The mutualism between siricid woodwasps and Amylostereum fungal symbionts has
long been considered to be species-specific. Recent studies from North America
have challenged this assumption, where native siricids and the introduced Sirex
noctilio are clearly swapping symbionts. Whether this pattern is a consequence of
invasion or an underappreciated property of siricid biology is unknown. Here we
show that the native Japanese siricid, Sirex nitobei, carries both Amylostereum
areolatum and Amylostereum chailletii, rather than only A. areolatum as long
assumed. Furthermore, all samples from a Urocerus sp. unexpectedly carried, A.
chailletii and not Amylostereum laevigatum. Vegetative compatibility group tests
revealed extensive clonality, with one VCG present amongst three A. areolatum
isolates and six VCGs present amongst 61 A. chailletii isolates. These results
contribute to the understanding of insect-fungal fidelity in the siricid-Amylostereum
association and, together with other studies, suggest that host tree influences
Amylostereum species occurrence, perhaps more strongly than wasps.