Growth, Differentiation and Sexuality

38 S.D. Harris
dle organization that accompany mitosis were accurately
described by the mid-20th century using
conventional light and electron microscopy. Key
pre-mitotic events include duplication and separation
of the spindle pole bodies (SPBs), the fungal
equivalent of microtubule-organizing centers
(MTOCs). These structures are embedded within
the nuclear envelope, and uniquely to fungi, appear
to be tethered to interphase chromosomes via
chromatin (Heath 1994). As they migrate, each SPB
generates a half-spindle composed of microtubules.
Once the SPBs are aligned opposite to each other,
the half-spindles come into register to form a full,
bipolar spindle. During the process of spindle assembly,
interphase chromosomes undergo marked
condensation near the developing spindle. Eventually,
each chromosome is captured by spindle microtubules
that bind to the kinetochore. Notably,
in contrast to animals and plants, the nuclear envelope
does not breakdown in preparation for mitosis
(Aist and Morris 1999).
Unlike animals and plants, filamentous fungi
do not display the classic metaphase alignment of
chromosomes at the mid-point of the mitotic spindle
(Fig. 3.1). Instead, metaphase chromosomes
appear to occupy the middle one-third to one-half
of the spindle. Chromosome segregation then
occurs in two steps – anaphase A and anaphase B
(Fig. 3.1). During anaphase A, sister chromatids
disjoin and move toward the opposite spindle
poles. Remarkably, microscopic examination of
this process revealed that chromosome disjunction
is not synchronous (Aist 1969). Accordingly,
during anaphase A, separating chromatids are
often aligned along the entire length of the spindle,
rather than moving in a synchronous wave toward
the pole. Anaphase B begins when the chromatids
reach the poles, and is characterized by rapid
elongation of the spindle and marked development
of spindle asters. Another distinct feature of fungal
mitosis is the absence, during anaphase B, of
a well-defined spindle mid-zone consisting of antiparallel
arrays of overlapping polar microtubules
(Aist and Bayles 1991). This may have important
implication for the subsequent regulation of
cytokinesis. Finally, midway during anaphase
B, the nuclear envelope collapses and re-forms
around the segregated chromosomes. Following
the completion of mitosis in fungal hyphae, nuclei
undergo a period of rapid oscillatory movement
within the hyphal cell. As a result, they achieve
proper alignment throughout the cell prior to
cytokinesis. At this point, it should be emphasized
Fig. 3.1. A–D Progression of mitosis in Aspergillus nidulans
hyphae. Mitotic spindles (A,C) are visualized using
a functional GFP-TubA (α-tubulin) fusion provided by X.
Xiang. Nuclei (B,D) are stained using Hoechst 33258. A,B
Adjacent nuclei engaged in anaphase A (anaA), anaphase B
(anaB), and telophase (telo). C,D Adjacent nuclei found in
metaphase (meta) and anaphase A. Insets Higher magnification
of the metaphase nucleus, showing that the nuclear
material occupies approximately one-half of the spindle.
Bar 4 μm (8 μm for insets)
that not all mitoses are necessarily coupled to
cytokinesis in hyphal cells (Clutterbuck 1970).
B. The Duplication Cycle
Because hyphal cells do not separate following cytokinesis,
the fungal cell cycle has been termed
the duplication cycle (Fiddy and Trinci 1976; Trinci
1978). Like the cell cycle, the duplication cycle is
definedbytheperiodoftimerequiredtoduplicate
and segregate cellular constituents. For example,
the Aspergillus nidulans cycle typically requires
120 min for completion, of which only about 5 min
is devoted to mitosis (Bergen and Morris 1983).