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Abstract
DNA sequences of the nuclear encoded small subunit (SSU) 18S ribosomal RNA gene and characters from a morphological/physiological dataset were analyzed by maximum parsimony to investigate phylogenetic relationships among and within two putative sister orders of Zygomycota that produce regularly septate hyphae with ultrastructurally similar plugged, flared septal pores, the Harpellales and Kickxellales. The most parsimonious trees (MPT) inferred from the molecular data, using Chytridium-Spizellomyces and Rhopalomyces as outgroups in separate analyses, were largely concordant. Cladistic analysis of nucleotide characters identified three strongly supported monophyletic lineages that formed an unresolved trichotomy in the 18S rDNA gene trees: the Harpellales, Kickxellales and a Spiromyces clade. Trees inferred from the nonmolecular data were largely unresolved and generally poorly supported by bootstrap and decay analyses. However, results of a partition-homogeneity test detected no positive conflict between the morphological and molecular datasets, indicating that they could be combined. Morphological evolution was investigated by optimizing non-molecular characters on the 18S rDNA gene tree. Systematic results support the recognition of a Spiromyces clade independent of the Kickxellales where it is currently classified and the apparent paraphyly of Coemansia with Spirodactylon derived within it. A formal diagnosis of Spiromyces aspiralis sp. nov. is provided together with an emendation of the genus.