Phylogenetic position of Phlyctochytrium planicorne (Chytridiales, Chytridiomycota) based on zoospore ultrastructure and partial nuclear LSU rRNA gene sequence analysis

Abstract

The genus Phlyctochytrium (Chytridiales, Chytridiomycota) is an assemblage of approximately 70 described taxa that exhibit morphological, ultrastructural, and molecular diversity. Although the type species Phlyctochytrium hydrodictyi is not available for study, a similar taxon, Phlyctochytrium planicorne, has been brought into pure culture. Like the type, P. planicorne is an apophysate algal parasite. In this study zoospore ultrastructure and nuclear large subunit ribosomal RNA gene sequences are used to infer the phylogenetic position of P. planicorne among the Chytridiales. This is the first use of a Phlyctochytrium in a molecular phylogeny and the first Phlyctochytrium to exhibit the zoospore type herein described. The chytridialean zoospore of P. planicorne contains a rumposome, a bundle of 5-7 microtubules in a microtubular root connecting the rumposome to the kinetosome, an electron-opaque saddle over the kinetosome, an electronopaque plug in the base of the flagellar axoneme, and a paracrystalline inclusion in the peripheral cytoplasm; in these features it is similar to the zoospore described for Chytridium lagenaria. In a parsimony analysis, Phlyctochytrium planicorne is nested as a lone taxon in a polytomy including the Lacustromyces, Nowakowskiella, and Rhizophydium clades. Because there are so many species of Phlyctochytrium for which the ultrastructure and molecular characters are unknown, further studies are necessary to determine whether the genus as described is monophyletic.