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United States Department of Agriculture

Agricultural Research Service

Title: Phylogenetic Relationships among the Harpellales (Trichomycetes) and Kickxellales (Zygomycetes), Two Orders of Zygomycota That Form Septate Hyphae

Authors
item O`DONNELL, KERRY
item Cigelnik, Elizabeth
item Benny, Gerald - UNIV OF FLORIDA

Submitted to: Mycologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 7, 1998
Publication Date: N/A

Interpretive Summary: Classification of fungal groups has been based primarily on morphological characters, especially those of the spore-bearing (reproductive) structures that display a diverse array of morphologies. All fungi that produce nonmotile spores within an enclosed spore case have been grouped within the Zygomycota which contains two classes, the Zygomycetes and Trichomycetes. Fungi within these classes include serious plant, animal and human pathogens, as well as industrially important species used for food and the fermentation of various agriculturally derived chemicals. Efforts to develop a natural classification of these fungi have proven difficult because of their morphological simplicity. The purpose of this study was to investigate the proposed genetic affinity of the Harpellales (Trichomycetes) and Kickxellales (Zygomycetes) which is based on shared anatomical features of their cell wall septa and their spore-bearing structures. Results of the molecular genetic and morphological analyses identified three genetically distinct groups: the Harpellales, Spiromyces, and a group containing most of Kickxellales. A more general conclusion from this study is that a molecular genetic approach is essential to develop a natural classification of the fungi that can be used to predict, for example, the production of industrially important compounds by these medically and agronomically important fungi.

Technical 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 nonmolecular 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.

Last Modified: 8/27/2014
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