Blastocladiomycota

Abstract

The Blastocladiomycota, a phylum of fungi that produce motile spores and gametes, live in water or in soils, where they are active when water is present and survive with thick‐walled resting sporangia when soils are dry. Some genera are saprobic, but the phylum also contains pathogens of invertebrates and plants. Life cycles can include alternation of haploid and diploid generations and different generations may have different hosts. Sexual reproduction begins by fusion of gametes and meiosis takes place during germination of resistant sporangia. Saprobic members have been used as model organisms and genome sequencing of Allomyces and Blastocladiella continues this trend. Coelomomyces species parasitise mosquito larvae and are capable of causing death of up to 90% of their hosts. Obligate parasitism, narrow host ranges, need for an alternate host and unpredictable rate of control have deterred development of Coelomomyces species for mosquito control.

Key Concepts:

  • Members of the Blastocladiomycota produce spores and gametes that are motile by means of a posteriorly directed whiplash flagellum.

  • Motile spores, which are recognisable by their centrally located nucleus that is apically surrounded by a nuclear cap consisting of ribosomes, are recognisable at the ultrastructural level by microtubules in groups of three that extend from the apex of the kinetosome and surround the nucleus.

  • Some saprobic species have an alternation of morphologically similar haploid and diploid generations and some pathogens have alternation of haploid and diploid generations with different morphologies and sometimes on different hosts.

  • Sexual reproduction is by fusion of motile gametes and meiosis takes place during the germination of resting sporangia.

  • The phylum contains pathogens of crustaceans, nematodes, algae and aquatic and semi‐aquatic plants.

  • Saprobic species, especially Allomyces macrogynus and Blastocladiella emersonii, are used for physiological and genetic studies and the genome of A. macrogynus has been sequenced.

Keywords: Allomyces; alternation of generations; aquatic fungi; pathogen; zoosporic fungi

Figure 1.

Cladogram of families and genera in the Blastocladiomycota based on phylogeny in Porter et al. with unsequenced genera (?) added. Position of Sorochytriaceae (on dashed line) unknown.

Figure 2.

Allomyces arbusculus. (a and b) Zoospores with nuclear cap (arrows). (c) Germling with basal rhizoids and apical growth of the thallus. (d) Dichotomously branched thallus with occasional septae. (e) Apical female gametangia subtended by orange male gametangia. (f) Sporothallus bearing empty zoosporangium and thick‐walled, pigmented resistant sporangia. (g) Close‐up of punctuate wall of resistant sporangium. Magnification bar in (a) also for (b) and (g). Magnification bar in (c) also for (d–f).

Figure 3.

Catenariaceae in pure culture. (a) Catenaria anguillulae seldom branched. (b) Catenophlyctis variabilis highly branched. Magnification bar in (a) also for (b).

Figure 4.

Life stages of Physoderma. (a and b) Germination of resistant sporangia. (c) Zoospore. (d and e) Mature epibiotic zoosporangia. (f) Zoospore release. (g) Endobiotic stage with turbinate cells (arrows). (h) Resistant zoosporangia (RS) developing from short branches of turbinate cells. Reproduced from Sparrow FK, Griffin JE and Johns RM (1961) Observations on chytridiaceous parasites of phanerogams XI. A Physoderma on Agropyron repens. American Journal of Botany48: 850–858, with permission from the Botanical Society of America.

Figure 5.

Polycaryum laeve in Daphnia pulicaria. (a) Fungal thalli in haemocoel of host. (b) Immature, unwalled thallus (arrow). (c) Mature, walled thalli. (d) Zoosporangium with ruptured wall preceding zoospore release. (e) Zoospore release. (f and g) Pairing zoospores. Magnification bar in (b) also for (c–f).

Figure 6.

Diagramatic ultrastructure of kinetesomal region of Blastocladiomycota zoospore. microtubules, mt; nucleus, N; nonflagellated centriole, NFC.

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How to Cite close
Longcore, Joyce E, and Simmons, D Rabern(Sep 2012) Blastocladiomycota. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0023623]