Growth, Differentiation and Sexuality

the standard Mendelian segregation pattern is realized
(Fabritius and Judelson 1997). Open reading
frames within the mating type locus have hitherto
not been identified.
Eight genes have been characterized which are
up-regulated during sexual development in P. infestans.
As the transcription level of two of these
increases during the early stages before physical
contact between the partners has been established,
they might be induced by the α hormones. Very
low expression was also found in vegetative hyphae,
which would not be uncommon in a hormone
response system. Three sequences resemble
that of proteins interacting with RNA: a ribonuclease
activator, an RNA-binding protein of
the Drosophila melanogaster pumilo protein family
(Puf), and Rnase H, and may thus be involved
in mating regulation (Fabritius et al. 2002). The
Puf-like protein was later found to be transcribed
not only during early sexual development but also
during the early asexual development of sporangia
(Cvitanich and Judelson 2003). The predicted
products of two of the up-regulated genes show
similarities to elicitins, and one to a glycoprotein
cell surface receptor (Fabritius et al. 2002). One of
the elicitin-like products belongs to a P. infestans
multigene family of putative extracellular transglutaminases
which are possibly involved in cell
wall strengthening or in enhancing adhesion. The
gene transcribed mating-specifically is the only one
of this gene family for which an elicitin activity
could not be predicted from the sequence data. The
variousputativeenzymesareconceivablyinvolved
in cell wall-related processes within the different
developmental processes leading to vegetative hyphae,
zoospores, sexual organs or haustoria (Fabritius
and Judelson 2003). Even with no evidence so
far for a direct interaction of one of the up-regulated
genes with the α hormone system, or with steroid
signals, the character of all putative gene products
suggests a possible involvement in mating-related
events.
For the oomycete sexual pheromone systems,
the complexity of the hormone response and of
its regulation becomes ever more apparent. The
pheromone-mediated part of the sexual reaction is
strongly interwoven with other developments and
influences, both intracellular and external, resulting
in tightly interconnected intracellular regulatory
events. Other signal compounds, the nutritional
status, and the availability of phospholipids
are certainly involved in the regulation of the sexual
reaction, too.
Pheromones 227
III. Conclusions
In the three organismic groups here considered,
one noticeable fact concerning pheromone action
recurs. The sexual interactions within a larger systematic
unit are mediated by the same or very similar
substances, within members of a given genus in
the Chytridiomycota and the Oomycota, and even
within a given class in the Zygomycota. Nevertheless,
different levels of specificity exist within each
group, enabling recognition of a compatible mating
partner of the same species. In Oomycota and
Zygomycota, species-specificity is, with high probability,
realized by using different derivates and isomers
of a common basic compound.
Genetic analyses of the pheromone systems
are largely missing, and true mating type loci are
not known for nearly all of these organisms. BesidesthePeronosporales,itisonlyinZygomycetes
that first attempts at genetic characterization have
been made. In this group, however, the cooperative
biosynthesis discussed above in this chapter
provides an uncomplicated mating type system
which essentially could do without a mating type
locus governing the complete differentiation programme.
Trisporic acid derivates, especially the esterised
and, thus, non-charged compounds, could
directly pass the cellular membrane and act immediately
as ligands at the level of transcriptional
regulation. Therefore, a mucoralean mating type
locus could be constituted by not more than the
genetic apparatus, structural and regulatory, warranting
that trisporoid biosynthesis be performed
obligatorily cooperatively between complementary
mating types.
This idea is based on the structural similarities
between trisporids and retinoids. Retinoids, which
are also degradation products of β-carotene,
function as intercellular communication systems
in vertebrates, especially in developmental regulatory
processes. Biologically active retinoids
bind to nuclear retinoid receptors belonging to
the steroid/thyroid hormone nuclear receptor
superfamily. Ligand-bound receptor complexes
function as transcription factors by binding to
DNA-binding sites, the retinoic acid response
elements. These are located among the transcriptional
regulatory sequences of target genes (Napoli
1999). Comparable effects of intracellularly localized
trisporoids, triggered by binding directly
to nuclear receptor proteins and thus acting as
transcription factors, are conceivable. According