The chytrids (Figure 29.14) include several distinct lineages of mostly aquatic microorganisms once classified with the protists. However, morphological evidence (cell walls that consist primarily of chitin) and molecular evidence support their classification as early-diverging fungi. In this book we use the term “chytrid” to refer to all three of the formally named clades listed as chytrids in Table 29.1, but some mycologists use this term to refer to only one of those clades, the Chytridiomycota. There are fewer than 1,000 described species among the three groups of chytrids.

Chytrids reproduce both sexually and asexually. Like the animals, chytrids that reproduce sexually possess flagellated gametes. The retention of this trait reflects the aquatic environment in which fungi first evolved. Chytrids are the only fungi that include species with flagella at any life cycle stage. Both the spores (called zoospores) and the gametes are flagellated (Figure 29.15A). There are two types of spores and gametes. Unlike in most fungi, one type of gamete is larger than the other, so the gametes can be distinguished as male (small gametes) and female (large gametes; see Figure 29.15A). Except for size, however, the two gametes are very similar.

Question

The two gametes differ in size; a male gamete is defined as the smaller of the two gametes, and the female is the larger gamete.

The chytrids are diverse in form. Some are unicellular, others have rhizoids, and still others have coenocytic hyphae. They may be parasitic (on organisms such as algae, mosquito larvae, nematodes, and amphibians) or saprobic. Some have complex mutualistic relationships with foregut-fermenting animals such as cattle and deer. Many chytrids live in freshwater habitats or in moist soil, but some are marine.