Growth, Differentiation and Sexuality

134 N.L. Glass and A. Fleissner
G1 growth arrest following exposure to pheromone
(Kemp and Sprague 2003). Far11p was shown to
interact with five other proteins (Far3, Far7, Far8,
Far9 and Far10). Mutations in any of these other
genes give phenotypes identical tothose ofthe far11
mutants (Kemp and Sprague 2003). It is possible
that the Far11 complex forms part of a checkpoint
that monitors mating cell fusion in coordination
with G1 cell cycle arrest. Homologs of genes encoding
several of the proteins in the Far11 complex
are lacking in N. crassa, including FAR3 and FAR7
(Glass et al. 2004). It is unknown whether, together
with ham-2, the other FAR homologs are required
for anastomosis in N. crassa.
Another N. crassa mutant, so, shows substantial
reduction in its ability to undergo both selfand
non-self hyphal fusions (Fleissner et al. 2005).
Compared to a wild-type strain, the so mutant
(allelic to ham-1; Wilson and Dempsey 1999)
showsanalteredconidiationpattern,shortened
aerial hyphae, and female infertility. The gene that
complements the so mutation in N. crassa encodes
a protein of unknown function, but that contains
a WW domain. WW domains are predicted to be
involved in protein–protein interactions (André
and Springael 1994). Homologs of the so gene are
present in the genomes of filamentous ascomycete
fungi, but are absent in ascomycete yeast and
basidiomycete species. As with ham-2, mak-2
and nrc-1 mutants, so mutants are deficient in
both germling and hyphal fusion (Fig. 7.2D).
Interestingly, fusion between the mating partners
during sexual development is not impaired in
the so mutant, indicating that so is specific for
vegetative cell fusion events.
VI. Physiological and Morphogenetic
Consequences of Anastomosis
Hyphae of ascomycete and basidiomycete fungi are
dividedintocompartmentsbytheformationof
septa. Although septa in ascomycete species are
not complete and cytoplasmic continuity remains,
the presence of septa decreases the rate of cytoplasmicflow.Sinceasystemwithhighinternalresistance
theoretically has a reduced capacity to explore,
it is possible that the formation of a network
cancounteracttherestrictiveeffectofseptation,
and thereby increase the throughput by connecting
parallel hyphae (Rayner 1996). The role that
anastomoses plays in the structure and function-
ing of a mycelium as a single, dynamic physiological
entity is unclear. Conceptual paradigms that
focusonhyphaltipsasindependentgrowthunits
become limiting when applied to understanding
mycelial networks (Davidson et al. 1996; Rayner
1996; Davidson 1998).
The formation of a hyphal network may be important
in influencing hyphal pattern formation
and morphogenesis in filamentous fungi. Hyphal
fusion may facilitate signaling within a colony (either
by molecules, proteins, or perhaps electric
fields; reviewed in Gow and Morris 1995), which
may also affect the behavior and development of
a filamentous fungal colony. Unlike the artificial
situationofgrowthinapetridish,innaturefungal
colonies exploit diverse environments with variable
distributions and types of nutrient sources.
Furthermore, a single fungal individual interacts
and competes for available nutrients with other microorganisms,
such as fungi and bacteria, as well
as insects. The ability to form a hyphal network
may be needed for coordinated behavior between
the different parts of a fungal colony. An example
of this coordination is illustrated during growth
of the basidiomycete species Hypholoma fasciculare.
If one part of a uniformly circular colony
comesintocontactwithaspatiallydiscretenutrient
source, that part of the colony opposite the nutrient
source ceases its extension and the colony shifts
its growth toward the source. While colonizing this
new resource, the portion of the colony distal to the
nutrient source degenerates (Dowson et al. 1989).
In environments showing a heterogeneous
distribution of resources, transfer of nutrients
occurs from the place of uptake (source) to parts
of the colony in need of these molecules (sinks).
For example, in the basidiomycete R. solani,
carbon from regions of the colony growing on
a carbon-rich medium is translocated to regions
lacking this resource (Jacobs et al. 2004). Another
example for the necessity of metabolite transport
across a colony is biotrophic plant pathogens, such
as Erisyphe graminis and Uromyces fabae, which
form haustoria within plant cells. Nutrients taken
up by these specialized structures in plant cells are
redistributed to the other parts of the mycelium
that subsequently undergo differentiation and
reproduction (Staples 2001; Voegele et al. 2001).
Presumably, the formation of a hyphal network
facilitates the transport of nutrients from one part
of the colony to another.
In some basidiomycete species, it has been
shown that the formation of a larger functional