Growth, Differentiation and Sexuality

enzymes/proteins secreted to the hyphal tip must
occur during hyphal fusion, from those associated
with cell wall formation to others associated with
cell wall degradation and adhesion. One or perhaps
both participating hyphae coordinates the cellular
machinery involved in hyphal adhesion and cell
wall breakdown. In S. cerevisiae,proteinsinvolved
in adhesion, the agglutinins and enzymes involved
in cell wall breakdown and re-synthesis, are associated
with formation of the mating pair (Cross
1988); many of these genes are transcriptionally
regulated by activation of the pheromone response
MAP kinase cascade.
D. Pore Formation and Cytoplasmic Flow
The formation of a pore, resulting in cytoplasmic
continuity between fusion hyphae, requires fusion
of plasma membranes. Plasma membrane fusion
associated with mating or with developmental processes
in multicellular eukaryotes is not well understood
(White and Rose 2001; Shemer and Podbilewicz
2003). In S. cerevisiae, anumberofmutants
have been identified that are capable of polarization
of the cytoskeleton and schmoo formation,
but fail to undergo mating cell fusion. These
include strains containing lesions in FIG1, FIG2,
FUS1, FUS2 and PRM1 (Elion et al. 1995; Erdman
et al. 1998; Heiman and Walter 2000); PRM1,
FUS1 and FIG1 encode plasma membrane proteins
that are localized to the schmoo tip during mating
(Trueheart and Fink 1989; Erdman et al. 1998;
Heiman and Walter 2000). Fus1 also interacts with
Cdc42 and the formin Bni1, and may function as
ascaffoldfortheassemblyofacomplexinvolvedin
polarized secretion of septum-degrading enzymes
(Nelson et al. 2004; Fig. 7.4). Fus2 is an intracellular
protein that interacts with Rvs161; this complex
also localizes to the schmoo tip (Elion et al. 1995;
Brizzio et al. 1998). Interestingly, many of these
late components, involved with septum degradation
and possibly membrane fusion, such as FUS1,
FUS2, FIG1 and FIG2, arenotconservedinthe
genome of filamentous ascomycete species such as
N. crassa (Glass et al. 2004).
Following pore formation, cytoplasmic mixing
occurs in the fusion region. Often, considerable
cytoplasmic flow through a fusion pore is
observed, possibly due to different turgor pressures
between fusing hyphae (Hickey et al. 2002;
http://www.icmb.ed.ac.uk/research/read/neurospo
ra/movies.html). New septa are frequently formed
Anastomosis in Filamentous Fungi 133
close to the fusion point (Buller 1933; Hickey et al.
2002), perhaps as a response to cytoplasmic flow.
Organelles, such as mitochondria, vacuoles and
nuclei, are capable of being transferred between
fusion hyphae as a result of hyphal fusion (Hickey
et al. 2002; Glass et al. 2004; Fig. 7.3C). It is
possible that hyphal compartments in the vicinity
of the fusion site have a mechanism to adapt
to changes in cytoplasmic flow and organelle
composition. Post-contact consequences of hyphal
fusion, involving physiological adaptation to
cytoplasmic mixing and cytoplasmic flow, are
virtually uncharacterized in filamentous fungi.
The end result of hyphal fusion events is that tips
that were growing along particular trajectories are
changed into conduits through which the contents
from different hyphal compartments mix and are
subsequently shuttled in various directions. It is
a process that terminates growth of specific hyphal
branches at specific locations and times during the
development of a mycelium.
V. Anastomosis Mutants in Filamentous
Fungi of Unknown Function
A number of mutants defective in anastomosis
have been identified in filamentous fungal species,
although the genetic cause of the fusion defect
in most cases has not been identified. These
mutants have been identified primarily by heterokaryon
tests. In Gibberella fujikuroi, amutant
(hsi-1) was identified that was heterokaryon selfincompatible
(Correll et al. 1989). In Verticillium
albo-atrum, four strains showed heterokaryon
self-incompatibility, which was associated with
the inability to undergo anastomosis (Correll
et al. 1988). In F. oxysporum f. sp. melonis, selfincompatible
isolates showed a highly reduced
number of fusion events (Jacobson and Gordon
1988).
In N. crassa, a gene required for hyphal anastomosis,
ham-2 (hyphal anastomosis) encodes a putative
transmembrane protein (Xiang et al. 2002).
The ham-2 mutants show a pleiotropic phenotype,
including slow growth, female sterility and homozygous
lethality in sexual crosses. In addition,
ham-2 mutants fail to undergo both hyphal and
germling fusion and are blind to self. Recently,
afunctionforahomologofham-2 in S. cerevisiae,
termed FAR11, was reported. Mutations in FAR11
result in a mutant that prematurely recovers from