Molecular phylogeny of Melanospora and Diaporthales, and population genetics of dogwood anthracnose fungus

Unresolved phylogenetic problems in pyrenomycetes and population structure of an asexual plant pathogen (Discula destructiva) were investigated. Analyses based on DNA sequences of nuclear encoded small and large subunit RNA genes (SSU and LSU nrDNA) and deduced amino acid sequences of a subunit of RNA polymerase II gene (RPB2) indicated that five genera previously considered in Ceratostomataceae of Sordariales were related to Hypocreales. Melanospora (including the type) and a genus of Ceratostomataceae formed a basal clade, but monophyly of the five genera and of the genus Melanospora was rejected. Discula destructiva and four other Discula species were derived from within Diaporthales according to the SSU and LSU nrDNA and RPB2 phylogenies. There were three clades in the LSU nrDNA phylogeny. The five Discula species were in one of the clades but they were not monophyletic. This delimitation is congruent with anamorph and pigmentation distributions in the diaporthalean taxa but is not congruent with family concepts based on other phenotypic characters. Taxa of Magnaporthaceae were excluded from the Diaporthales, but its phylogenetic position remains unresolved. Preliminary studies of the evolutionary mechanisms of D. destructiva involved investigation of its population structure. Two distinct groups of D. destructiva isolates, one from the western U.S. and the other from the eastern U.S., were identified with amplified fragment length polymorphism (AFLP) markers and sequences of several genes. Discula destructiva is thought to have been introduced to North America in late 1970s. The remarkably low genetic diversity compared to other asexual fungi indicated that D. destructiva is still under intense selection pressure and that episodic selection may still be in effect. However, the transition to a less virulent, heterogeneous population might have begun in the New York City area, a possible epidemic center in the east, which had relatively higher genetic diversity than the samples from other areas.