Chytridiomycetes

Some chytrid zoospores get married; others do not. This is due to the alternation of generations.

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      Water molds belong to a fungal class which some botanists call Chytridiomycetes. The name Chytridiomycetes literally means “little pot fungi.” Evidently the botanist who thought up this name thought that the reproductive structures of this organism looked like little pots. Judging from the pictures, they actually look more like perfectly round Christmas tree ornaments. But it would be hard to express that concept in a single convenient name.

      Some of these chytrids are parasites. The species Chytridium confervae feeds on a broad spectrum of aqueous organisms: algae, protozoa, or even other fungi. But Rhizophydium planktonicum is a very finicky eater. Only Asterionella formosa, a delicious diatom, will satisfy its fastidious taste. After attaching itself to its hapless victim, this fungal gourmet feeds voraciously, and the diatom soon dies.

      Other chytrids object to such a wanton destruction of life and feed exclusively on decaying plants. Water molds of the genus Allomyces are good examples.

       Chytrids possess an unusual characteristic that distinguishes them from most other fungi. Their reproductive pots or globes eventually fill with spores. But while most fungal spores look like little pieces of dust which the wind must blow to a suitable location before germination can take place, chytrids spores are zoospores that have the power to move without any help. They have a whip-like structure called a flagellum by which they move through the water with ease.

      Some zoospores do not want to get married. They have all the chromosomes they need; so they merely hunt around for a convenient food source and settle down on it. Thereafter they lead a sedentary life.

      But other zoospores have only half the normal number of chromosomes. Some of these prefer a bachelor’s life and immediately look around for some delicious rotting plant on which it can live out its life in luxury. Other zoospores first look around for a female and get married. In this case, the twain literally become one: one body with two flagella instead of one. This spliced zoospore then looks around for a food source and settles down.

      Actually, the chytrid’s life cycle is not as erratic as it sounds. Allomyces, whose life cycle is described above, has two different generations that alternate with one another. The first generation has the full number of chromosomes. This is called the diploid generation. It produces some diploid zoospores and others that have only half the full complement of chromosomes. These latter are called haploid. Both types settle down without further ado. Each diploid zoospore develops into another diploid chytrid, while the haploid zoospores become haploid fungi. These haploid organisms are the second of the two alternating generations. Eventually these haploid fungi produce male and female zoospores, which get married, unite their chromosomes, and start a new diploid generation.

      Hopefully they will live happily ever after.

Note on fungal flagella: Fungi of the class Oomycetes also have flagella, but some botanists no longer regard them as fungi.

References:

“Botany: An Ecological Approach” by William A. Jensen and Frank B. Salisbury

“Biology” by Neil A. Campbell, Jane B. Reece, and Lawrence G. Mitchell

Wikipedia: Chytridiomycota  http://en.wikipedia.org/wiki/Chytridiomycota

University of Jena: Chytridium confervae

http://www2.uni-jena.de/biologie/mikrobio/fportraits/zchytridium.html

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