Growth, Differentiation and Sexuality

44 S.D. Harris
astral microtubules seem to play a key role in
this process (Aist and Morris 1999). In particular,
astral microtubules emanating from the SPB make
cortical contacts that appear to be necessary
for post-mitotic nuclear movement (Inoue et al.
1998b). The motor proteins that mediate the
processremainunknown,althoughdyneinisan
obvious candidate, based on its likely role in assembling
astral microtubules (Inoue et al. 1998b).
In addition, as characterized in A. nidulans, ApsA
represents a putative cortical anchor for astral
microtubules (Xiang and Fischer 2004).
VI. Coordination of Mitotic Events
in a Multinucleate Hyphal Cell
A characteristic feature of filamentous fungi is the
presence of multinucleate hyphal cells. Nuclear
numbers per multinucleate cell range from two
in dikaryotic basidiomycetes to as many as one
hundred in the primary hyphae of Neurospora
crassa. Fungi appear to have evolved two distinct
strategies for coordinating mitosis within multinucleate
hyphal cells (Fig. 3.4). One approach,
observed in fungi such as A. nidulans, Alternaria
solani, and Fusarium oxysporum (Rosenberger
and Kessel 1967; Aist 1969; King and Alexander
1969; Clutterbuck 1970; Fiddy and Trinci 1976),
is characterized by a parasynchronous wave of
mitosis that progresses through the hyphal cell,
such that neighboring nuclei are simultaneously
engaged in mitosis. By contrast, fungi such as
N. crassa and A. gossypii (Serna and Stadler
1978; A. Gladfelter, personal communication)
employ an alternative approach characterized by
asynchronous mitoses, where individual nuclei
undergo mitosis in an autonomous manner. The
mechanisms underlying either mode of mitotic
behavior remain unknown. However, parasynchronous
mitoses are presumably triggered by
a wave of mitosis-inducing factors such as activated
CDKs or NimA. Coupling this wave to
regulated nuclear transport might allow nuclei to
sequentially enter mitosis as the wave propagates
through the hyphal cell. Moreover, asynchronous
mitoses could conceivably be established by merely
eliminating the wave, such that mitotic regulators
are uniformly distributed throughout the hyphal
cell. In this scenario, tightly regulated nuclear
transport and/or localized nuclear autonomous
translation might permit individual nuclei to enter
Fig. 3.4. Comparison of synchronous and asynchronous
mitoses. During a synchronous mitosis, a mitotic wave progresses
through the hyphal tip cell (arrow). The degree of
shading correlates with extent of progression through mitosis
(i.e., darkly shaded nuclei adjacent to the tip are in
anaphase, whereas non-shaded nuclei near the septum are
stillinG2).Nowaveisobservedduringasynchronousmitoses.
Instead, nuclei appear to enter mitosis (darkly shaded
nuclei) in a completely autonomous manner
mitosis regardless of the mitotic status of their
neighbor. Preliminary evidence supporting this
model for asynchronous mitoses was recently
obtained in A. gossypii, wheretheentirecellcycle
can be completed despite the presence of uniform
levels of mitotic B-type cyclins (A. Gladfelter,
personal communication).
At this point, it is not clear why mitosis occurs
in a synchronous manner in some filamentous
fungi,whereasitisasynchronousinothers.Anadditional
complication is that mitotic synchrony can
break down under sub-optimal growth conditions,
as observed in A. nidulans (Rosenberger and Kessel
1967). To some extent, the explanation may lie in
the relative advantages of mitotic asynchrony. For
example, in fungi that possess a high ratio of nuclei
perunitofcytoplasm,synchronousmitosesmay
not be sufficient to ensure that all nuclei divide
within one round of the duplication cycle. Accordingly,
asynchrony may be the only way to maintain
the appropriate ratio. Alternatively, mitotic asynchrony
may allow hyphal cells to better cope with
DNA damage by limiting the number of actively
dividing nuclei at any given time. This may not
be important for fungi such as A. nidulans, where