Introduction to Fungi, Third Edition

110 STRAMINIPILA: OOMYCOTA
Fig 5.26 Phytophthora cactorum.
Development of oogonium, antheridium
and oospore. (a) Initials of oogonium
and antheridium. (b) Oogonium and
antheridium grown to full size: the
oogonium has about 24 nuclei and the
antheridium about 9. (c) Development
of a septum at the base of each, and
degeneration of some nuclei in each until
the oogonium has 8 or 9 nuclei and the
antheridium 4 or 5. (d) A simultaneous
division of the surviving nuclei in
oogonium and antheridium.The
protoplast has large vacuoles.
(e) Separation of oosphere from
periplasm. Nuclei divide in the periplasm
prior to degeneration.The oogonium
presses into the antheridium. (f) Entry of
one antheridial nucleus by a fertilization
tube.The protoplasm and remaining
nuclei of the antheridium degenerate.
(g) Development of oospore wall.
(h) The oospore enters its dormant
period with exospore formed from dead
periplasm, endospore deposited inside
it, and paired nuclei in association but
not yet fused. (a h) are composite
drawings of eight stages in sequence
(after Blackwell,1943).
a resting state. Oospore differentiation proceeds
from the outside inwards (centripetal development).
The oospore has a thin outer wall
(epispore) which is derived from the periplasm
and appears to consist of pectic substances. The
inner oospore wall (endospore) is rich in b-1,3-
glucans which form a major storage reserve and
are mobilized by glucanases just prior to germination
(Erwin & Ribeiro, 1996). Within the
developing oospore, the numerous small lipid
droplets coalesce into a few large ones. Lipids are
undoubtedly the major endogenous storage
reserve in the spores of Oomycota (Dick, 1995)
and many other fungi. Later, the dense body
vesicles which are rich in mycolaminarin and
phosphate fuse together, giving one large structure,
the ooplast. Like the endospore, the ooplast
is consumed during germination whereas some
lipid droplets are saved and are translocated into
the germ tube (Hemmes, 1983). Considering their
thick walls and abundant storage reserves, it is
not surprising that oospores are the longest-lived