Growth, Differentiation and Sexuality

266 R. Fischer and U. Kües
resources. Pine needle colonisation correlates
with spore dispersal. Macro- and microclimate
effects, nutritional content of the substrate, and
fungal successions and interactions with other
organisms in suppressing or promoting spore
production are seen to influence the outcome
of spore dispersal (Gourbière et al. 1999, 2001;
van Maanen et al. 2000; Gourbière and Debouzie
2003), demonstrating how complex even minute
ecosystems are for fungal spore formation.
Spore dispersal gradients are usually steep
(Gregory 1973; Horn et al. 2001), supporting the
idea that the large mass of fungal spores is destined
for entering new substrate and/or for long-term
survival in a given biotope. Nevertheless, spores
have been reported to be transferred over long
distances, also across oceans, and to successfully
invade new territories (Brown and Hovmøller
2002; Hovmøller et al. 2002).
C. Sexual Versus Asexual Reproduction
Verified by various mechanisms (see Sect. II.2,
and The Mycota, Vol. IV, Chap. 1), asexual reproduction
is very common in mycelial fungi, and
asexuality even defines the class Deuteromycota.
The other extreme are fungi, such as Sordaria
macrospora, which reproduce exclusively sexually
(Chap. 16, this volume). However, due to
current fungal sequencing programmes, there
is mounting evidence that several species previously
classified as strictly asexual indeed have
the genetic inventory for sexual reproduction
(Tzung et al. 2001; Pöggeler 2002; Dyer et al. 2003;
Johnson 2003; Wong et al. 2003; Galagan 2005).
The opportunistic diploid pathogen Candida
albicans was long believed not to have a sexual
cycle. The key genes for sexual development
were, however, found in the genomic sequence
of this fungus, which greatly stimulated research
in this field (Johnson 2003). In the laboratory, it
is now well established that mating and nuclear
fusion occur in this species, with formation of
tetraploids (Bennett et al. 2005). Several other
species considered as asexual were shown to
have a mating type locus organisation with two
alternate idiomorphs, as is typical for heterothallic
ascomycetes (see Chap. 15, this volume). The two
mating type idiomorphs of the barley pathogen
Rhynchosporium secalis, anasexuallyreproducing
species with no known teleomorph, have
been found in equal frequencies in populations
throughout the world. Such frequency-dependent
selection of mating types is consistent with sexual
reproduction occurring in nature, too (Linde
et al. 2003). It is not clear yet how general these
experimental findings are, and whether most, or
even all species classified today as being asexual
have the ability to reproduce sexually, or do so
even in nature.
From an evolutionary point of view, it appears
to be advantageous to reproduce not only asexually
but also sexually, although sexual reproduction disrupts
favourable gene combinations, and requires
energy and time from the organism. It was proposed
long ago that sexual reproduction increases
genetic variation (Weismann 1904). Recently, this
hypothesis was nicely tested by using strains of the
yeast Saccharomyces cerevisiae able to reproduce
sexually in competition assays with purely asexual
strains in which the sexual cycle was prohibited by
two mutations (Goddard et al. 2005). These experiments
revealed that sexual strains adapted faster
to harsh environmental conditions than asexual
ones. In an earlier study with Aspergillus nidulans,
sexual development was shown to slow down
the accumulation of deleterious mutations. Thus,
even for a homothallic fungus like A. nidulans,
propagating both in a sexual and in an asexual
reproduction mode, sexual development is advantageous
(Bruggeman et al. 2003). Although asexual
development can be regarded as very useful for
thelifeofanorganismtospreadintheenvironment
(Sect. II.B), a combination with sexual development
promotes continued existence over evolutionary
time.
III. Endogenous and Environmental
Factors Trigger Spore Formation
Spore formation is an energy-consuming process,
and requires strict regulation of the morphogenetic
pathway to most efficiently use resources to reproduce
and guarantee the survival of the species. Most
spores are destined for distribution through the
air and, in such cases, the fungus has to “ensure”
that the spores are indeed delivered into the air or
produced only when the sporulation structures are
exposed to the air. For these reasons, spore generation
is regulated by a number of endogenous and
environmental factors.
A primary factor for induction of spore formation
is the nutritional status of the mycelium