Intergenic Spacer Single Nucleotide Polymorphisms for Genotyping Amylostereum areolatum (Russulales: Amylostereacea) Symbionts of Native and Non-native Sirex Species

In North America Amylostereum areolatum (Chaillet ex Fr.) Boidin is a fungal symbiont associated with both the non-native Sirex noctilio Fabricius (Hymenoptera: Siricidae) and less commonly the native Sirex nigricornis Fabricius (Hymenoptera: Siricidae) woodwasps. The relationship between S. noctilio and A. areolatum constitutes a serious threat to pine plantation in the southern hemisphere. Studies have shown evidence of exchange of symbionts between non-native and native Sirex species. Our objectives were 1) to identify and assemble a panel of rDNA intergenic spacer–single nucleotide polymorphisms (IGS-SNPs) for genotyping strains of A. areolatum symbionts associated with Sirex species in North America, and 2) to develop genetic markers for monitoring the spread of specific A. areolatum haplotypes associated with S. noctilio across regions. The IGS-SNPs panel analyzed included haplotypes B1, B2, D1, D2 (from known IGS type B and D), E, and F. Genetic markers and haplotype-specific primers were designed to detect the IGS haplotypes D and E of A. areolatum. We found that haplotype D was absent in A. areolatum from S. nigricornis in Louisiana, while haplotype E was detected in all A. areolatum from S. nigricornis in Canada and Louisiana. Both haplotype D and E were co-detected in approximately 5% of samples from Canada. The IGS-SNP markers detected specific haplotypes accurately. Observing haplotype D in any A. areolatum from the native S. nigricornis likely indicates the presence of the potentially harmful S. noctilo-A. areolatum complex. The work highlights how IGS-SNPs can help in early detection without direct occurrence/observations of the non-native species of concern.

Molecular identification of Deladenus proximus Bedding, 1974 (Tylenchida: Neotylenchidae), a parasite of Sirex nigricornis (Hymenoptera: Siricidae)

Parasitic nematodes were isolated from Sirex nigricornis (Hymenoptera: Siricidae) females collected in baited traps across pine forests of Arkansas and Mississippi, USA. We examined 650-720 bp of cytochrome c oxidase subunit I and compared Arkansas and Mississippi sequences to sequences from nematodes collected in Illinois, Louisiana and New York that were positively identified as Deladenus proximus. We propose that a single species, D. proximus, occurs across Arkansas and Mississippi. Moreover, all sequences, which spanned a large geographic area (Louisiana to New York), exhibited only minor genetic variation (mean genetic distance of 0.004). This suggests that all examined specimens are a single species and could suggest continuous gene flow across eastern North America. Additional genes should be examined to discuss more fully the prospect of continuous gene flow.

Lack of fidelity revealed in an insect–fungal mutualism after invasion

Symbiont fidelity is an important mechanism in the evolution and stability of mutualisms. Strict fidelity has been assumed for the obligate mutualism between Sirex woodwasps and their mutualistic Amylostereum fungi. This assumption has been challenged in North America where the European woodwasp, Sirex noctilio , and its fungal mutualist, Amylostereum areolatum , have recently been introduced. We investigate the specificity of the mutualism between Sirex and Amylostereum species in Canada, where S. noctilio co-infests Pinus with native Sirex nigricornis and its mutualist, Amylostereum chailletii . Using phylogenetic and culture methods, we show that extensive, reciprocal exchange of fungal species and strains is occurring, with 75.3 per cent of S. nigricornis carrying A. areolatum and 3.5 per cent of S. noctilio carrying A. chailletii . These findings show that the apparent specificity of the mutualism between Sirex spp. and their associated Amylostereum spp. is not the result of specific biological mechanisms that maintain symbiont fidelity. Rather, partner switching may be common when shifting geographical distributions driven by ecological or anthropogenic forces bring host and mutualist pairs into sympatry. Such novel associations have potentially profound consequences for fitness and virulence. Symbiont sharing, if it occurs commonly, may represent an important but overlooked mechanism of community change linked to biological invasions.

Sirex species (Hymenoptera: Siricidae) and their parasitoids in Pinus sylvestris in eastern North America

Siricids and their parasitoids were reared from Scots pine (Pinus sylvestris L. (Pinaceae)) trees infested by Sirex noctilio F. in central New York State. Sirex noctilio accounted for 94.3% of the siricid specimens emerging, totaling 1313 specimens from six trees, with a maximum of 495 from one tree. Of the individuals emerging per tree, 20.6 ± 5.2% were female. Two native siricids, Sirex nigricornis F. and S. edwardsii Brullè, also emerged from trees but in low numbers. Three hymenopteran parasitoid species that attack siricids emerged, totaling 21.8 ± 6.4% parasitism per tree. Ibalia leucospoides ensiger Norton (Ibaliidae) was by far the most abundant parasitoid, at 20.5 ± 6.3% parasitism per tree. The percentage of female S. noctilio emerging was positively correlated with wood diameter, whereas percent parasitism by I. l. ensiger was negatively correlated with wood diameter.