Introduction to Fungi, Third Edition

INTRODUCTION
129
In the inoperculate chytrids such as Olpidium,
Diplophlyctis and Cladochytrium, the sporangium
forms a discharge tube which penetrates to the
exterior of the host cell and its tip becomes
gelatinous and dissolves away. In operculate
chytrids such as Chytridium and Nowakowskiella,
the tip of the discharge tube breaks open at a
special line of weakness and is seen as a special
cap or operculum after discharge (see Fig. 6.4b).
6.1.1 The zoospore
The number of zoospores formed inside zoosporangia
of chytrids varies with the size of the
spore and sporangium. Although the zoospore
size is roughly constant for a given species, the
size of the sporangium may be very variable. In
Rhizophlyctis rosea, tiny sporangia containing only
one or two zoospores have been reported from
culture media deficient in carbohydrate, whereas
on cellulose-rich media large sporangia containing
many hundred spores are formed. The release
of zoospores is brought about by internal
pressure which causes the exit papillae to burst
open. In studies of the fine structure of mature
sporangia of R. rosea and Nowakowskiella profusa
(Chambers & Willoughby, 1964; Chambers et al.,
1967), it has been shown that the single
flagellum is coiled round the zoospore like a
watch spring. The zoospores are separated by a
matrix of spongy material which may absorb
water and swell rapidly at the final stages
of sporangial maturation. When the internal
pressure has been relieved by the ejection of
some zoospores, those remaining inside the
sporangium escape by swimming or wriggling
through the exit tube. In some species the spores
are discharged in a mass which later separates
into single zoospores, but in others the zoospores
make their escape individually.
The form of the zoospore is similar in all
chytrids (with the exception of the multiflagellate
members of the Neocallimastigales). There
is a spherical or ellipsoidal body which in some
forms is capable of plastic changes in shape, and
a long trailing flagellum. When swimming, the
zoospores show characteristic jerky or ‘hopping’
movements; additionally, abrupt changes in
direction are sometimes made. The internal
structure of the zoospore as revealed by light
and electron microscopy is variable, but characteristic
of particular genera (Lange & Olson,
1979). In view of the plasticity in morphology of
the thallus under different growth conditions,
zoospore ultrastructure is regarded as a more
satisfactory basis of classification (D. J. S. Barr,
1990, 2001). Two features are of taxonomic
importance, the flagellar apparatus and an assemblage
of organelles termed the microbody lipid
globule complex (MLC) (D. J. S. Barr, 2001).
The flagellar apparatus
The whiplash flagellum resembles that of other
eukaryotes, with a smooth membrane enclosing
a cylindrical shaft, the axoneme, made up
internally of nine doublet pairs of microtubules
surrounding two central microtubules. As shown
in Fig. 6.2, the base of the axoneme comprises
three regions, the flagellum proper, the transitional
zone and the kinetosome. The function
of the kinetosome is to generate the flagellum.
An interesting feature found in several species
is a second kinetosome or the remainder of
one, the dormant kinetosome. Its presence
has led to the suggestion that the ancestors
of the Chytridiomycota may have had biflagellate
zoospores, the second flagellum having been
lost in the course of evolution (Olson & Fuller,
1968).
In section, the kinetosome resembles a cartwheel
(Fig. 6.2f), because to each of the nine
outer microtubule doublets seen in the flagellum
proper, a third microtubule is attached.
This is called the C-tubule; in the doublets,
that tubule with extended dynein arms is the
A-tubule, and its partner is labelled B. These
flagellar microtubules radiate as kinetosome
props into the zoospore, perhaps providing
structural support and anchorage of the flagellum
(D. J. S. Barr, 2001). Microtubules may also be
attached laterally to the kinetosome, contributing
to the flagellar root system (Figs. 6.2c, 6.19).
In the innermost (proximal) part of the transitional
zone, the nine microtubule triplets of
the kinetosome are converted into the doublets
of the flagellum proper; concentric fibres,
possibly arranged helically, surround the nine
doublet pairs. Also within the transitional zone,