Introduction to Fungi, Third Edition

640 USTILAGINOMYCETES: SMUT FUNGI AND THEIR ALLIES
Fig 23.3 Teliospore germination
in Ustilago avenae. (a) Germinating
teliospore showing the four-celled
promycelium, each cell of which
is producing sporidia. Budding of
detached sporidia is also shown.
(b) Germinating teliospore showing
fusion of the two terminal cells
to initiate a dikaryon. (c) Fusion
of conjugation tubes from two
basidiospores to initiate a dikaryon.
these cereal smuts, we prefer to keep their
original names for the time being. The corn
smut pathogen, U. maydis, is not closely related
to the core Ustilago spp. and instead occupies an
intermediate position between Ustilago and
Sporisorium (Stoll et al., 2003). In total, there are
about 230 Ustilago spp. and 190 Sporisorium spp.
(Kirk et al., 2001), although such numbers
obviously vary with the species concept adopted.
The surface of teliospores is an important aid
in identification. Ustilago hordei has a smooth
teliospore surface whereas most other cereal
smuts have spiny surfaces (Figs. 23.2a,b). Huang
and Nielsen (1984) have shown that this difference
between smooth and spiny surfaces is due
to only two genes. Many of those Ustilago spp.
now grouped with Microbotryum have teliospore
surfaces with conspicuous reticulations or
striations. There are also variations in the
processes of teliospore germination in Ustilago
(Ingold, 1983c; Vánky, 1994). The classical pattern
is shown by U. avenae (Fig. 23.3), U. hordei and
U. maydis in which the promycelium is threeseptate.
All four cells give rise to sporidia, and
compatible sporidia fuse following pheromone
stimulation (see p. 643). When attached to a host
plant surface, teliospores of U. avenae germinate
in a different way; instead of producing sporidia,
adjacent compatible cells of the promycelium
fuse directly (Fig. 23.3b; Vánky, 1994). In U. nuda
(Fig. 23.4), direct fusion of promycelium cells
occurs both on agar and in nature. However,
following synchronous division of the two nuclei
in the fusion cell, septa are laid down such that
a mosaic of both mono- and dikaryotic cells
results. Compatible monokaryotic cells may fuse