Shiitake mushrooms and molecular clocks: historical biogeography of Lentinula
Fungi make up a large, ecologically important group of eukaryotes that has been neglected in historical biogeography. In this study, molecular phylogenies and molecular clock dating were used to infer historical biogeography of the shiitake genus, Lentinula Earle, which occurs in both the Old World and New World. The major goals of this study were to determine if the Old World and New World groups are monophyletic, and whether the Old World/New World disjunction is the result of vicariance or long distance dispersal.Location
Collections studied are from Asia, Australasia, the Gulf Coast region of North America, the Caribbean, Central America, and northern South America.Methods
Phylogenetic trees were inferred with nuclear large subunit (nuc-Isu) ribosomal DNA (rDNA) and internal transcribed spacer (ITS) sequences. Molecular clock dating was performed using trees derived from nuc-Isu rDNA and mitochondrial small-subunit (mt-ssu) rDNA sequences. Rate constancy was tested with maximum likelihood, and clocks were calibrated using evidence from fossils or other molecular clock studies, or according to expected ages under different historical biogeographic scenarios.Results
Analyses of ITS and nuc-Isu rDNA sequences suggest that there are seven species of Lentinula worldwide, which occur in two main clades, one in the New World the other in the Old World. Rate constancy cannot be rejected in nuc-Isu rDNA, but can be rejected in mt-ssu rDNA. Both genes suggest that the Old World/New World disjunction could be due to fragmentation of an ancient Laurasian range. An alternative Gondwanan hypothesis is not supported by the molecular clock age estimates.Main conclusions
Only one long distance dispersal event must be invoked in Lentinula, that being between Australia and New Zealand. Despite having airborne spores, it appears that long distance dispersal is rare in Lentinula. This may also be true in other fungi, which are therefore excellent candidates for historical biogeographic studies using molecular characters.