Growth, Differentiation and Sexuality

7 Re-Wiring the Network: Understanding the Mechanism and Function
of Anastomosis in Filamentous Ascomycete Fungi
N.L. Glass 1 ,A.Fleissner 1
CONTENTS
I. Introduction ......................... 123
II. Germling Fusion ...................... 125
III. Hyphal Fusion ........................ 125
IV. Mechanistic Aspects of Anastomosis ...... 126
A. Competency....................... 127
B. Pre-Contact ....................... 127
1.SignalingMolecules............... 127
2.Receptors ....................... 128
3.G-Proteins ...................... 130
4.MAPKinasePathways .............
5. Initiation of Branch Formation/Pegs
130
andPolarization..................
C. Contact, Adhesion
131
andCellWallBreakdown............. 132
D. Pore Formation and Cytoplasmic Flow . .
V. Anastomosis Mutants in Filamentous Fungi
133
of Unknown Function ..................
VI. Physiological and Morphogenetic
133
Consequences of Anastomosis ........... 134
VII. Conclusion ........................... 135
References ........................... 135
I. Introduction
Afilamentousfungalcolonyconsistsofanetwork
of interconnected multinucleate hyphae that
grows by hyphal tip extension, branching and
anastomosis (hyphal fusion). Vegetative hyphae
within a colony, such as that of the filamentous
ascomycete Neurospora crassa, can be separated
into two morphologically distinct regions. The
first is the peripheral portion of the colony,
whereapicalhyphaegrowoutwardandexhibit
a subapical branching pattern (Buller 1933;
Hickey et al. 2002). Hyphae at the periphery of
the colony exhibit negative autotropism and are
refractory to anastomosis. By contrast, within
theinteriorofacolony,thehyphalmorphology
is distinctly different from that of the peripheral
hyphae, and is highly reticulated in appearance.
Branch initiation occurs at irregular intervals,
1 Department of Plant and Microbial Biology, The University of
California, Berkeley, CA 94720-3102, USA
and these hyphae often show positive tropic
responses associated with hyphal fusion events.
Both hyphal tip growth and the formation of lateral
branches are associated with a membrane-dense
organelle, the Spitzenkörper, found at the tips
of all hyphae, branches and pegs; directionality
of growth of a hypha has been associated
with the position of the Spitzenkörper at the
hyphal tip (Girbardt 1957; Lopez-Franco and
Bracker 1996; Riquelme et al. 1998; Hickey
et al. 2002; Riquelme and Bartnicki-Garcia
2004).
The ability to make an interconnected hyphal
network that is characteristic of a filamentous fungal
colony has certain advantages. Anastomosis between
hyphae within a single colony enables fungi
to establish complex functional units that show coordinated
growth and exploration of their environment
(Buller 1933; Rayner 1996). Cytoplasmic
continuitycanberestoredbygrowthofhyphae
through dead hyphal compartments, followed by
hyphal fusion with living sectors (Buller 1933).
Anastomosis between hyphae of genetically different,
but heterokaryon-compatible genotypes can
lead to genetic diversity via parasexual recombination
and formation of novel genotypes (Pontecorvo
1956; Swart et al. 2001). Parasexuality is thought
to contribute to the high adaptability of fungi in
species that lack sexual reproduction and genetic
diversity generated via meiosis.
Anastomosis between different colonies
also has potential disadvantages. Hyphal fusion
between different individuals increases the risk
of transfer of deleterious infectious elements or
resource plundering (Debets et al. 1994; Debets
and Griffiths 1998; van Diepeningen et al. 1998;
Chu et al. 2002; Bruggeman et al. 2003). Protection
against these threats is mediated by a mechanism
of nonself recognition via heterokaryon incompatibility.
During heterokaryon incompatibility,
hyphal fusion between individuals that have
alternative specificities at nonself recognition loci,
The Mycota I
Growth, Differentation and Sexuality
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© Springer-Verlag Berlin Heidelberg 2006