Growth, Differentiation and Sexuality

216 C. Schimek and J. Wöstemeyer
Fig. 12.1. A Structure of sirenin, the sesquiterpenoid gamete
attractant produced by female gametangia of Allomyces sp.
B Sexual differentiation in A. macrogynus: paired female
and male gametagia develop on the same hypha. Bar =
50 μm (Photograph by S. Münch)
uniflagellate gametes develop in “male” and “female”
gametangia, although these are usually located
on the same haploid hypha. In A. macrogynus,
the gametangia are paired, the male being
terminal and the female subterminal (Fig. 12.1A).
The gametes express a mutual attraction system.
The larger and colourless gametes, regarded as female,
attract the faster swimming, smaller male
gametes (Pommerville 1978, 1981), which show an
intense orange colour due to their high amount ofγcarotene.
Fusion results in the formation of a biflagellate
zygote which, in turn, gives rise to the diploid
vegetative stage of the complex life cycle. Attraction
of the gametes is mediated by a sesquiterpene,
the female pheromone sirenin (Fig. 12.1; Machlis
1958). Male gametes release a different compound
to the surrounding water, exerting an attractive
function to female gametes. It is usually termed
parisin (Pommerville and Olson 1987); its chemical
structure has not been elucidated. Female gametes
never react to sirenin or any of its artificial
derivates (Carlile and Machlis 1965a; Pommerville
and Olson 1987; Pommerville et al. 1988).
Fig. 12.2. Structures of trisporic acid and its biosynthetic
precursors, sex pheromones, in Zygomycetes. Only major
metabolites are shown. All structures represent the Bderivate
with a keto function at C13, the different substitutions
and substitution sites of the other derivates being
indicated in the lower right corner. Each species produces
a differently composed mixture of these derivates
As sirenin and several derivates are available
by chemical synthesis, the structural requirements
for biological activity were elucidated. There are
mandatory requirements for the primary hydroxyl
group in the aliphatic side chain (Machlis 1973) and
for the general geometry of the bicyclic ring system
(Pommerville et al. 1988). The hydroxymethyl
groupatthebicyclicringsystemcanbeomitted
without affecting activity in the picomolar concentrationrange.Itcannotbesubstituted,however,by
introducing large hydrophobic groups; introducing
a benzyl ether (-OCH2-C6H5) atthisposition
drops the activity by a factor of 10 6 ,from10pM
to 10 μM. Acomparabledropinactivityisseen
when the ring system is removed (Pommerville
et al. 1988).
The sirenin receptor, still not identified at the
molecular level, depends on the l-form of sirenin
(Fig. 12.1). The d-form is completely inactive, pos-