Microbody-like organelles as taxonomic markers among Oomycetes

Zoospores of Oomycetes contain a variety of microbody-like organelles with highly structured matrices. Although in general their function is unknown, the appearance of similar organelles in related taxa suggests the ultrastructural differences could be used as taxonomic characters. This study surveys microbody-like organelles of oomycetous zoospores to determine if this is an additional criterion by which the phylogeny of these fungi can be evaluated. In zoospores of the order Saprolegniales, kinetosome-associated organelles (K-bodies) are found which typically consist of tubular and/or granular matrices. K-bodies are not found associated with kinetosomes in zoospores of the Peronosporales, but microbodies containing tubules, and in some genera marginal plates, are located near the kinetosomes, along the groove, and in other peripheral areas. K-bodies have been reported in only one member of the order Lagenidiales. These K-bodies lack a granular matrix, but contain a single curved plate from which tubules arise, forming a cone. In the one genus of the Leptomitales examined, a similar K-body contains a plate and scattered tubules. Organisms with similar microbody-like organelles are probably more closely related than those with different types of microbody-like organelles. The presence of an organelle resembling K-bodies in zoospores of an alga in the Tribophyceae supports the phylogenetic association between algae and Oomycetes. A complete survey of Oomycete genera may well reveal intermediates between the structurally different types of microbody-like organelles, allowing the reconstruction of the phylogenetic history of an organelle.