Growth, Differentiation and Sexuality

shows the same activity in M. mucedo, but not in
P. blakesleeanus (Sutter et al. 1996). The amount of
zygophores formed is usually much higher in partnersoftheoppositematingtype.Severalderivates
exhibit strictly mating type-specific effects by
triggering reactions only in the opposite mating
type. The most active compound in bioassays
using M. mucedo (−) is trisporic acid B, followed
by 4-dihydro-methyltrisporate B, where 27 and
107 pmol (10 and 30 ng) ofsubstancerespectively
are sufficient to induce a response (Sutter and
Whitaker 1981b). A comprehensive model of the
actual events in trisporoid action is still lacking.
As zygophore induction can be observed when
the mating partners are separated by a gap or
barrier, volatile precursors have been postulated.
4-dihydro-methyltrisporate and trisporin are the
candidates proposed to be volatile. Trisporic acid
and trisporol, on the other hand, are certainly
not volatile, their action depending on diffusion
contact between the partners. The precursors are
also said to be involved in zygotropism, i.e. the
directed growth of zygophores leading to contact,
adhesion and fusion of sexual structures of the two
mating partners (Fig. 12.3C; Mesland et al. 1974).
As all trisporoid effects are triggered by
trisporic acid as well as its precursors at more
or less the same order of magnitude, the true
function of trisporic acid in sexual reactions is not
yet fully understood. The precursors have been
described as interhyphal chemical messengers or
hormones, contrasting with the trisporic acids
which are intrahyphal chemical regulators (Sutter
1975). Some findings indicate that all precursors
are internally converted into trisporic acid, which
then would be the only regulatory molecule.
Another chain of thought leads to the idea that the
precursors effect their own regulatory functions,
and that trisporic acid fulfils different functions
or, as might also be the case, no specific function
at all in the regulation of sexual events. The ability
oftrisporicacidtoinducezygophoresmightbe
due to the inherent structural similarities with
the precursor compounds. Trisporic acid and its
precursors might also each cause a different set of
effects. All trisporoids will certainly be involved in
more general regulation networks, as is the case for
the structurally similar compounds retinoic acid
in animals and abscisic acid in plants, which both
are involved in the regulation of gene expression
on a large scale. Directly and by interaction with
a number of other signal molecules and/or binding
proteins, these compounds ultimately act at the
Pheromones 221
transcription level (e.g. Bastien and Rochette-Egly
2004; Chung et al. 2005). A correlation to cAMP
levels was observed in M. mucedo (Bu’Lock et al.
1976). Hitherto, no trisporoid receptor or trisporid
binding protein/motif has been characterized.
Retinol (vitamin A) was found to act as structural
analogue to trisporoids in enhancing carotene
synthesis (Eslava et al. 1974) but it is inactive
in zygophore induction (Bu’Lock et al. 1976, C.
Schimek, personal observation).
It should be kept in mind that trisporoid
actions might be somewhat differentiated between
the divers species, according to their inherent
differences in structuring the mating reactions.
In Absidia glauca and many other species, distinct
zygophores are not formed and, therefore,
zygotropism cannot occur. Rather, progametangia
apparently develop between strong aerial hyphae
of the mating partners which either get in close
contact or even touch each other accidentally
during vegetative growth (peg-to-peg interaction).
The importance of secreted precursors, volatile
or not, in these events has never been analysed
at all. In other species, e.g. P. blakesleeanus and
B. trispora, zygophores are formed in the upper
substrate layer or at the interface between the
substrate and the air. Under such circumstances,
diffusing signal molecules seem much more
plausible, and efficient, than volatiles.
From early studies of zygomycete sexuality,
and based on numerous observations of partial
interspecific reactions, it has repeatedly been
argued that the same signal system is active
throughout a large group of organisms. Trisporoid
action was established for the sexual reactions of
homothallic species (Werkman and van den Ende
1974). With the identification of trisporoids in the
genera Blakeslea, Mucor and Phycomyces, theuse
of trisporic acid across family borders became
a certainty. Meanwhile, trisporoid activity has also
been described in a different order, the Mortierellales
(Schimek et al. 2003), and preliminary data
indicate the presence of these signals, although
not their functions, in Chytridiomycota (S.
Münch, C. Schimek, unpublished data). Moreover,
trisporoids are involved in host recognition in
a special type of parasitism occurring in Zygomycota.
The biotrophic fusion parasite Parasitella
parasitella (Mucorales, Mucoraceae) depends on
the pheromones for identifying suitable hosts.
In some cases, e.g. in crossings with A. glauca,
this dependency is restricted to strictly mating
type-specific infestations (Wöstemeyer et al. 1995).