Adaptation during the transition fromOphiocordycepsentomopathogen to insect associate is accompanied by gene loss and intensified selection

Abstract

AbstractFungal and bacterial symbiosis is an important adaptation that has occurred within many insect species, which usually results in the relaxation of selection across the symbiont genome. However, the evolutionary pressures and genomic consequences associated with this transition are not well understood. Pathogenic fungi of the genusOphiocordycepshave undergone multiple, independent transitions from pathogen to associate, infecting soft-scale insects trans-generationally without killing them. To gain an understanding of the genomic adaptations underlying this transition, long-read sequencing was utilized to assemble the genomes of bothParthenolecanium corniand itsOphiocordycepsassociate from a single insect. A highly contiguous haploid assembly was obtained forPart. corni, representing the first assembly from a single Coccoidea insect, in which 97% of its 227.8 Mb genome was contained within 24 contigs. Metagenomic-based binning produced a chromosome-level genome forPart. corni’sOphiocordycepsassociate. The associate genome contained 524 gene loss events compared to free-living pathogenicOphiocordycepsrelatives, with predicted roles in hyphal growth, cell wall integrity, metabolism, gene regulation and toxin production. Contrasting patterns of selection were observed between the nuclear and mitochondrial genomes specific to the associate lineage. Intensified selection was most frequently observed across nuclear orthologs, while selection on mitochondrial genes was found to be relaxed. Furthermore, scans for diversifying selection identified associate specific selection within three adjacent enzymes catalyzing acetoacetate’s metabolism to acetyl-COA. This work provides insight into the adaptive landscape during the transition to an associate life history, along with a base for future research into the genomic mechanisms underpinning the evolution ofOphiocordyceps.