Growth, Differentiation and Sexuality

comparative observations made on different ascomycete
and basidiomycete species, he concluded
that vegetative fusions require two growing tips.
Depending on the involvement of hyphal tips or
short lateral branches, called pegs, he categorized
fusion events into four types: hypha-to-hypha,
hypha-to-peg, peg-to-peg and hook-to-clamp or
clamp-connection fusions. Later studies proved
that direct tip-to-side fusions are common in fungi;
hyphal tips can fuse laterally with a hypha in the
absence of a recognizable peg (Aylmore and Todd
1984; Todd and Aylmore 1985; Hickey et al. 2002).
Mechanistically, the process of hyphal fusion can
be divided into three steps: (1) pre-contact, (2)
contact, adhesion and cell wall breakdown, and (3)
pore formation and cytoplasmic flow (Glass et al.
2000, 2004; Hickey et al. 2002). In the following sections,
we describe aspects of the fusion process and
discuss recent literature on possible mechanisms
regulating anastomosis. We also compare and
contrast mating cell fusion (using Saccharomyces
cerevisiae as a model) to germling and hyphal
fusion in filamentous ascomycete species.
A. Competency
Vegetative hyphal fusion is a highly orchestrated
process initiated by a physiological switch within
the growing and developing mycelium that renders
a portion of the colony fusion competent. What is
required or involved with the developmental switch
from fusion refractory hyphae (hyphae at the periphery
of the colony) to fusion-competent ones
(within the interior of the colony) is currently unknown.
It is also not clear what controls the frequency
of, or the spatial and temporal distribution
of hyphal fusion events within the fusioncompetent
region of a fungal colony. Fusions within
the interior of a colony are not uniform, indicating
that microenvironmental factors within the colony
may play an important role in influencing the distribution
of fusion events within a colony.
B. Pre-Contact
Early observations that fusion hyphae alter their
trajectoryinresponsetoproximityledtotheidea
thatthefusionpartnersarecapableofremotesensing.
In his description of anastomosis in Botrytis,
Ward (1888) notes: “When one sees a hypha deflected
from its previous course through nearly
a right angle (. . .) and I have seen cases in an-
Anastomosis in Filamentous Fungi 127
other fungus where the deflection amounts to considerably
more than a right angle, it seems to me
impossible to avoid the impression that some attraction
is exerted” (Ward 1888). Numerous other
studies have described pre-contact attraction of hyphae
that eventually undergo fusion (Köhler 1930;
Buller 1933; Hickey et al. 2002). Hyphae involved in
fusion thus show a chemotactic response, such that
reorientation of each hypha toward its neighbors
resultsinacommongrowthtrajectory;thepresence
of a fusion-competent hypha also often results
in the formation of a peg in a receptive neighboring
hypha.
In N. crassa, the reorientation of hyphae destined
to fuse is associated with alterations in the
position of the Spitzenkörper, or results in the formation
of a new Spitzenkörper associated with peg
formation in the receptive hypha (Hickey et al.
2002; Fig. 7.3A). The initiation of branching events,
which is also associated with the formation of a new
Spitzenkörper, has been hypothesized to be regulatedbyanexcessintherateofformationofvesicles
associated with tip growth in relation to the rate of
deposition of these vesicles at the apex (Watters
and Griffiths 2001; Riquelme and Bartnicki-Garcia
2004). However, it is unlikely that such a mechanism
is functioning during peg formation in a receptive
fusion hypha. Presumably, a fusion hypha
must be able to sense a gradient in chemotactic
signals, which results in a change in Spitzenkörper
localization within the hyphal apex and an alteration
in growth trajectory. The localization of the
Spitzenkörper in the hyphal apex has been associated
with directionality of growth (Riquelme et al.
1998). Upon physical contact, the two Spitzenkörper
of the fusion hyphae are juxtaposed at the point
of contact (Fig. 7.3B; Hickey et al. 2002). During
fusion-hypha growth and peg formation, the function
of the Spitzenkörper is believed to be similar
to its function at hyphal tips – secretion of cell wall
and membrane material associated with growth
via membrane-bound vesicles (Grove and Bracker
1970; Howard 1981; Bartnicki-Garcia et al. 1989;
Gierz and Bartnicki-Garcia 2001).
1. Signaling Molecules
The nature of the substances that act at a distance
and which are involved in hyphal or germling fusion
is not known. Microscopically, events associated
with germling and hyphal fusion are similar,
although it is possible that different signaling
molecules may be involved in each process. Sig-