Division Chytridiomycota

This group grows in aquatic habitats. Members show isomorphic alternation of generations, where there is an alternation between a diploid, spore-producing stage (the sporophyte) and a haploid, gamete-producing stage (the gametophyte). The two stages are nearly identical in form, hence the term isomorphic alternation of generations. Meiosis occurs in the sporophyte (diploid) stage, producing haploid spores, which give rise to the haploid gametophyte stage. Gametes are produced, then, by mitosis in the gametophyte stage. These gametes fuse to restore the diploid chromosome number, and the resulting zygote develops into the sporophyte stage. A diagram illustrates the typical life cycle of chytrids. Both spores and gametes are flagellated; this is suitable for an aquatic organism, and stands in contrast to the "higher" terrestrial fungi that we will study in the next exercise. Gametes are anisogamous, since there are smaller male gametes and larger female gametes. Except for the differences in the reproductive organs, it is difficult to tell the haploid from the diploid stage. Finally, other features of the chytrids are their coenocytic hyphae (no walls separating the cells), and their chitinous outer cell walls.

(A) Allomyces.

Examine Allomyces with a compound microscope.

(1) Hyphae
Note the colorless hyphae. Are cross walls (septa) present? If so, where? (Sieve like pseudoseptae occur at frequent intervals.)

(2) Haploid of Diploid?
Determine whether you are examining a haploid or diploid phase fungus as follows: If it is haploid there will be two large, more or less round, adjoining cells with papillae attached along the thallus or branches of the fungus. These large cells are gametangia or gamete-bearing cells. One is a male gametangium and the other a female. The male gametangium appears orange when close to maturity and the female is clear. These produce flagellated anisogamous gametes that are released into the water to find each other. The female gametes release a hormone called sirenin that attracts the male gametes.

If it is diploid, there will be only a single, large, thin-walled cell with one or more papillae attached to the thallus or branch at any given point. In addition there may be numerous thick walled, orange or reddish-brown colored cells attached to the thallus or branches.

(3) Mitosporangia or Meiosporangia?
The large, thin-walled cells are zoosporangia or mitosporangia. By mitosis they produce diploid flagellated zoospores that become new diploid fungi after germination. This is a form of asexual reproduction.

The thick-walled colored cells are meiosporangia. Meiosis occurs inside these cells to produce the haploid zoospores (meiospores) that germinate to become the haploid organism.

• Keep a freshwater mount of this fungus and observe periodically. Look for the release of gametes or spores from the gametangia or sporangia.