Zoospores of the nematode-parasitic Catenaria anguillulae
(Chytridiomycota) were studied by videomicroscopy in sealed films of
water on microscope slides in the presence or absence of freeze-inactivated
nematodes (Panagrellus redivivus). Zoospores swam for
more than 1 h at a mean velocity of 104 μm s−1,
interspersed with repeated phases (1–2 min) of amoeboid crawling
on glass or
nematode surfaces. They were attracted to and encysted near the mouth,
excretory pore, and anus of nematodes, or eventually
encysted at random on glass and nematode surfaces. The single posterior
flagellum
was immobile during amoeboid crawling but
resumed rapid beating when the last pseudopodium was being retracted. Zoospores
encysted by adhesion of the anterior of an
amoeboid cell to a surface; then the cell posterior was raised above the
anterior
so that the flagellum projected perpendicular to the
surface, and the flagellum was retracted by rotation of the cell contents.
Cysts
germinated within 20–60 min by a narrow germ-tube
at the site of adhesion. The germ-tube grew a short distance, then formed
an
intercalary vesicle into which the cyst contents
emptied by expansion of a cyst vacuole. In several cases the germ-tube
penetrated
a nematode and formed the vesicle inside the
host. Rhizoids or assimilative hyphae developed from the vesicle or by
growth
of the germ-tube tip. An increasing proportion of
zoospores that remained motile after 1 h in water films had a globose body
in
contrast to the normal elongated form. This seemed
to be caused by damage during repeated transitions between the amoeboid
and
swimming phases, because pseudopodia sometimes
remained firmly attached to a glass surface.
C. anguillulae showed consistent orientation (polarity) of
zoospore encystment and cyst germination. This parallels the behaviour
of other zoosporic fungi or fungus-like organisms (Plasmodiophora
brassicae, Rozella allomycis, Pythium, Phytophthora
and Saprolegnia
spp.) suggesting that it is a common feature of zoosporic parasites.
Surface-recognition for encystment by C. anguillulae was
mediated by the zoospore soma, not the flagellum. In addition, we redefine
the
early development of C. anguillulae, including
flagellar retraction by rotation of cell contents, non-specific adhesion
of
zoospores and cysts to surfaces, and evacuation of cyst
contents into a vesicle from which further growth occurs.