Searching for homologous ultrastructural characters in zoosporic fungi

One aim of ultrastructural studies of motile cells of Oomycetes and Chytridiomycetes, two groups of organisms recognized to have evolved along distinct lineages, has been to compile data bases of characters for reconstruction of phylogenetic hypotheses. Because little is known about the ontogeny, composition, and function of many structures that might be useful, assuming homology owing to similarity in ultrastructural form is problematic. In this paper we explore approaches to elucidating homologies between single membrane bounded organelles in zoospores. We use K-bodies of Oomycete zoospores as an example of an organelle for which ontogenic studies have revealed that certain morphological forms are analogous to other forms. Results of these studies demonstrate that as a morphological character K-body structure can be valuable among subgroups of Oomycetes, but convergence in structure makes it unreliable as a comparative character across the class. Although comparative morphology as an avenue to understanding phylogeny is sometimes challenged, this approach can provide valuable insights into processes involved in organellar evolution. Key words: Chytridiomycetes, Oomycetes, organelles, systematics, ultrastructure, zoospores.