Introduction to Fungi, Third Edition

EXAMPLES OF MUCORALES
183
Fig 7.15 Zygorhynchus moelleri.
(a) Zygospore and sporangium.
(b) Young sporangiophores.
(c) Dehisced sporangia. (d g) Stages
in zygospore formation. Note that
the fungus is homothallic and that the
suspensors are unequal.
human pathogens associated with mucormycosis
(Rinaldi, 1989). Most species of Rhizopus are
heterothallic, but R. sexualis is homothallic and
forms zygospores freely within 2 days in the
laboratory (see Fig. 7.10).
Rhizopus microsporus causes rice seedling
blight in which root growth is strongly impaired
by a toxin, rhizoxin, excreted by the soil-borne
pathogen. The toxin binds to b-tubulin, thereby
interfering with mitosis. Intriguingly, it is
synthesized not by R. microsporus but by bacteria
(Burkholderia spp.) living endosymbiotically
within the cytoplasm of Rhizopus hyphae
(Partida-Martinez & Hertweck, 2005). Bacterial
endosymbionts have been reported only rarely
from fungi, e.g. in the zygomycete Geosiphon
pyriforme (p. 221), or within hyphae of the
ascomycete Morchella elata (p. 427) and the
basidiomycete Laccaria bicolor (p. 552).
Absidia
There are some 20 species growing in soil
(Domsch et al., 1980). Characteristic features are
pear-shaped sporangia arising in partial whorls
along stolon-like branches which produce
rhizoids at intervals but not opposite the