Growth, Differentiation and Sexuality

124 N.L. Glass and A. Fleissner
termed het loci, triggers a cell death response in
the fusion cell and often in subtending hyphal
compartments (Labarère and Bernet 1977; Jacobson
et al. 1998; Biella et al. 2002; Sarkar et al. 2002;
Marek et al. 2003).
Hyphal fusion within a fungal colony occurs in
filamentous basidiomycete and ascomycete species,
as well as in species in the basal lineage to the
Ascomycota and the Basidomycota, the Zygomycota.
Anastomosis in species of zygomycetes are
apparently not common (Gregory 1984), but were
observed in cultures of Mortierella sp. (Griffin and
Perrin 1960). Recent evidence describes anastomosis
in the zygomycete arbuscular mycorrhizal fungal
species Glomus mosseae, G. caledonium, and G.
intraradices (Giovannetti et al. 1999, 2001). Hyphal
fusionhasalsobeenobservedinspecieswithinthe
Oomycota, such as Phytophthora capsici (Stephenson
et al. 1974), which grow like filamentous fungi,
but are more closely related to algae.
In filamentous ascomycete species, hyphal
fusionoccursduringallstepsofthefungallife
cycle. These fusion events serve different purposes
during the establishment and development of
mycelial colonies. Under certain conditions and
in some species, anastomosis can occur between
germinating and even apparently ungerminated
conidia (Köhler 1930; Mesterhazy 1973; Roca et al.
2003, 2005; Fig. 7.1A), perhaps providing cooperation
during germination to establish a colony
among like genotypes. Hyphal fusion within a mature
colony results in the formation of a network of
interlinked hyphae, which presumably facilitates
communication and resource exploitation (Buller
1933; Rayner 1996; Fig. 7.1B). In the sexual phase
of the life cycle, cell fusion is essential for mating
in out-breeding species. Sexual reproduction can
be initiated by fusion between specialized hyphae
from female reproductive structures (trichogynes)
and a male cell of the opposite mating type,
which may be a hypha or asexual spore, such
as a conidium or microconidium (Bistis 1981;
Fig. 7.1C). In other species, such as Aspergillus
nidulans, out-crossing is believed to require
either transient heterokaryon formation or an
undescribed fertilization event (Bruggeman et al.
2003). After initiation of the sexual cycle, cell
fusion is also associated with ascogenous hyphae
development and crozier formation (Fig. 7.1D),
a process that occurs just prior to karyogamy and
meiosis (Raju 1980; Davis 2000).
In this review, we focus on the phenomenon
of germling/vegetative hyphal fusion between like
Fig. 7.1. Stages in the life cycle of Neurospora crassa where
fusion occurs. A Conidia at sufficient cell density undergo
fusion between germlings. Bar = 10 μm. B Hyphae within
the interior of the colony show chemotropism and hyphal
fusion. Adapted from (Hickey et al. 2002), reprinted with
permission. Bar = 10 μm. C The sexual cycle is initiated
by cell fusion involving a fertile receptive hypha (t trichogyne)
emanating from a female reproductive structure,
the protoperithecia (out of view). The trichogyne shows
chemotropism toward a conidium of the opposite mating
type (c). Arrow indicates fusion point. Bar = 10 μm. D Following
fertilization, nuclei of opposite mating types (mat A
and mat a) proliferate in the ascogenous hyphae. Opposite
mating-type nuclei pair off and migrate into the crozier.
In N. crassa, karyogamy occurs in the penultimate cell of
the crozier (cr). Hyphal and nuclear fusion occurs between
the terminal cell and the subtending cell of the crozier (fc).
Karyogamy, meiosis and an additional mitotic division occur
in the ascus, resulting in an eight-spored ascus. The ascospores
are initially binucleate. Asci, ascogenous hyphae
and croziers treated with DAPI, a nuclear stain. Bar = 20 μm
genotypes in filamentous ascomycete species, particularly
on N. crassa as a model for this process.
We will describe the morphology of these events,
review the known molecular factors involved in
fungal anastomosis, compare them to events associated
with mating cell fusion, and speculate about
thepossibleroleofanastomosisbetweenindividuals
of like genotype. The genetics and molecular
basis of heterokaryon incompatibility, as a consequence
of hyphal fusion between individuals of unlike
genotype, has recently been reviewed (Leslie
1993; Bégueret et al. 1994; Glass et al. 2000; Saupe
2000; Glass and Kaneko 2003) and thus will not be
covered here.