The slippery nature of ectomycorrhizal host specificity: Suillus fungi associated with novel pinoid (Picea) and abietoid (Abies) hosts.

Suillus is among the best-known examples of an ectomycorrhizal (ECM) fungal genus that demonstrates a high degree of host specificity. Currently recognized host genera of Suillus include Larix, Pinus, and Pseudotsuga, which all belong to the pinoid clade of the family Pinaceae. Intriguingly, Suillus sporocarps have been sporadically collected in forests in which known hosts from these genera are locally absent. To determine the capacity of Suillus to associate with alternative hosts in both the pinoid and abietoid clades of Pinaceae, we examined the host associations of two Suillus species (S. punctatipes and S. glandulosus) through field-based root tip sampling and seedling bioassays. Root tip collections underneath Suillus sporocarps were molecularly identified (fungi: nuc rDNA internal transcribed spacer region ITS1-5.8S-ITS2 [ITS barcode]; plant: trnL) to assess the association with multiple hosts. The bioassays contained both single- and two-species treatments, including a primary (Larix or Pseudotsuga) and a secondary (Picea, Pinus, or Abies) host. For the S. punctatipes bioassay, an additional treatment in which the primary host was removed after 8 mo was included to assess the effect of primary host presence on longer-term ECM colonization. The field-based results confirmed that Suillus fungi were able to associate with Abies and Tsuga hosts, representing novel host genera for this genus. In the bioassays, colonization on the primary hosts was detected in both single- and two-species treatments, but no colonization was present when Picea and Abies hosts were grown alone. Removal of a primary host had no effect on percent ECM colonization, suggesting that primary hosts are not necessary for sustaining Suillus colonization once they are successfully established on secondary hosts. Collectively, our results indicate that host specificity is more flexible in this genus than previously acknowledged and help to explain the presence of Suillus in forests where recognized hosts are not present.

Does fungal competitive ability explain host specificity or rarity in ectomycorrhizal symbioses?

Two common ecological assumptions are that host generalist and rare species are poorer competitors relative to host specialist and more abundant counterparts. While these assumptions have received considerable study in both plant and animals, how they apply to ectomycorrhizal fungi remains largely unknown. To investigate how interspecific competition may influence the anomalous host associations of the rare ectomycorrhizal generalist fungus, Suillus subaureus, we conducted a seedling bioassay. Pinus strobus seedlings were inoculated in single- or two-species treatments of three Suillus species: S. subaureus, S. americanus, and S. spraguei. After 4 and 8 months of growth, seedlings were harvested and scored for mycorrhizal colonization as well as dry biomass. At both time points, we found a clear competitive hierarchy among the three ectomycorrhizal fungal species: S. americanus > S. subaureus > S. spraguei, with the competitive inferior, S. spraguei, having significantly delayed colonization relative to S. americanus and S. subaureus. In the single-species treatments, we found no significant differences in the dry biomasses of P. strobus seedlings colonized by each Suillus species, suggesting none was a more effective plant symbiont. Taken together, these results indicate that the rarity and anomalous host associations exhibited by S. subaureus in natural settings are not driven by inherently poor competitive ability or host growth promotion, but that the timing of colonization is a key factor determining the outcome of ectomycorrhizal fungal competitive interactions.