Biological Species in Pleurotus
RONALD H. PETERSEN, KAREN W. HUGHES, AND NADEZHDA PSURTSEVA
Phylogenetic Reconstructions
Phylogenetic reconstructions based on DNA sequences are based on the premise that the sequence of base pairs in DNA is altered by mutation (i.e. that every now and then, one base is exchanged for another, or one or more bases are deleted, repeated or otherwise altered). Alteration of the bases is known to take place at different rates,in different parts of the genome. Where DNA is actively coding for RNA product, change in the DNA is thought to be rare and surely selected against, because DNA uniformity is essential for the RNA product to be repetitively identical. Such lengths of DNA are termed "conserved," and are best used for phylogenetic reconstruction where organisms are not closely related (i.e. intergeneric phylogenies). When taxa are closely related, a more mutable length of DNA is better used where mutations are not so strongly selected against.
A second characteristic of molecular phylogenetic reconstructions is the choice of the "out group" or "root" for the phylogeny. Although phylogenies can be produced which show only the relationships of representatives within the group to each other (termed an "unrooted tree"), it is often thought wise to compare those relationships to an "out group," usually chosen for its close relationship (usually via morphotaxonomy) to the group in question. Choice of the out group can significantly alter positions of OTUs (i.e. operational taxonomic units) within the phylogeny.
Intending to show the relationship of Pleurotus to out groups Hohenbuehelia and Resupinatus, Vilgalys et al (1996) used a sequence within a gene (large subunit ribosomal RNA) to generate an intergeneric phylogenetic reconstruction. There they could identify several clades, but especially clear were the "P. ostreatus clade," the "P. djamor clade," and the "P. cystidiosus clade." All of Pleurotus was exclusive of the out groups.
A favorite DNA sequence for within-genus phylogenies is the length of DNA which encompasses an ITS as well as a gene (5.8S ribosomal RNA). An ITS sequence phylogeny within Pleurotus showed that the P. ostreatus complex was internally coherent, but related to the P. pulmonarius clade (including some representatives under the name P. sajor-caju), and that P. levis was only relatively distantly related (Vilgalys, et al., 1996).
See http://www.biology.duke.edu/fungi/mycolab/primers.htm for primers and a map of the ribosomal gene repeat.
Other phylogenetic reconstructions (M. Binder, Regensburg; R.G. Thorn, Univ. Wyoming; both pers. comm.) confirm the infrageneric coherence of Pleurotus, and recognition of morphological complexes as congruent to molecular phylogenetic clades.
Hibbett and Vilgalys (1993) postulated that Pleurotus was more closely related to agarics in the family Tricholomataceae than it was to the polypores (Singer, 1986), based on dimitic tissue construction in some species of Pleurotus). Still other phylogenies group Pleurotus with Hohenbuehelia on the one hand, but relatively close to Omphalotus, Nothopanus ss Singer, and Lampteromyces. The latter three genera are characterized by bioluminescence of basidiomata (Moncalvo et al. 2002. One hundred and seventeen clades of agarics. Molecular phylogenetics and evolution 23:357-400).