Chytridiomycetes

If dead leaves or onion bulb scales are immersed in a little water containing a bit of soil, an abundance of chytrids will likely appear in a short time. Their spores seem to be everywhere. They are the simplest of the fungi, reproducing primarily by zoospores. Some species exhibit alternation of generations. The simple, globe-shaped Chytridium sphaerocurpum (figure 18-11, which extends rhizoids into the substrate and produces unicellular sporangia and zoospores, and Allomyces arbuscula (figure 18-2), which shows alternation of generations, are two examples of chytrids. Chytrids consist of only a spherical cell and a few rhizoids that penetrate the host tissue.

Figure 18-1 Chytridium sphaemrpum growing on a dead leaf in water.

Figure 16-2 Allomyces arbuscula. Two kinds of sporangia are produced: thin-walled sporangia, which produce mitospores, and thick-walled sporangia, which produce meiospores. The mitospores produce sporophytes, and the meiospores produce gametophytes. The antheridia are orange in color. A fertilized egg grows into a sporophyte.

Figure 18-2 shows two types of sporangia on the Allomyces sporophyte plant: thick-walled sporangia and thin-walled sporangia. In the thick-walled sporangia, meiosis occurs and the resulting spores, called meiospores, are haploid. The meiospores grow into gametophytes. In the thin-walled sporangia, mitosis occurs, and the resulting spores, called mitospores, are diploid. Mitospores do not produce gametophytes but, rather, grow into sporophytes. If a culture of Allomyces were to be dried out, the thin-walled sporangia would not survive and the thick-walled sporangia would. Such treatment would therefore ensure alternation of generations. The rendering of the gametophyte in figure 18-2 shows two kinds of gametangia: the antheridia, situated below, and the oogonia, located above. Whereas the antheridia (male gametangia) are orange in color, the female gametangia are colorless. When the gametes unite, a biflagellated zygote forms, which, in turn, grows to a sporophyte thallus. Sperm cells are attracted to the egg by a hormonal substance having the romantic name sirenin.