Fidelity Among Sirex Woodwasps and Their Fungal Symbionts

Abstract

We report that associations between mutualistic fungi and their economically and ecologically important woodwasp hosts are not always specific as was previously assumed. Woodwasps in the genus Sirex engage in obligate nutritional ectosymbioses with two species of Amylostereum, a homobasid\iomycete genus of white rot fungi. In the present study, the Amylostereum species and genotypes associated with three species of Sirex native to eastern North America and one relatively recent invasive Sirex from Europe were investigated by comparing intergenic spacer regions (IGS). Sirex spp. were sampled over 6 years from 23 sites in six US states, ranging from Maine in the northeast to Louisiana in the southeast, to obtain samples of Amylostereum from mycangia of adult females. Two of the native Sirex species (Sirex nigricornis and Sirex nitidus) were associated with either Amylostereum chailletii or Amylostereum areolatum, refuting the hypothesis of strict species-specific relationships. However, the invasive Sirex noctilio and the native Sirex cyaneus were each collected with only A. areolatum or A. chailletii, respectively, although S. noctilio was associated with two different IGS genotypes of A. areolatum and S. cyaneus occurs sympatrically with the other native Sirex. In Pinus, the preferred host tree of S. nigricornis and S. noctilio, these species co-occurred in 25.9 % of trees sampled, and horizontal transmission of fungal strains from S. noctilio to S. nigricornis was documented, although only in one tree. The extent that further spread and establishment of S. noctilio will alter the composition of symbionts carried by native Sirex is unknown but will depend in part on the degree of flexibility in these host–symbiont associations.

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

“…This skewed directionality of transfer could result from temporal patterns in oviposition and emergence, or disproportionate utilization of S. noctilio-weakened trees by S. nigricornis. The lack of host -symbiont fidelity detected in this study, also shown in an independent concurrent study [19], calls into question the mechanisms maintaining the apparent fidelity of symbiont associations in the native range of S. noctilio. It is possible that geographical, host or temporal segregation among native siricids may be sufficient to maintain the low rates of transfer observed in Europe and elsewhere [5].…”

“…These findings suggest that this genotype could have been introduced from a previously unsampled S. noctilio source population, as identified by a recent analysis of a global S. noctilio collection [22], or that ENA harbours an unsampled native A. areolatum population. A concurrent study of woodwasp-fungal fidelity in eastern USA revealed that two native woodwasps, S. nigricornis and S. nitidus, carried A. areolatum in their mycangia [19]. The majority of these fungal isolates were shown to contain IGS type BE, which was identified in MLG2 in this study.…”

“…Both MLG1 and MLG3 corresponded with previously isolated samples (MLG3 and MLG2 in Bergeron et al [18], respectively). MLG2 was unique in this study, although ITS (MLG2a and MLG2b) and IGS sequences (MLG2a) have been previously isolated [8,19].…”

Diversity, population genetics, and evolution of macrofungi associated with animals

“…Comparisons between the phylogenies of the woodwasps and their associated Amylostereum fungi indicated little species-specific relationships between the two interacting partners (Hajek et al 2013). For example, A. areolatum can be carried by various wasp species, while Sirex nigricornis and Sirex nitidus can be found associated with either A. chailletii or A. areolatum (Gaut 1970; Hajek et al 2013).…”

“…For example, A. areolatum can be carried by various wasp species, while Sirex nigricornis and Sirex nitidus can be found associated with either A. chailletii or A. areolatum (Gaut 1970; Hajek et al 2013). Therefore, the Amylostereum fungi have shown themselves capable of finding new insect partners in various conditions.…”