Straminipile “Fungi” – Taxonomy

Abstract

The phylogeny and taxonomy of the mastigonate ‘fungal‐like’ organisms (stramenopiles) are described. The marine Labyrinthulomycota occur in the same stramenopile clade as the bioecids and opalinids, whilst the hyphochytrids and biflagellate oomycetes occur in a sister clade, together with the ochrophyte algae. The uniflagellate hyphochytrids are a small group whose closest relative appears to be the phagotrophic flagellate . The largest and best documented of the straminipile fungi are the oomycetes. There are a number of early diverging oomycete clades (Classes incertae sedis) consisting of mostly holocarpic marine species that are parasites of algae and invertebrates. The more advanced, largely freshwater and terrestrial fungal‐like oomycetes, fall into two main classes, the Saprolegniomycetes and Peronosporomycetes. The former largely encompasses the saprotrophic water moulds, whilst the latter contains the majority of the economically important plant pathogens, including the white blister rusts (Albuginales), and the downy mildews (Peronosporales).

Key Concepts

  • Molecular phylogenetics reveals most bioflagellate fungal‐like organisms traditionally studied by mycologists and occur in the stramenopile (heterokont) clade within the SAR (formerly chromalveolate) superkingdom.
  • These fungal‐like organisms have probably evolved from a photosynthetic flagellate ancestor and there have been several independent plastid loss events.
  • The traditional taxonomy of all of these organisms has had to be substantially re‐evaluated in the light of recent molecular studies.

Keywords: Chromista; Heterokonta; hyphochytrid; labyrinthulid; oomycete; phylogeny; stramenopile; thraustochytrid

Figure 1. Schematic phylogenetic tree summarising the likely phylogenetic relationships between the diverse members of the chromalveolate (including stramenopiles) superkingdom. The photosynthetic lines are shaded in orange and postulated plastid loss events indicated by the red bars. The terminology is taken from Cavalier‐Smith and Chao () and the tree is based on a phylogenetic analysis of conserved protein genes by Tsui et al. (). Reproduced with permission from Beakes et al. (2011) ©Springer.
Figure 2. Labyrinthulomycota. Diagrams summarising representative life cycles in the Thraustochytriaceae (a–c), Aplanochytriaceae (d) and Labyrinthulaceae (e), showing differences in the mode of thallus division and spore formation. Adapted from the original drawings of Daiske Honda. (f) Photograph showing the ultrastructure of the unique membranous plug separating thallus from rhizoid/slime track systems in thraustochytrids and labyrinthulids (Honda, unpublished).
Figure 3. Hyphochytrids. (a–c) Series of drawings showing mature thallus and spore release in the monocentric hyphochytrid Note the chytrid‐like appearance and absorpative rhizoid system (a) and release of cytoplasm into a thin‐walled vesicle (b) in which zoospore differentiation takes place (c). Drawings based on photographs of Gauriloff and Fuller in . (d) Line drawing of an electron micrograph of a zoospore of showing single anterior flagellum with mastigoneme hairs. (e, f) Line drawings of developing and mature polycentric thalli of (d–f) Reproduced with permission from Karling, 1977 © Koeltz Scientific Books. Scale bar = 10μm
Figure 4. Oomycota Diagram summarising the evolutionary trends in the morphology of the oomycete thallus mapped onto the proposed evolutionary hierarchy. Adapted from Beakes et al. (2011) © Springer.
Figure 5. Photographic montage summary of the various stages in the asexual and sexual stages of the life cycle of the water mould Photographs and micrographs by Gordon Beakes, except for the photographs of zoosporangia by which were taken by the late AD Greenwood.
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Beakes, Gordon W, Thines, Marco, and Honda, Daiske(Feb 2015) Straminipile “Fungi” – Taxonomy. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001984]