Developments in fungal taxonomy

Abstract

Fungal infections, especially those caused by opportunistic species, have become substantially more common in recent decades. Numerous species cause human infections, and several new human pathogens are discovered yearly. This situation has created an increasing interest in fungal taxonomy and has led to the development of new methods and approaches to fungal biosystematics which have promoted important practical advances in identification procedures. However, the significance of some data provided by the new approaches is still unclear, and results drawn from such studies may even increase nomenclatural confusion. Analyses of rRNA and rDNA sequences constitute an important complement of the morphological criteria needed to allow clinical fungi to be more easily identified and placed on a single phylogenetic tree. Most of the pathogenic fungi so far described belong to the kingdom Fungi; two belong to the kingdom Chromista. Within the Fungi, they are distributed in three phyla and in 15 orders (Pneumocystidales, Saccharomycetales, Dothideales, Sordariales, Onygenales, Eurotiales, Hypocreales, Ophiostomatales, Microascales, Tremellales, Poriales, Stereales, Agaricales, Schizophyllales, and Ustilaginales).

FIG. 1

Phylogenetic tree showing relationships of eukaryotes, based on the nucleotide sequence of 18S DNA. Reprinted from reference with permission of the publisher.

FIG. 2

Diagnostic features of the fungal phyla of clinical relevance. Zygomycota: a, coenocytic hypha; b, zygospore; c, sporangiophore; d, sporangiospores. Basidiomycota: e, basidiomata; f, basidium; g, basidiospores; h, hypha with clamp connections. Ascomycota: i, ascomata; j, ascus; k, ascospores; l, septate hypha. Deuteromycetes: m, pycnidium; n, conidiophore; o, conidiogenous cells; p, conidia. Oomycota: q, zoospore; r, gametangia; s, oospores.

FIG. 3

Typical characteristics of zygomycetes. a, mycelial colony; b, zygospore; c, gametangium; d, sporangium; e, sporangiole; f, merosporangia; g, columella; h, sporangiospores; i, merospores; j, stolon; k, rhizoids; l, coenocytic hypha; m, section of a hypha; n, sporangiophore.

FIG. 4

Typical characteristics of ascomycetes. a, mycelial colony; b, yeast colonies; c, perithecium; d, cleistothecium; e, ascus; f, unitunicate asci; g, bitunicate ascus; h to m, anamorph types; n, conidiophore; o, conidiogenous cell; p, hyphae; q, detail of a uniporate septum; r, detail of a multiporate septum.

FIG. 5

Diagnostic unrooted phylogenetic tree showing the relative positions of fungal clades of clinical importance by 18S rRNA sequences.

FIG. 6

Typical characteristics of basidiomycetes. a, mycelial colony; b, yeast colony; c, basidiomata; d, section of a basidiomata; e, detail of structure d; f, basidium; g, sterigma; h, basidiospores; i to l, different anamorph types; m, ballistoconidium; n, hypha with clamp connections (structure o); p, detail of structure n with a dolipore septum.