Introduction to Fungi, Third Edition

128 CHYTRIDIOMYCOTA
N-acetylglucosamine, is also found in the walls
of other Eumycota (i.e. Zygomycota, Ascomycota
and Basidiomycota), whilst the cell walls of
members of the Oomycota contain cellulose.
Cellulose and chitin occur together in the walls
of species of Hyphochytrium and Rhizidiomyces,
members of the Hyphochytriomycota (Fuller,
2001; see Section 4.3).
The form of the thallus in the
Chytridiomycota is varied. In biotrophic species
such as Olpidium and Synchytrium, where the
whole thallus is contained within the host cell,
there is no differentiation into a vegetative
and a reproductive part. At maturity the entire
structure, except for the wall which surrounds it,
is converted into reproductive units, i.e. zoospores,
gametes or resting sporangia. Such thalli
are termed holocarpic (Fig. 6.1). More usually, the
thallus is differentiated into organs of reproduction
(sporangia and resting sporangia) arising
from a vegetative part which often consists of
rhizoids. These serve in the exploitation of the
substratum and the assimilation of nutrients.
Thalli of this type are eucarpic. Eucarpic thalli
may have one or several sporangia and are then
termed monocentric or polycentric, respectively
(Fig. 6.1). In some species there are both monocentric
and polycentric thalli, so that these terms
have descriptive rather than taxonomic significance.
A further distinction has been made,
especially in monocentric forms, between those
in which only the rhizoids are inside the host
cell whilst the sporangium is external (epibiotic),
in contrast with the endobiotic condition in
which the entire thallus is inside the host cell
(Fig. 6.1). In monocentric thalli, the rhizoids
usually radiate from a single position on the
sporangium wall, but in polycentric forms a
more extensive, branched rhizoidal system, the
rhizomycelium, develops.
The zoosporangium is generally a spherical
or pear-shaped sac bearing one or more discharge
tubes or exit papillae. The method of
zoospore release has been used in classification.
Fig 6.1 Types of thallus structure in the
Chytridiales, diagrammatic and not to scale.