Growth, Differentiation and Sexuality

C. Oomycota
In the non-mycotan phylum Oomycota, the mating
types cannot be discriminated easily because
clear defining features do not exist. Moreover, the
terms “sex” and “mating type”, commonly understood
to establish a distinct and fixed set of features
and actions, do not seem to apply to all members
of this group. Rather, at least within the Saprolegniales,
a system of “relative sexuality” exists (besides
a number of solely “male” and solely “female”
strains), allowing the strain producing the highest
amount of the “female” sex pheromone, antheridiol,
to define the mating behaviour of the partner
strain(Barksdale 1967;Barksdale andLasure 1973).
Traditionally, the mating types are termed “female”
and “male”. Many strains are homothallic, or selffertile,
the two types of sexual organs, oogonia and
antheridia, being formed on proximal hyphae of
the same thallus (Raper 1952).
Research on the sexual signal system has
been performed almost exclusively on several
heterothallic strains of Achlya ambisexualis or A.
bisexualis, and the homothallic A. heterosexualis
(Saprolegniales), and has been reviewed by
Raper (1952), Barksdale (1969), McMorris (1978),
Gooday (1983, 1994), Gooday and Adams (1993),
and Mullins (1994). Since the early 1990s, little
supplementary information on the biochemistry
of the signal system was added. Another source of
information, especially for structural considerations,
addresses the sexual processes of the plant
pathogens Phytophthora sp. (Elliott 1983) and
Pythium sp. (Peronosporales; Knights and Elliott
1976).
1. Structures
The substance named hormone A by Raper in his
detailed studies of signal exchange during sexual
processes in Achlya in the 1930s and 1940s (reviewed
by Raper 1952) was subsequently isolated
from a female strain of A. bisexualis by McMorris
and Barksdale (1967), and termed antheridiol.
A molecular formula of C29H42O5 was established,
and the existence of hydroxyl and carbonyl functionsaswellasthepresenceofanα-β
unsaturated
γ-lactone and an α-β unsaturated ketone were deduced.
A structure was proposed by Arsenault and
co-workers in the following year (Arsenault et al.
1968), and these authors also first determined the
steroid nature of the pheromone. This structure
(Fig. 12.4) was confirmed by data derived from fully
Pheromones 223
synthetic isomers (Edwards et al. 1969). Based on
a standard tetracyclic steroid nucleus, with a side
chain inserted at C17, the major difference between
antheridiol and mammalian steroid hormones lies
in the length of the side chain. In antheridiol, the
long (10 carbon) side chain also encompasses the
lactone ring. The structure contains two C=C double
bonds, one at C5−6 of the nucleus, the other at
C24−25 of the side chain, as well as two carbonyl
functions at C7 and C26, and two hydroxyl groups
at C3 and C22 respectively.
The structural feature most important for activity
is the stereochemistry at C22 and C23. From
thefourpossiblestereoisomers,onlythefunctional
antheridiol (22S, 23R) exhibits activity at 6 pg/ml,
whereas the activity levels in the 22R, 23S and the
22S, 23S isomers are reduced by a factor of 1000
andactivityisprobablyevenfurtherreducedin
the 22R, 23R stereoisomer (Barksdale et al. 1974).
By contrast, structural changes at the ring system
seem to be of minor importance. Removal of the
C7 ketogroupandexchangeofthefreehydroxyl
group at C3 with its acetate only reduce activity by
a factor of 20. By contrast, changes in the oxidation
status at C22 or C23 lead to dramatic reduction of activity,
whereas further structural divergence of the
side chain yields completely inactive compounds.
A number of other steroids, including mammalian
steroid hormones, have been proved to be equally
inactive (Barksdale et al. 1974).
Themalepheromone,originallytermedhormone
B by Raper (1952), was later renamed oogoniol
(McMorris et al. 1975). The oogoniols turned
outtobeamixtureofactivesteroids.Anumber
of these have been characterized, including the
unesterified oogoniol, its isobutyrate, propionate
and acetate esters, oogoniol-1, -2, and -3 respectively,
and their 24(28)-dehydro-analogues (Mc-
Morris et al. 1975; McMorris 1978). Similarly to
antheridiol, the oogoniols are 29-carbon steroids
containing a Δ 5−7 -ketone chromophore (Fig. 12.4).
They carry an ester substituent at C3, hydroxyl
functions at C11 and C15, andaprimaryhydroxyl
group, too, at C29 attheendofthesidechain.
Oogoniols do not contain a lactone ring (McMorris
et al. 1975) but, analogously to antheridiols,
the physiological activity is strongly dependent on
the structure of the side chain. In fact, the minor
compounds, 24(28)-dehydro-oogoniols, are about
100 times more effective than the saturated analogues
and may represent the physiologically active
compounds (McMorris 1978; Preus and Mc-
Morris 1979). Possibly, biosynthetic intermediates