Introduction to Fungi, Third Edition

90 STRAMINIPILA: OOMYCOTA
was the first to react. Raper (1939) postulated that
the development of the antheridial branches was
in response to a hormone, termed Hormone A,
secreted by vegetative female colonies. By further
experiments of this kind, he showed that the later
steps in the sexual process were also regulated
by means of diffusible substances. He postulated
that the antheridial branches secreted a second
substance, Hormone B, which resulted in the
formation of oogonial initials on the female
colony. The oogonial initials in their turn
secreted a further substance called Hormone C,
which stimulated the antheridial initials to grow
towards the oogonial initials and also resulted
in the antheridia being delimited. Having made
contact with the oogonial initials, the antheridial
branches secreted Hormone D which resulted
in the formation of a septum cutting off the
oogonium from its stalk, and in the formation
of oospheres. The original scheme (Table 5.2)
therefore implicated four hormones, but confusion
arose subsequently because the effect of
Hormone A can be modulated by amino acids
and other metabolites released from the hemp
seeds (Barksdale, 1970; Schreurs et al., 1989).
Since Hormone A is active at extremely low
concentrations of 2 10 11 M (Barksdale, 1969),
purification of this substance was extremely challenging,
and 6000 l of culture fluid had to be
extracted to obtain 20 mg crystalline Hormone A
(Barksdale, 1967). It was eventually identified as
the steroid antheridiol (Fig. 5.11b). Soon after, the
structure of Hormone B was elucidated and
found also to be a steroid, oogoniol (Fig. 5.11c),
which is, in fact, present as three chemically
closely related forms (McMorris et al., 1975). The
effect postulated by Raper (1939) to be due to
Hormone C is now thought to be mediated by
antheridiol activity, whereas Hormone D may not
exist (Carlile, 1996b). Both antheridiol and the
oogoniols are derived from fucosterol (Fig. 5.11a),
the principal sterol in Achlya (see Elliott, 1994).
The physiological roles of antheridiol and
the oogoniols are several-fold and include induction
or suppression of sexuality (Thomas &
McMorris, 1987), directional growth of gametangial
tips (McMorris, 1978), and stimulation of
the production of cell wall-softening enzymes
(especially cellulase) at points of branching and
contact between gametangia (Mullins, 1973; Gow
& Gooday, 1987). A cytoplasmic receptor protein
for antheridiol has been detected (Riehl et al.,
1984), and the hormone probably acts like
its equivalents in mammalian cells, by the
receptor hormone complex moving to the
nucleus and binding specifically to DNA, increasing
transcription rates of certain genes (Elliott,
1994).
There is evidence that the co-ordination of
sexual reproduction by hormonal control is not
confined to heterothallic forms of Achlya, but
also takes place in homothallic species. The fact
that it is possible to initiate sexual reactions
between homothallic and heterothallic species
of Achlya shows that some of the hormones are
common to more than one species, although
Table 5.2. Postulated effects of hormones on sexual reactions in Achlya ambisexualis.
Hormone Produced by Affecting Specific action
A Vegetative hyphae Vegetative hyphae Induces formation of antheridial branches.
B Antheridial branches Vegetative hyphae Initiates formation of oogonial initials.
C Oogonialinitials Antheridial branches (1) Attracts antheridial branches.
(2) Induces thigmotropic response and
delimitation of antheridia.
D Antheridia Oogonialinitials Induces delimitation of oogonium
by formation of basal septum.
After Raper (1939). So far, only hormones A and C (antheridiol) and B (oogoniol) have been shown
to exist.