Phylogenetic studies of Saprolegniomycetidae and related groups based on nuclear large subunit ribosomal DNA sequences

To reveal phylogenetic relationships within the Peronosporomycetes (Oomycet es), we sequenced a part of the nuclear rDNA coding for the ribosomal large subunit of 46 Peronosporomycetes species and one representative of the Xan thophyta. The main emphasis of our study was put on the phylogenetic relati onships within the Saprolegniomycetidae. We supplemented our data with a se quence of Phytophthora megasperma Drechsler from GenBank. Two sets of seque nces were analysed using the neighbor-joining method, statistically support ed by the bootstrap method, as well as the maximum parsimony method. Our re sults are well compatible with the tripartite subclassification of the Pero nosporomycetes into Saprolegniomycetidae, Rhipidiomycetidae and Peronosporo mycetidae, as well as with the placement of the orders Saprolegniales and L eptomitales in the Saprolegniomycetidae. Pachymetra chaunorhiza Croft & Dic k, which has been placed in the Sclerosporales, was grouped within the Sapr olegniales. Within the Peronosporomycetidae, the orders Peronosporales and Pythiales could not be separated. There are indications that Phytophthora d e Bary and the Peronosporales form a common natural group. The genus Achlya Nees proved to be a heterogeneous group.