Joyce E Longcore, University of Maine, Orono, Maine, USA
The Chytridiomycota is a phylum in the kingdom Fungi, whose members produce unwalled, asexual spores that swim by means of a single, posteriorly directed flagellum. Members of this group are microscopic saprophytes or parasites that inhabit fresh and saline water, soils and the guts of herbivores.
. Definition and Overview
. Zoospore . Morphogenesis . Economic Importance . Ecology
Definition and Overview
The Chytridiomycota is the most ancient lineage of the ﬁve phyla in the kingdom Fungi, the others being the Zygomycota, Glomeromycota, Ascomycota and Basidiomycota. In contrast to other members of the kingdom, members of the Chytridiomycota (chytrids orchytridiomycetes) form unwalled, motile, asexual reproductive spores (zoospores). Chytrids, along with other zoospore-producing fungal organisms such as oomycetes, thraustochytrids and hyphochytrids, at one time were classiﬁed as Phycomycetes and later in the Mastigomycotina. More recently, the chytrids and other groups of zoosporic fungi were classiﬁed as separate phyla of protists. See also: Fungiand the history of mycology; Fungal spores Chytrids resemble other members of the kingdom Fungi in having chitinous walls, mitochondria with plate-like cristae, and in synthesizing lysine by the a-amino adipic acid pathway. Their position within the Fungi was postulated based on these physiological characters and has been supported by analyses of mitochondrial deoxyribonucleic acid (DNA) andnuclear ribosomal DNA (rDNA). Although chytrid zoospores require free water for dispersal, these organisms are not limited to lakes, streams, ponds and marine habitats. Free water is present in soils at certain times and many chytrids have short life cycles and drought-resistant resting stages that make them well adapted to live in terrestrial habitats. Members of the order Neocallimastigales, commonlyknown as ‘rumen fungi’ or ‘anaerobic gut fungi’, grow in a unique aquatic habitat: the rumens and hindguts of mammalian herbivores. See also: Fungal cell walls; Vertebrate food pathway and gut: overview Many of the ﬁrst-named chytrids were discovered by phycologists, who found them growing on freshwater algae. Other natural substrates for chytrids include pollen, cast exuviae of aquatic insects,protozoans and small invertebrates, other fungi, moribund pieces of plants, fruits and waterlogged twigs. Besides examining such substrates microscopically, chytrids can be found by ‘baiting’. Pieces of organic material small enough to be examined on a microscope slide are added to dishes with samples of pond or lake water containing a small amount of debris, or to a gram or two of soil to whichsterilized water has been added. Common baits include puriﬁed shrimp exoskeleton (chitin), snake skin (keratin); cellophane, lens paper and
white onion skin (cellulose); pollen (especially pine pollen, which is easy to collect), seeds and fruits. After a day or two, pollen grains are usually infested with chytrid sporangia, and after several days chytrids andother zoosporic fungi can be found on the other baits. From these baited ‘gross cultures’, most chytrids can be brought into pure culture by using specialized techniques that often involve wiping the small reproductive bodies of the fungus through agar to clean them. Although they can be found almost anywhere, chytrids have not been well known, perhaps because they are microscopic and are not isolatedinto pure culture with methods designed for other fungal groups. Also, few are associated with economically signiﬁcant, agricultural diseases. This group has become better known, however, because of a chytrid that infects the keratinized skin cells of amphibians and causes a disease that seems to be one of the reasons for the decline in amphibian populations on several continents. See also:...