Abstract
Amylostereum areolatum is the symbiotic fungus of the Eurasian woodwasp, Sirex noctilio, a globally invasive species. The mutualistic symbiont is associated with the woodwasp, assisting the damage process and providing nutrition for its insect partners. Colonization and growth of A. areolatum have essential impacts on the development and spread of S. noctilio, though the mechanism of interaction between the two has been poorly described. In this study, the first genome of this symbiotic fungus was sequenced, assembled, and annotated. The assembled A. areolatum genome was 57.5 Mb (54.51% GC content) with 15,611 protein-coding genes. We identified 580 carbohydrate-active enzymes (CAZymes), 661 genes associated with pathogen-host interactions, and 318 genes encoding transport proteins in total. The genome annotation revealed 10 terpene/phytoene synthases responsible for terpenoid biosynthesis, which could be classified into three clades. Terpene synthase gene clusters in clade II were conserved well across Russulales. In this cluster, genes encoding mevalonate kinase (MK), EGR12 (COG1557), and nonplant terpene cyclases (cd00687) were the known biosynthesis and regulatory genes. Genome sequence analysis of this fungus would prove the possibility of A. areolatum volatiles affecting the host selection of S. noctilio on a molecular basis. We further clarified that A. areolatum was a strict obligate symbiotic fungus. The wasps might protect the fungus before it was introduced into a suitable host substrate by oviposition, while the fungus would provide S. noctilio with a suitable environment and nutrients for the larval growth. These results would lay a foundation for our understanding of the mechanism of this entomogenous symbiosis.IMPORTANCESirex noctilio (F.), together with Amylostereum areolatum, a wood-decaying symbiotic fungus, causes severe damage to Pinus species worldwide. In China, it causes extensive death of Mongolian pine (Pinus sylvestris var. mongolica). There is an obligate dependency mutualism between the woodwasp and its fungus. Studies have suggested that the fungal growth rate affected the size of the wasps: larger adults emerged from sites with a higher fungus growth rate. This genome is the first reported genome sequence of a woodwasp symbiotic fungus. Genome sequence analysis of this fungus would prove the possibility of A. areolatum volatiles affecting the host selection of S. noctilio on a molecular basis. We further clarified that A. areolatum was a strict obligate symbiotic fungus and that it would provide S. noctilio with a suitable environment and with nutrients for the larval growth. These results would lay a foundation for our understanding of the mechanism of this entomogenous symbiosis.