INTRODUCTION TO THE BLASTOCLADIOMYCOTA

This phylum was once considered part of the chytrids; however, most of the true chytrids (Chytridiomycota) produce a limited mycelium (text with tooltip) Filamentous, vegetative bodies of fungi; consist of hyphae. while the Blastocladiomycota usually make extensive mycelia. Thus, they superficially resemble the water molds to which they were thought to have been affiliated. Like the chytrids, the Blastocladiomycota and the Neocallimastigomycota are the only members of the fungi in which motility has been retained. In overall growth habit, the blastocladiomycetes tend to be eucarpic (text with tooltip) Thallus consists of rhizoids and sporangium. , in which there is an extensive vegetative growth habit in which some of the organism participates in reproduction (asexual and sexual).

Members of this phylum do exhibit a complete alternation of generation between a haploid gametophyte (text with tooltip) A gametophyte is haploid, gamete-producing phase of an organism that exhibits alternation of generation. (Figure 1) and a diploid sporophyte (text with tooltip) A sporophyte is the diploid phase of an organism that exhibits alternation of generation. This phase produces spores usually in specialized sporangia as the immediate products of meiosis. (Figure 2), the phase in which meiosis occurs (see Figure 3). Allomyces, a eucarpic taxon, forms cottony mycelia in both phases such that they would be indistinguishable without seeing gametangia or sporangia on the hyphae. The gametophyte of Allomyces (Figure 1) has tandem paired gametangia that terminate the hyphae such that the terminal gametangium (usually smaller of the two) is the “male”, that is, it produces the smaller of the two motile anisogametes. The larger proximal gametangia release larger motile anisogametes (text with tooltip) Anisogamous (adj.) describes sexual reproduction in which the gametes are structurally siimilar, but not identical. . The sporophyte of Allomyces (see Figure 2) produces terminal single sporangia within which meiosis occurs and zoospores are formed. The released zoospores (text with tooltip) A zoospore is an asexual spore that is motile. Zoo- (pronoumced zo-o) is a prefix that means moving. germinate and develop into the gametophyte.

FIGURE 1. Allomyces gametophyte with terminal male gametangia and proximal female gametangium. Image from http://www.csupomona.edu/~jcclark/classes/bot125/resource/graphics/	FIGURE 2. Allomyces sporophyte showing sporangia. Image from http://www.csupomona.edu/~jcclark/classes/bot125/resource/graphics/

A
B	FIGURE 3. LIFE HISTORY OF ALLOMYCES. A. Diagram of the isomorphic alternation of generation in Allomyces. B. Diagrams of the terminal gametangia. The smaller terminal gametangium forms the smaller male (M) anisogamete. The larger proximal gametangium forms the larger female (F) anizogamete. Z = germinating zygote. Images from Bold et al. (1987)

SYSTEMATICS OF THE BLASTOCLADIOMYCOTA

Alexopoulos (1966) united all of the flagellated fungi into a class called Chytridiomycetes. In general, this group was treated as a protist taxon (e.g. Barr 1990; Margulis and Schwartz 1988, Pr-26 and 1998, Pr-29), or as a group of simple or basal fungi (e.g. Sleigh et al. 1984; Alexopoulos and Mims 1979; and Alexopoulos et al. 1996). Although the blastocladiomycetes were treated as chytrids, the differences between eucarpic and holocarpic (text with tooltip) Entire thallus functions as sporangium. chytrids implied a very real difference between them.

Bruns et al. (1992) in their examination of 18S rRNA nucleotide sequences, confirmed that the chytrids and the Zygomycota were basal groups within the clade of the Kingdom Fungi and seemed to close the book on the question of the chytrids. More recently, further molecular evidence (e.g. Tudge 2000, Patterson 1999, and Baldauf 2003 for a synopsis) has confirmed their position near the root of the fungi [see also Lang, The Fungal Mitochondrial Genome Project], which is part of a larger clade called the opisthokonts, a group that includes the choanoflagellates and the metazoans (Patterson 1999). Margulis and Schwartz (1998) still maintain that the fungi are a kingdom of conjugating taxa, and, therefore, continue to exclude the chytrids.

More recent molecular evidence has called into question the monophyly of the Chytrids as defined here (Lutzoni et al. 2004). The Blastocladiales appear to be the sisters of some of the orders in the “Zygomycota” (Figure 4). Subsequent research (James et al. 2006a and 2006b) confirmed the uniqueness of the Blastocladiomycetes and their placement as a phylum (James et al. 2006a). That was used in the Linnaean classification presented by Hibbett et al. (2007).