Chromosomal-level assembly of Bactericera cockerelli reveals rampant gene family expansions impacting genome structure, function and insect-microbe-plant-interactions.

Lineage specific expansions and gene duplications are some of the most important sources of evolutionary novelty in eukaryotes. Although not as prevalent in eukaryotes compared to bacteria, horizontal gene transfer events can also result in key adaptations for insects, especially for those involved in insect-microbe interactions. In this study we assemble the first chromosomal assembly of the psyllid Bactericera cockerelli and reveal that the B. cockerelli genome has experienced significantly more gene expansion events compared to other Hemipteran representatives with fully sequenced genomes. We also reveal that B. cockerelli's genome is the largest psyllid genome (567 Mb) sequenced to date and is ~15% larger than the other two psyllid species genomes sequenced (Pachypsylla venusta and Diaphorina citri). Structurally, B. cockerelli appears to have an additional chromosome compared to the distantly related psyllid species P. venusta due to a previous chromosomal fission or fusion event. The increase in genome size and dynamic nature of the B. cockerelli genome may largely be contributed to the widespread expansion of type I and II repeat elements that are rampant across all of B. cockerelli's. chromosomes. These repeat elements are distributed near equally in both euchromatic and heterochromatic regions. Furthermore, significant gene family expansions and gene duplications were uncovered for genes that are expected to be important in its adaptation to insect-plant and microbe interactions, which include transcription factors, proteases, odorant receptors, and horizontally transferred genes that are involved in the nutritional symbioses with their long-term nutritional endosymbiont Carsonella.