Growth, Differentiation and Sexuality

222 C. Schimek and J. Wöstemeyer
4. Genetic Consequences
Although sexuality and the trisporoid communication
system have been the focus of research on and
off at least for the last five decades, only fragments
are known about the genetic basis of these features.
From the elucidation of the cooperative synthesis
pathway, two hypotheses concerning the genetic
situation have been formulated: first, a single mating
type gene would exist, with the two alleles (+)
and (−). The alleles would determine which of the
two mating type-specific enzyme activities, i.e. the
(−)-specific C4-dehydrogenase or the (+)-specific
C1-oxygenase, would become repressed or constitutively
expressed (Bu’Lock et al. 1976). The second
hypothesis states that all necessary genes would existinbothmatingtypes,butthatthesynthesisof
one set of mating type-specific enzymes would be
blocked (Nieuwenhuis and van den Ende 1975). In
a side argument, the sex-specific enzymatic steps
were proposed to be repressed in single growing
cultures, but to become derepressed in mating situations
(Bu’Lock et al. 1973, 1976). Various independent
observations led to the consensus that a true
mating type locus might not exist in Zygomycetes.
A genetic difference between mating types reflected
at the protein level relates indeed to a plasmid
encoding a non-glycosylated 15-kDa external
membrane protein which was found exclusively in
the (+) mating type of Absidia glauca (Teepe et al.
1988; Hänfler et al. 1992). Monoecious mutants,
obtained by protoplast fusion of complementary
mating types, do not express the protein, indicating
that the protein might not be necessary for the
mating process (Wöstemeyer et al. 1990).
The only gene identified from the trisporoid
synthesis system in M. mucedo (Czempinski et al.
1996; gene referred to as TDH) and P. parasitella
(Schultze et al. 2005), TSP1, codes for the 4dihydro-methyltrisporate
dehydrogenase (TDH),
the enzyme with the strictly (−) type-specific
activity (Fig. 12.3). Enzyme activity was first detected
in (−) zygophores of M. mucedo and in the
(−) equivalents of the homothallic Zygorhynchus
moelleri (Werkman 1976). Enzyme activity was
also documented in several species of Mortierella,
belonging to the Mortierellales, thus establishing
the use of trisporoids also in that order (Schimek
et al. 2003). In addition, the study revealed
enzyme activity throughout sexually stimulated
(−)-type mycelium. Enzyme activity in mycelium
beyond the sexually committed hyphae was also
observed in M. mucedo and A. glauca (C. Schimek,
unpublished data). As was already suggested by
Mesland et al. (1974), trisporoid biosynthesis is
evidently not restricted to hyphae undergoing
sexual morphogenesis. Trisporoids are usually
extracted from mycelia growing submerged. The
biosynthesis pathway is therefore fully active
under conditions where no zygophores and, in
fact, no aerial structures at all are formed.
The TSP1 gene for TDH was isolated and
characterized by Czempinski et al. (1996), and was
found to exist in both mating types of M. mucedo,
A. glauca, P. parasitica and B. trispora. In the
meantime, the gene has been identified in about
30 species, belonging to more than 10 families
of three orders of the Zygomycetes, including P.
blakesleeanus and Thamnidium elegans and, wherever
both mating types were analysed, the gene was
detected in each of these (K. Voigt, unpublished
data). Another set of experiments confirms the
interpretation presented by Sutter et al. (1996)
concerning the existence of two independent
dehydrogenase enzymes in P. blakesleeanus. Our
data clearly show two biochemically separable
enzymatic activities catalysing the conversion of 4dihydrotrisporin
and 4-dihydro-methyltrisporate
respectively in M. mucedo (C. Walter, unpublished
data). Two recent studies address the regulation
of TSP1, encodingTDH,inP. parasitica (Schultze
et al. 2005) and M. mucedo (Schimek et al. 2005).
From these results, it appears that the regulation
of trisporoid biosynthesis enzymes varies between
the species. The TSP1 gene is part of a complex
gene cluster in both species, but the ORFs found
in the P. parasitica gene cluster are not all present
in the M. mucedo cluster (Schimek et al. 2005;
Schultze et al. 2005). TSP1 itself is constitutively
transcribed in both species, indicating that enzyme
activity is regulated post-transcriptionally.
In P. parasitica, gene expression and regulation
follow the same pattern in intraspecific sexual
interactions as well as in parasitic interactions
with the host A. glauca. With the exception of
a not yet fully characterized putative HSP gene
(K. Schultze, A. Burmester, unpublished data),
the other ORFs in the gene cluster seem not to
be involved directly in TSP1 regulation. In the
partially sequenced genome of Rhizopus oryzae
(http://www.broad.mit.edu/annotation/fungi/rhiz
opus_oryzae/),TSP1 is situated in a similar genetic
context as that in M. mucedo. Whether or not the
different organization of the gene cluster is related
to the parasitic life style of P. parasitica remains to
be studied in more detail in the future.