Putative origin of clonal lineages of Amylostereum areolatum, the fungal symbiont associated with Sirex noctilio, retrieved from Pinus sylvestris, in eastern Canada

The Eurasian Sirex noctilio–Amylostereum areolatum complex was discovered and has become established close to the North American Great Lakes in the 2000s. This invasive forest insect pest represents a very high risk to native and exotic pines in North America. We investigated the geographical origin of clonal lineages of the fungal symbiont A. areolatum in the recently pest-colonized eastern Canadian region by analyzing mitochondrial and nuclear sequence variations and comparing the genetic diversity of a worldwide collection of fungal symbionts among six countries where the Sirex complex is native and four countries from which the insect–fungal complex has been introduced. In total, 102 isolates were analyzed. While 12 multilocus genotypes (MLGs) are observed in the areas where S. noctilio is native, only two MLGs are retrieved from areas where S. noctilio is not native, indicating the wide spread of clonal lineages in the introduced fungal symbiont of S. noctilio. MLG2 comprises 26 % of the Canadian isolates and is also observed in Chile and South Africa, where the insect–fungal complex has also been introduced. MLG3 comprises 74 % of the Canadian isolates and is also observed in the USA, but nowhere else in our worldwide collection. Thus, at least one of the Canadian clonal lineages shares a common origin with A. areolatum isolates from the Southern Hemisphere. The source of the second clonal lineage is still unknown, but phylogenetic analyses show that MLG3 is isolated. More extended sampling is necessary to determine the origin of this fungal clonal lineage and investigate its probable symbiotic association with native North American Sirex.

► The fungus Amylostereum areolatum is widely spread by its symbiont, the Sirex woodwasp.
► Two unique fungal multilocus genotypes were found in areas where the pest complex has been introduced.
► The predominant clonal lineage in Canada is also observed in the USA but nowhere else.
► The other Canadian clonal lineage shares a common origin with those from the Southern Hemisphere.