Molecular phylogenetics of the Chytridiomycota supports the utility of ultrastructural data in chytrid systematics

Abstract

The chytrids (Chytridiomycota) are morphologically simple aquatic fungi that are unified by their possession of zoospores that typically have a single, posteriorly directed flagellum. This study addresses the systematics of the chytrids by generating a phylogeny of ribosomal DNA sequences coding for the small subunit gene of 54 chytrids, with emphasis on sampling the largest order, the Chytridiales. Selected chytrid sequences were also compared with sequences from Zygomycota, Ascomycota, and Basidiomycota to derive an overall fungal phylogeny. These analyses show that the Chytridiomycota is probably not a monophyletic group; the Blastocladiales cluster with the Zygomycota. Analyses did not resolve relationships among chytrid orders, or among clades within the Chytridiales, which suggests that the divergence times of these groups may be ancient. Four clades were well supported within the Chytridiales, and each of these clades was coincident with a group previously identified by possession of a common subtype of zoospore ultrastructure. In contrast, the analyses revealed homoplasy in several developmental and zoosporangial characters.Résumé : Les chytrides (Chytridiomycota) sont des champignons aquatiques morphologiquement simples qui se caractérisent par la présence de zoospores typiquement munies d'un unique flagelle dirigé vers l'arrière. Cette étude porte sur la systématique des chytrides en présentant une phylogénie basée sur les séquences de l'ADN ribosomale du gène codant pour la petite sous-unité, chez 54 chytrides, en mettant l'accent sur un échantillonage de l'ordre le plus important, les Chytridiales. Les auteurs ont également comparé des séquences sélectionnées de chytridiales avec des séquences provenant de Zygomycota, d'Ascomycota, et de Basidiomycota afin d'obtenir une phylogénie générale. Ces analyses montrent que le Chytridiomycota n'est probablement pas un groupe monophylétique; les Blastocladiales se regroupent avec le Zygomycota. L'analyse ne définit pas les relations entre les ordres de chytrides, ou entre les clades parmi les Chytridiales, ce qui suggère que les moments de divergences entre ces groupes pourraient être anciens. Quatre clades sont bien définis au sein des Chytridiales, et chacun de ces clades coïncide avec un groupe précédemment identifié par la possession d'un sous-type commun d'ultrastructures zoosporales. A l'opposé, les analyses révèlent la présence d'homoplasie chez plusieurs caractères au niveau du développement et des zoosporanges.

Hidden diversity among aquatic heterotrophic flagellates: ecological potentials of zoosporic fungi

“…2), which is typical of fungal zoospores (cf. James et al, 2000). Forms with two equal flagella such as some taxa of Cercozoa (Cavalier-Smith & Chao, 2003) were also present in relatively high number, as well as forms with two unequal flagella such as some stramenopiles or heterokonts (Fig.…”

Assessing the microsporidia-fungi relationship: Combined phylogenetic analysis of eight genes

“…Indeed, the AFTOL analyses placed the blastocladialean chytrid representative, Allomyces, within one of several otherwise zygomycete clades. This result is not unique: an analysis conducted on ribosomal DNA sequences from dozens of chytrids placed the blastocladialean chytrid fungi within a combined zygomycete/blastocladialean clade (James et al, 2000). Ideally, several representatives of both the zygomycetes and the chytrids would have been included in this study, especially given the somewhat tenuous position of the Blastocladiales within the fungal tree.…”

Molecular phylogeny of the Blastocladiomycota (Fungi) based on nuclear ribosomal DNA

“…With the rise of electron microscopy, classification shifted to emphasize zoospore ultrastructure (Fuller 1977;Barr 1980). The ultrastructure of zoospores, which is conserved and informative for defining the Blastocladiales and orders in the Chytridiomycetes (Fuller 1977;Barr 1978Barr , 1980Barr , 1981Powell 1978;Lange & Olson 1979;James et al 2000;Letcher et al 2006;Letcher et al 2008;Mozley-Standridge et al 2009;Simmons et al 2009;V elez et al in press) led to the transfer of the Physodermataceae from the Chytridiales to the Blastocladiales (Lange & Olson 1980b). Ultrastructural zoospore features also led to the classification of Sorochytriaceae within the Blastocladiales (Dewel et al 1985) and, along with molecular evidence led to placing Polycaryum laeve in the Blastocladiales (Johnson et al 2006).…”

“…Although earlier molecular analyses yielded uncertain results about the relationship of the Blastocladiales with other zoosporic fungi (James et al 2000), the Blastocladiales recently was reclassified from the Chytridiomycota to a new phylum, the Blastocladiomycota. The new phylum is based on a molecular rDNA phylogeny and ultrastructural characters (James et al 2006).…”