Order Spizellomycetales was delineated based on a unique suite of zoospore ultrastructural characters and currently includes five genera and 14 validly published species, all of which have a propensity for soil habitats. We generated DNA sequences from small (SSU), large (LSU) and 5.8S ribosomal subunit genes to assess the monophyly of all genera and species in this order. The 53 cultures analyzed included isolates on which all described species were based, plus other spizellomycetalean cultures. Phylogenetic placement of these chytrids was explored with maximum parsimony and maximum likelihood analyses, both of which yielded comparable topologies. Kochiomyces, Powellomyces and Triparticalcar were monophyletic, while Gaertneriomyces and Spizellomyces were polyphyletic. Isolates, distinct from described species, clustered among each of the five genera, indicating that species diversity in genera is greater than currently recognized. One isolate formed a clade that included no described species, representing a new genus. Zoospore ultrastructural features and architecture seem to be good indicators of phylogenetic relationships, but finer scrutiny of characters such as kinetosome-associated structures (KAS) is needed to understand more clearly the diversity within this order as it is revised.
