Introduction to Fungi, Third Edition

PYTHIALES
101
Fig 5.19 Life cycle of Phytophthora infestans.This fungus is heterothallic, and the asexual part of the life cycle (left of diagram) is
shown only for one mating type (A1). Nuclei in vegetative states are diploid.When two compatible mycelia meet, multinucleate
oogonia and antheridia are differentiated, and one meiotic event in each results in the transfer of one haploid nucleus from the
gametangium to the oogonium. Karyogamy is delayed until shortly before oospore germination.Open and closed circles represent
haploid nuclei of opposite mating type; diploid nuclei are larger and half-filled. Key events in the life cycle are meiosis (M),
plasmogamy (P) and karyogamy (K).
some heterothallic species are known, e.g.
P. sylvaticum, P. heterothallicum and P. splendens.
In these cases, mating is a complicated affair
under hormonal control, and with relative
sexuality (see p. 95).
Oogonia arise as terminal or intercalary
spherical swellings which become cut off from
the adjacent mycelium by cross-wall formation.
In some species, e.g. P. mamillatum, the oogonial
wall is folded into long projections (Fig. 5.18b).
The antheridia arise as club-shaped swollen
hyphal tips, often as branches of the oogonial
stalk (monoclinous) or sometimes from separate
hyphae (diclinous). In some species, e.g.
P. ultimum, there is typically only a single antheridium
to each oogonium, whilst in others, e.g.
P. debaryanum, there may be several (Fig. 5.18a).
The young oogonium is multinucleate and
the cytoplasm within it differentiates into a
multinucleate central mass, the ooplasm from
which the oosphere develops, and a peripheral
mass, the periplasm, also containing several
nuclei. The periplasm does not contribute to
the formation of the oosphere.
As soon as the gametangia have become
delimited by the basal septum, mitotic divisions
cease. Nuclei may be aborted at this stage, and
in oogonia of P. debaryanum 1 8 nuclei undergo
meiosis (Sansome, 1963). Meiotic divisions are
synchronous in the antheridium and the oogonium,
although no protoplasmic continuities
exist at this stage (Dick, 1995). In the antheridium
of P. debaryanum and P. ultimum, all nuclei
but one degenerate prior to meiosis, so that four