Introduction to Fungi, Third Edition

266 HEMIASCOMYCETES
Fig10.3 Saccharomyces cerevisiae. (a) Vegetative yeast cells
reproducing by budding. (b) Yeast asci containing mostly four
spores, sometimes with only three spores in focus. (c) Ascus
showing a budding ascospore (arrow). (d) Ascus in which two
spores have fused together and are budding. (e) Two ascospores
fusing (top left), and two fused ascospores forming a
diploid bud (right).
and vitamins of the B group. Such a medium
results in well-grown cells which will sporulate
on transfer to a sporulation medium. Sporulation
occurs best on media in which budding
is inhibited. Low concentrations of an assimilable
carbon source are necessary to provide
energy for the sporulation process. Acetate agar
(1 g glucose, 8:2 g Na acetate3H 2 O and 2.5 g yeast
extract l 1 ) is a good sporulation medium
(Yarrow, 1998).
Diploid yeast cells convert directly into
asci within 12 24 h of incubation in a suitable
sporulation medium. The frequency of ascus
formation in most isolates is quite low, usually
less than 10% (Vaughan-Martini & Martini,
1998b). The cytoplasm differentiates into four
thick-walled spherical spores, although the
number of spores may be fewer (see Fig. 10.3d).
The nuclear divisions which precede ascus
formation are meiotic. As is also the case in
mitosis, the nuclear envelope remains intact
during meiosis, taking up a lobed shape as the
nuclear spindles draw the chromosomes into two
and then four corners of the envelope (Fig. 10.4).
The mechanism of ascospore formation has been
extensively reviewed by Neiman (2005). It is very
similar to that of higher ascomycetes, the main
difference being that there is no common vesicle
enclosing all nuclei prior to ascospore delimitation.
Instead, a cup-shaped double-membrane,
the prospore membrane, associates with each of
the four spindle-pole bodies, and this gradually
encapsulates its nuclear lobe until the four
nuclei separate (Figs. 10.4g,h). As in other ascomycetes,
the ascospore wall is then laid down
in the lumen between the two membranes
surrounding the developing ascospore.