104 CHAPTER 5
not the development of the sex organs themselves. In
a few cases, notably in Achlya, a single colony can show
“relative sexuality” – it will behave as a male (fertiliz-
ing the oogonium from its antheridium) or “female”
depending on the strain with which it is paired.
The hormonal control of mating has been studied
intensively in Achlya. The hormones are steroids
(Fig. 5.22) derived from the isoprenoid pathway, like
sirenin discussed above. They were discovered by
growing “strong female” and “strong male” strains in
separate dishes of water, then passing the water over
colonies of the opposite strain. The hyphae of the female
produce the hormone antheridiol, which causes the
male strain to increase its rate of cellulase enzyme pro-
duction and to form many hyphal branches (the
antheridial branches – see Fig. 2.34). Once it has been
triggered by antheridiol, the male strain produces
other steroid hormones, termed oogoniols, which
trigger the development of oogonia (sex organs) in the
female strain. In normal conditions the antheridial
hyphae then grow towards the oogonia, clasp onto
them, and produce fertilization tubes to transfer the
“male” nuclei into the oogonium (see Fig. 2.34). Similar
hormonal systems might occur in other Oomycota
such as Pythiumand Phytophthoraspp., but they have
not been characterized. A notable feature of Pythium
and Phytophthorais that they cannot synthesize sterols
from nonsterol precursors, and often do not need
sterols for somatic growth, but they always require trace
amounts of sterols for both sexual reproduction and
asexual reproduction. They would be able to obtain
them from a plant host or other natural sources.Zygomycota
The Zygomycota can be homothallic, or heterothallic
with two mating types (termed “plus” and “minus”).
One of the roles of the mating-type genes is to regu-
late the production of hormone precursors (prohor-
mones) from β-carotene, which is another product
of the isoprenoid pathway already mentioned. This
hormonal system is shared by many members of
the subgroup Mucorales because, for example, a plus
strain of Mucorcan elicit a sexual response from a minus
strain of Rhizopus, even though the development of a
hybrid is blocked at a later stage due to incompatibility.
As shown in Fig. 5.23, the prohormones of the plus
and minus strains are similar molecules but they
differ slightly, owing to the different enzymes of the
two strains. The prohormones are volatile compounds
which diffuse towards the opposite mating type; then
they are absorbed and converted to active hormones,
the trisporic acids, by the complementary enzymes
of the opposite strain. Initially, only low levels ofCH 3
CH 2 OH
CH 2 OH
Sirenin
(Allomyces)Antheridiol
(Achlya)Oogoniol
(Achlya)OH
OH
O
O
O
OH
O
O OCCH 2 OH
HO
HO
Fig. 5.22Three pheromones that regulate
sexual reproduction in zoosporic organisms.
Sirenin is released by female gametes of
Allomyces spp. Antheridiol and oogoniols
regulate sexual attraction and mating in
Achlyaspp. (Oomycota) and possibly in other
Oomycota.