Phylogenetic and biogeographic relationships of eastern Asian and eastern North American disjunct Suillus species (fungi) as inferred from nuclear ribosomal RNA ITS sequences.

Species of Suillus produce fleshy, pored mushrooms. They are important symbiotic (ectomycorrhizal) partners of many coniferous trees. The genus includes several putative eastern Asian and eastern North American disjunct species, i.e., the S. americanus-S. sibiricus and S. decipiens-S. spraguei complexes. Phylogenetic relationships among the groups were determined to further understand the biogeographic pattern. Analyses were based on 40 sequences of the ITS region of the nuclear ribosomal RNA tandem repeats, representing 18 distinct species/populations. Our phylogenetic analyses suggested that: (1) Chinese and United States' (U.S.) S. spraguei plus S. decipiens form a strongly supported monophyletic group, with North American S. decipiens and Chinese S. spraguei being sister taxa; (2) S. americanus, Asian and U.S. S. sibiricus, plus S. umbonatus form a clade supported by a high bootstrap value; and (3) little ITS sequence divergence exists within the latter group compared to the S. decipiens-S. spraguei clade. Phylogenetic patterns revealed by this study imply a close phylogenetic relationship between eastern Asian and eastern North American disjunct population/species of Suillus. These fungi display relatively high host fidelity (at least to the host subgenus level), suggesting potential coevolutionary/comigratory trends.