Introduction to Fungi, Third Edition

PYTHIALES
109
Fig 5.25 Oogonial
development in
Phytophthora.
(a f) Stages of
development in
P. erythroseptica.
(g i). Stages of
development in
P. cactorum.
Sexual reproduction
Oospore formation is dependent on sterols and
mating hormones (p. 95) and may be homo- or
heterothallic. Phylogenetic studies have indicated
that the former is ancestral, heterothallism
having arisen repeatedly within the genus
Phytophthora (Kroon et al., 2004). Two distinct
types of antheridial arrangement are found.
In P. fragariae, P. megasperma and a number of
other species, antheridia are attached laterally to
the oogonium and are described as paragynous
meaning ‘beside the female’ (Figs. 5.22c, 5.25g i).
In other Phytophthora species such as P. infestans, P.
cinnamomi and P. erythroseptica, the oogonium,
during its development, penetrates and grows
through the antheridium (Hemmes, 1983). The
oogonial hypha emerges above the antheridium
and inflates to form a spherical oogonium, with
the antheridium persisting as a collar around its
base (Figs. 5.25a f). This arrangement of the
antheridium is termed amphigynous (‘around
the female’). In some species (e.g. P. cactorum,
P. clandestina, P. medicaginis), both types of arrangement
may be found (Figs. 5.22c,d); one or the
other may predominate, depending on strain
and culture conditions (Erwin & Ribeiro, 1996).
Both the oogonia and antheridia contain
several diploid nuclei, but as the oosphere
matures only a single nucleus remains at the
centre while the remaining nuclei are included
in the periplasm, i.e. the space between the
oosphere and the oogonial walls (see Fig. 5.2).
Meiosis occurs in the antheridium and oogonium
(Shaw, 1983). Fertilization tubes have been
observed and a single haploid nucleus is introduced
from the antheridium into the oosphere
(Fig. 5.26). Fusion between the oosphere nucleus
and the antheridial nucleus is delayed. Even
mature, dormant oospores may still be binucleate,
karyogamy usually occurring after breakage
of dormancy as a first step towards germination
(Jiang et al., 1989).
Following fertilization, the physiology
and ultrastructure of the oospore change to