Introduction to Fungi, Third Edition

164 CHYTRIDIOMYCOTA
Fig 6.25 Monoblepharis macrandra reproduction. (a) Terminal
zoosporangium containing cleaved zoospores. (b) Solitary
terminal antheridium. (c) Solitary terminal oogonium with
apical receptive area. (d) Oogonium with hypogynous
antheridium. (e) Spermatozoid release from solitary terminal
antheridium. (f) Exogenous oospore on empty oogonium with
bullations on the wall of the oogonium and lipid inclusions
in the cytoplasm. (a e) to same scale.Traced from Whisler
and Marek (1987), with permission by Southeastern Publishing
Corporation.
tips of the hyphae (Figs. 6.25b,c) or in pairs,
with the antheridium in a hypogynous position
(Fig. 6.25d). The antheridium often releases
sperm before the adjacent oogonium is ripe.
Each antheridium forms about four to eight
posteriorly uniflagellate swarmers which resemble,
but are somewhat smaller than, the zoospores.
The oogonium contains a single spherical
uninucleate oosphere, and when this is mature
an apical receptive papilla on the oogonial wall
breaks down. A spermatozoid approaching the
receptive papilla of the oogonium becomes
caught up in mucus and fusion with the
oosphere then follows, the flagellum of the
spermatozoid being absorbed within a few
minutes. Following plasmogamy, the oospore
secretes a golden-brown wall around itself
and nuclear fusion later occurs. In some species,
e.g. M. sphaerica, the oospore remains within
the oogonium (endogenous) but in others, e.g.
M. macrandra and M. polymorpha, the oospore
begins to move towards the mouth of the
oogonium within a few minutes of fertilization,
and remains exogenous, i.e. attached to it
(Fig. 6.25f). In the exogenous species, nuclear
fusion is delayed but finally fusion occurs and
the oospore becomes uninucleate. In some
species the oospore wall remains smooth, but
in others such as M. macrandra the wall may be
ornamented by hemispherical warts or bullations
(Fig. 6.25f). The oospore germinates after
a resting period which coincides with frozen
winter conditions or summer drought by producing
a single hypha which branches to form
a mycelium. The cytological details of the life
cycle are not fully known but it seems likely
that reduction division occurs during the germination
of the overwintered oospores.