78 CHAPTER 4
Fig. 4.13The duplication cycle of
Basidiobolus ranarum, a fungus that grows
as hyphae with complete, unperforated
septa on agar plates. (a,b) An apical cell
extends, synthesizes new protoplasm,
and continually draws the protoplasm
forwards. (c) When the protoplasmic
volume attains a critical size the nucleus
divides and a septum is formed. (d,e)
The new apical cell grows on and repeats
the process; the subapical cell produces
a branch, and the protoplasm and nucleus
migrate into this, producing a second
apical cell.cell. When the cell has reached a critical volume the
nucleus divides, and tags then direct the production of
a septum at the site where the cell will divide (stages
5–7). Both Saccharomycesand Schizosaccharomyces can
undergo mating under the influence of pheromones
produced by cells of opposite mating types, so this
represents yet another developmental stage involving
the establishment of polar sites (Fig. 4.12c,d).
The CDC42gene is a key component in the estab-
lishment of polarized growth of Saccharomyces. It
encodes a GTP (guanosine triphosphate)-binding pro-
tein, which acts in concert with the protein products
of other genes (CDC24and BEM1) to recruit septins and
actin microfilaments to the sites where localized wall
growth will occur. Less is known about the establish-
ment of polarity in fission yeast, but homologs of
some of the key budding yeast genes have been
identified in Schizosaccharomyces– for example, the
fission yeast genes ral1/sdc1(homologous to CDC24)
and ral3/scd2(homologous to BEM1). Some human
genes also show base sequence homology with CDC
genes of Saccharomyces, indicating a degree of gene con-
servation through evolutionary time. Thus, the yeast
cell cycle can be used to investigate cellular events of
much wider significance, including the regulation (or
deregulation) of normal cell division.
Do mycelial fungi have a cell cycle?The essence of the yeast cell cycle described above is
that, during a certain length of time (which varies
depending on nutrient availability and other environ-mental factors) one cell produces two cells, while
the biomass doubles, and the number of nuclei also
doubles. This raises the question of whether an
equivalent cycle occurs in mycelial fungi. The first
clear evidence that there is such a cycle in mycelial fungi
was obtained with an unusual fungus, Basidiobolus
ranarum(previously assigned to the Zygomycota, but
now transferred to Chytridiomycota based on its small
subunit rRNA gene sequence). This fungus usually
grows as single large cells in the hind gut of frogs or
other amphibiams, where it causes little or no damage.
But it will grow as hyphae with complete, unperforated
septa on agar plates. B. ranarumalso has extremely large
nuclei (about 20μm diameter) which are easily seen by
light microscopy and which are arranged regularly, one
per cell. As shown in Fig. 4.13, the hyphae extend
by tip growth on agar, synthesizing protoplasm and
drawing it forwards as the tip grows. When a critical
volume of protoplasm has been synthesized, the large
central nucleus divides and a septum is laid down at
the point of nuclear division, creating two cells, each
with one nucleus. The new apical cell then grows on,
and it repeats the whole process when enough proto-
plasm has been synthesized. The penultimate cell,
which has been isolated by the complete septum, pro-
duces a new branch apex and its protoplasm flows into
this. In effect, therefore, two hyphal tips (each with a
given protoplasmic volume) are formed from the
original single tip cell, and there is a clear relationship
between cytoplasmic volume, nuclear division, and
branching, exactly like the cell cycle of yeasts. How-
ever, it is termed a duplication cyclerather than a cell
cycle, and the cells remain attached to one another.