Growth, Differentiation and Sexuality

II. Germling Fusion
Even during the early days of mycology, microscope
analysis of fusion between germinating
asexual spores was described in detail. In 1930,
Köhler described interactions between germinating
conidia of the same or of different species.
Köhler (1930) also reported that conidia of Botrytis
allii and Fusarium sp. fuse by small hyphal
bridges (“Fusionshyphen”), which are significantly
narrower than germ tubes (Fig. 7.2A). In Fusarium
oxysporum, Mesterhazy (1973) also observed that
ungerminated macroconidia can be connected by
short fusion bridges while still in asexual reproductive
structures, termed sporodochia. Recent studies
on Colletotrichum species and on N. crassa showed
that specialized structures connect germlings as
well as ungerminated conidia (Roca et al. 2003,
2005). These conidial anastomosis tubes (CATs)
differ in size and growth behavior from germ tubes.
In addition to conidia, Köhler (1930) described
fusion between germ tubes within single isolates
of Botrytis, Sclerotinia, Neurospora and Fusarium
species (Fig. 7.2B). More recent reports describe
germling fusion in Fusarium sp., Venturia inaequalis
and N. crassa (Leu 1967; Mesterhazy 1973;
Pandey et al. 2004; Roca et al. 2005; Fig. 7.2C). It
Fig. 7.2. A–D Germinating conidia in close proximity undergo
germling fusion. A Germling fusion between conidia
from Fusarium sp. Adapted from Köhler (1929). B Anastomosis
between germinating conidia from N. crassa. Adapted
from Köhler (1929). C Germling fusion (arrow) between
germinating conidia of N. crassa. Bar = 10 μm. D Lack of
germling fusion in the N. crassa anastomosis mutant so.
Although contact between germ tubes is evident, fusion
(cell wall breakdown and membrane fusion) does not occur
(arrow)
Anastomosis in Filamentous Fungi 125
is currently unclear whether mechanistic or signaling
differences occur between conidial fusion
involving CATs versus germling fusion.
Köhler (1930) also observed interactions
between conidia of different species, although
complete germling fusions were not observed. For
example, conidia of N. sitophila and N. tetrasperma
formed small pegs when confronted. Interactions
between species that were not closely related were
also observed, e.g., hyphae of Botrytis allii formed
small pegs in response to approaching hyphal
tips of N. sitophila. Compared to intra-species
interactions, however, these interactions between
different species were weak.
The frequency of germling fusion in a number
ofspeciesisapparentlyaffectedbybothnutritional
and environmental factors. Generally speaking,
the following rule applies: the fewer nutrients, the
more anastomoses. In Leptosphaeria coniothyrium,
Sclerotinia fructigena, Botrytis and Fusarium sp.,
the frequency of germling fusion was reduced on
rich media (such as 1% malt or potato dextrose
agar), but higher on diluted media (Laibach 1928;
Köhler 1930). However, Leu (1967) reported that
the number of fusions formed between germlings
of Venturia inaequalis was significantly greater
on complete medium than on basal medium or
water agar. These results suggest that factors other
than nutrient availability also contribute to the
frequency of germling fusion between conidia.
In addition to nutritional factors that affect the
frequency of germling fusion, a number of studies
have suggested that the initiation of fusion events
among conidia shows a conidial density-dependent
function. In V. inaequalis, fusion was not observed
between two isolated conidia mounted close to one
another (Leu 1967). However, if multiple conidia
were placed close to one another under the same
conditions, numerous germling fusion events were
observed. Fusion between an isolated conidial pair
could be induced by the addition of culture filtrate
fromthesameordifferentisolates.Interestingly,
culture filtrates from isolates that were anastomosis
deficient did not induce fusion in this assay. The
observed fusion induction was apparently speciesspecific
because culture filtrates of V. pirina did
not induce fusions between V. inaequalis conidia.
III. Hyphal Fusion
Like conidial germling fusion, hyphal fusion
attracted the attention of early mycologists. In