extensive pseudogenization, and loss of genes in important free-living functions, S-MEX is in the early stages of establishing a host-dependent symbiosis.