Recent Developments Taxonomy and Phytogeny

Historically, members of the kingdom Fungi (Eumycota) are recognized in four phyla: Ascomycota, Basidiomycota, Chytridiomycota, and Zygomycota. In contrast to Ascomycota and Basidiomycota, which form regularly septate mycelia, most members of the phylum Zygomycota are characterized by possessing coeno-cytic mycelia (lacking cell septa). The sexual reproduction is by formation of highly resistant zygotes after fusion of isogamic sex organs and the asexual reproduction is by nonmotile spores (de Hoog & Guarro, 2000).

The phylum Zygomycota consists of two classes, the Trichomycetes (associated within living arthropods) and Zygomycetes (common in nature and surviving on decaying vegetation). Ten orders of Zygomycetes with diverse morphology and ecology have been described in the recent edition of the Dictionary of the Fungi (Kirk et al., 2001). These include saprobes obligate parasite of insects or microorganisms, endocommensals in crustacean guts and ecto- or endo-mycorrhizae. The ten orders of Zygomycetes are composed of 32 families, 124 genera, and 870 species. Organisms of clinical significance are mostly included in three orders, the group of Entomophtorales (having spores forcibly discharged), Mucorales and Mortierellales (spores liberated after cleavage of sporangial plasma). Therefore, species implicated in pathology are included in the Mucorales within the Mucoraceae (18 species covering genera such as Absidia, Apophysomyces, Chlamydoabsidia, Mucor, Rhizomucor, and Rhizopus), the Thamnidiaceae (Cokeromyces recurvatus), the Cunninghamellaceae (Cunninghamella bertholletiae), the Syncephalastraceae (Syncephalastrum racemosum), and the Saksenaeaceae (Saksenaea vasiformis); and in the Mortierellales within the family Mortierellaceae (2 species) (de Hoog & Guarro, 2000).

Although the taxonomy of the Zygomycetes is still mainly based on morphology, molecular phylogenetic data have contributed to the understanding of the evolution and taxonomy of the group. Over the past years, phylogenetic interactions among members of the order of Mucorales and Entomophtorales have been studied by sequencing ribosomal DNA (rDNA) genes as well as actin elongation factor of translation EF-1 genes.

An 18S and 28S rDNA phylogeny of clinically relevant species of 42 Zygomycetes was provided by Voigt et al. (1999). The analysis of the 18S data indicated that Absidia corymbifera was misplaced taxonomically. The genus Absidia harbors species producing small, apophysate sporangia with deliquescent walls, the production of stolons and rhizoids and appendage zygospores suspen-sors. Instead, A. corymbifera possess non-appendaged suspensors. Results of the molecular phylogeny also supported the transfer of Rhizomucor variabilis to Mucor. This species seemed to be more closely related phylogenetically to Mucor mucedo and Mucor hiemalis. Species of Rhizomucor are thermophilic and form simple or weakly branched rhizoids from hyphae, stolons, and sporangia while R. variabilis fails to do this.

For a better resolution of closely related species of some genera the same group (Voigt & Wostemeyer, 2001) used intron-containing nuclear genes (actin and translation elongation factor EF-1) to evaluate the phylogenetic positions of the Mucorales and Mortierellales. Results of this study indicated that a taxonomic revision of these orders is necessary.

A large-scale phylogenetic analysis of the Mucorales was carried out by O'Donnell and collaborators (2001). The analysis was based on partial nucleotide sequences from 18S and 28S rDNA genes together with elongation factor-1 exons and morphological data. Relationships within 13 morphologically defined families (including 54 genera and 63 species) were investigated. The results suggested that several of the largest families (Thamnidiaceae, Mucoraceae, and Chaetocladiaceae) as well as the genera Absidia and Mucor were polyphyletic.

Rhizopus species are classically divided by phenotypical criteria into three groups: the microsporus-, oryzae-, and stolonifer-group. A recent study investigated the molecular phylogeny of this genus using rDNA sequencing (18S, ITS, and 28S regions). The results indicated that phylogenetic relationships were similar to the morphological grouping, except for R. schipperae, a newly described human pathogen belonging to the microsporus group and R. stolonifer var. lyococcos, a nonpathogenic species. The microsporus-, oryzae-, and stolonifer-groups were supported by the analysis of 18S, ITS, and 28S sequences. Nevertheless, these molecular data placed R. schipperae distantly from the other members of this group. Similarly, R. stolonifer var. lyococcos clustered independently from R. stolonifer var. stolonifer in all trees suggesting an eventual reclassification as an independent species. These genetic data are not in accordance with the phenotypic taxonomy because these two varieties differ only by the morphology of the sporiangophore and are sexually compatible indicating that they should belong to the same species (Abe et al., 2006).

The evolutionary history of Zygomycetes and the interrelations among the different orders continue to be investigated by various groups which will contribute to a better knowledge of the organisms.

Cure Your Yeast Infection For Good

Cure Your Yeast Infection For Good

The term vaginitis is one that is applied to any inflammation or infection of the vagina, and there are many different conditions that are categorized together under this ‘broad’ heading, including bacterial vaginosis, trichomoniasis and non-infectious vaginitis.

Get My Free Ebook


Post a comment