Growth, Differentiation and Sexuality

14 Asexual Sporulation in Mycelial Fungi
R. Fischer 1 ,U.Kües 2
CONTENTS
I. Introduction ......................... 263
II. Spore Formation ...................... 263
A. VariabilityofSpores
andSporeProduction ............... 263
B. Ecological Aspects of Spore Production . . 264
C. Sexual Versus Asexual Reproduction . . . . 266
III. Endogenous and Environmental Factors
Trigger Spore Formation ................ 266
IV. Genetics of Asexual Spore Formation ...... 268
A. Aspergillus nidulans ................. 268
1.Cytology........................ 268
2.Genetics........................ 268
a) Seven-Transmembrane
and Other Signal Receptors . . . . . . 269
b) HeterotrimericG-Proteins ....... 270
c) MAP-Kinase and Other Signalling
Modules ..................... 271
d) cAMPSignalling............... 272
e) SmallG-Proteins............... 272
f) Transcription Factors
andOtherRegulators ........... 273
g) TargetGenes.................. 275
h) Interdependence of Vegetative
GrowthandDevelopment........ 275
B. OtherAscomycetes.................. 276
C. Basidiomycetes..................... 280
1.Cytology........................ 280
2.Genetics........................ 281
D. ConservedGeneticPathways .......... 283
V. Concluding Remarks ................... 284
References ........................... 285
I. Introduction
Thefilamentousgrowthformistypicalformany
fungi. By indefinite polarized growth, fungi are able
to populate enormous areas of a substrate, even
though most of them are non-motile organisms.
Renowned are the gigantic Armillaria individuals
covering several hundreds of hectares of for-
1 Max-Planck-Institute for Terrestrial Microbiology, Marburg, and
Institute for Applied Life Sciences, Applied Microbiology, University
of Karlsruhe, Hertzstr. 16, 76187 Karlsruhe, Germany
2 Institut für Forstbotanik, Abteilung Molekulare Holzbiotechnologie,
Georg-August-Universität Göttingen, Büsgenweg 2, 37077 Göttingen,
Germany
est grounds in Northern America (Ferguson et al.
2003). However, growth of hyphae requires constant
feeding with organic matter from the environment,
although the mycelium can cope with short
distances of nutrient-depleted areas by means of
hyphal transport (Cairney 2005). One strategy with
which sessile fungi adapt to conditions of limited
resources at a given place is spore formation. Typically,
different types of spores, sexually or asexually
derived, are generated, depending on the purpose
of the spores. Asexual spores are obtained from mitosis
(mitospores). Usually, they are produced in
high numbers as genetically identical propagules
of the individual they come from. Various modes
of production and different types of asexual spores
have evolved in fungi, emphasizing the high significance
they have within fungal life cycles, for fungal
life styles and for survival in the environment.
Often, asexual spores fulfil not only one distinct
function but have several different roles. Asexual
spores can serve as a means of reproduction and
dispersal to new substratum, for the purpose of
mating with other individuals and for perennation
as resting bodies under unfavourable conditions
(Kendrick 1979a,b; Kirk et al. 2001; see also Andrews
and Harris, The Mycota, Vol. IV, Chap. 1).
II. Spore Formation
A. Variability of Spores and Spore Production
Asexual fungal spores are produced by different
mechanisms (Kendrick 1979a,b; Esser 2001; Kirk
et al. 2001).
1. In thallic spore formation, arthrospores or
oidia are generated by fragmentation of
pre-existing hyphal filaments. If production
of a specific hyphal branch directly precedes
fragmentation, spores are also called arthroconidia.
The Mycota I
Growth, Differentation and Sexuality
Kües/Fischer (Eds.)
© Springer-Verlag Berlin Heidelberg 2006