Growth, Differentiation and Sexuality

following sequence of events has been established:
the initial step is the dehydrogenation at C22/C23,
followed by an oxidation series at C29. Beginning
withamethylfunction,thefinalcarboxylgroupis
synthesized via an ethyl group and the carbonyl as
intermediates. After these modifications, C22/C23
are oxidized again and, finally, cyclization occurs,
yielding the unsaturated γ-lactone ring of the
antheridiol side chain (Fig. 12.4; Popplestone and
Unrau 1974; McMorris 1978).
The oogoniols, despite being derived from the
same precursor, are synthesized by a different pathway.
For a start, fucosterol is oxidized to give an
aldehyde at C29. In the subsequent steps, hydroxylation
occurs at C11 and C15,oxidationoccursatC7,
and the hydroxyl group at C3 becomes esterified.
Finally, after all the ring modifications have taken
place, the C24−28 double bond is reduced (McMorris
and White 1977). No data whatsoever exist on
the enzymatic mechanism of these conversions.
3. Mode of Action and Cellular Consequences
To fully understand the role of steroids in the
regulation of sexual development in Phytophtora
spp., it is necessary to strictly distinguish their
effects from those of other steroid-mediated
regulatory mechanisms. Early observations of the
effects of externally added steroids promoting
vegetative growth (Hendrix 1964, 1965; Elliott
et al. 1966), concomitantly with these being
incorporated to a large extent into the cellular
membranes (Langcake 1974), emphasized the
function of steroids as necessary regulatory and
structural cellular components. In the case of
sexual processes, studies performed on compatible
male and female strains elucidated a sequence
of pheromone responses. Similar reactions were
observed in crossings between homothallic and
one compatible heterothallic strain. For antheridiol,
the same sequence was also obtained with
increasing concentration of externally added
pheromone. Antheridiol itself is produced constitutively
at low concentrations by the female.
As the first, decidedly sexual reaction, apical
growth in the male stops after exposure to a female
strain or within an hour after antheridiol addition
(Gow and Gooday 1987). In the next step, the
characteristic antheridial initials are formed in
a dose-dependent manner at the proximal ends of
male vegetative hyphae (Barksdale 1967). Within
30 min to exposure, certain reactions are also
induced in the mating partner. So, exposure to
Pheromones 225
antheridiol induces the synthesis and release of
oogoniol in male-reacting strains (Barksdale and
Lasure 1974; McMorris and White 1977), and the
formation of oogonial initials. These release higher
amounts of antheridiol and, thus, chemotrophically
attract the antheridial branches (Barksdale
1963, 1967). Higher concentrations of antheridiol
also effect the differentiation of the antheridia
by septum formation, and are also thought to
be involved in the onset of meiosis (Barksdale
1963, 1967). Continual exposure to antheridiol
for at least 30 min also induces conversion of this
compound into less active metabolites in the male
strains of the heterothallic A. ambisexualis or
A. bisexualis, and as well as in the homothallic
A. americana and A. conspicua (Musgrave and
Nieuwenhuis 1975). In female strains, metabolism
of antheridiol does not occur, neither is any antheridiol
metabolism observed in those oomycete
species which are not responsive to antheridiol
in their sexual reactions (Musgrave et al. 1978).
The authors suggest a regulatory function of this
metabolism. Inactivation of antheridiol would
serve in steepening the gradient of antheridiol and,
thus, facilitating directed growth and promotion of
gamete formation. As the antheridia also serve to
direct male gametangial nuclei through specialized
tubes directly to the oospheres enclosed in the
oogonium (Raper 1952), a function of antheridiol
in fertilisation may also be proposed. By strongly
inducing the formation of antheridial hyphae in
homothallic species growing adjacent to a female
Achlya strain, which thereby waste resources,
antheridiol also acts as inhibitor to both sexual
and asexual development of the homothallic
strain (Barksdale 1967; Thomas and McMorris
1987).
A number of studies exist on molecular
changes accompanying the onset of sexual development.
Most of these observations are rather
general in nature, and so the data obtained
might not reflect pheromone action per se but
rather general changes in cellular regulation
following the switch of commitment. Thus,
exposure to antheridiol was found to increase
the activity and the release of cellulases in
males. This increase occurs concomitantly to
the formation of antheridial branches (Thomas
and Mullins 1967, 1969; Mullins and Ellis 1974;
Mullins 1979), implying that it is a necessary
prerequisite to branching and the development
of new apical growth sites. A similar increase
in cellulase activity also accompanies massive