Introduction to Fungi, Third Edition

EXAMPLES OF MUCORALES
191
Thamnidium
The only species is T. elegans (Fig. 7.24), which
grows in soil in cold and temperate regions, and
on the dung of many different animals (Benny,
1992). It is psychrophilic, continuing to grow
at 1 2°C, with an optimum 18°C and a maximum
at 27 31°C (Domsch et al., 1980). It has
been reported from meat in cold storage.
In culture, large terminal columellate sporangia
are produced on tall sporangiophores which
may also have repeatedly dichotomous lateral
branches bearing fewer-spored columellate or
non-columellate sporangiola. The sporangiola
may also be borne on separate branch systems.
Low temperature and light induce the formation
of sporangia as opposed to sporangiola. During
the development of the sporangiophores, spiral
growth occurs as in Phycomyces (see Fig. 7.6).
Electron microscopy studies of the development
of sporangia and sporangiola show that
they develop in essentially the same way
(J. Fletcher, 1973a,b). At maturity the columellate
sporangia become converted into sticky sporangial
drops. In contrast, the sporangiola are easily
detached in wind tunnel experiments. A change
from damp to dry air leads to increased liberation
of sporangiola (Ingold & Zoberi, 1963).
Thamnidium elegans is heterothallic and forms
zygospores resembling those of Mucor or Rhizopus,
but they are produced best at low temperatures
such as 6 7°C and not at 20°C (Hesseltine &
Anderson, 1956).
7.3.4 Chaetocladiaceae
The family Chaetocladiaceae contains two
genera, the facultatively mycoparasitic Chaetocladium
and the saprotrophic Dichotomocladium.
Their fertile hyphae are branched and bear
monosporous sporangiola on fertile vesicles.
The main branches terminate in sterile spines
(Benny & Benjamin, 1993). Whilst species of
Chaetocladium are believed to be psychrophilic
and are rarely collected within the tropics, all
known species of Dichotomocladium have been
recorded only in tropical areas (Kirk, 1993).
Chaetocladium
In Chaetocladium (Fig. 7.25) there are no Mucor-like
sporangia. Sporangiola, each containing a single
spore, are borne on lateral branches which
end in spines. Such monosporous sporangiola
are sometimes termed conidia. There are two
species, C. jonesii and C. brefeldii, both parasitic
on other Mucorales (Benny & Benjamin, 1976),
especially on Mucor or Pilaira growing on dung.
At the point of attachment to the host there
are numerous yellow galls. These are unique
bladder-like outgrowths which contain nuclei
of both the host and the parasite in a common
cytoplasm. Chaetocladium does not form haustoria
and has been described as a fusion biotroph
(Jeffries & Young, 1994). Both Chaetocladium spp.
can, however, be cultured on standard agar
media in the absence of a host. They are
heterothallic. Chaetocladium brefeldii is heterogametangic,
forming zygospores resembling
Zygorhynchus. Burgeff (1920, 1924) has claimed
that a given strain of Chaetocladium can only
parasitize one of the two mating type strains
of heterothallic Mucor spp., suggesting that the
parasitic habit of fungi such as Chaetocladium
may have originated from attempted copulation
with other members of the Mucorales. Jeffries
and Young (1994) believed that contact is truly
mycoparasitic and not pseudosexual.
7.3.5 Choanephoraceae
This is probably the only current family in the
Mucorales to be monophyletic (O’Donnell et al.,
2001). Members of the Choanephoraceae are
essentially tropical in their distribution. There
are three genera of which the best-known are
Blakeslea and Choanephora (Kirk, 1984). Asexual
reproduction is by sporangia and sporangiola.
The sporangia which have brown persistent walls
are usually columellate and often hang downwards.
They contain dark brown sporangiospores
with a striate wall and bristle-like appendages at
each end. The sporangiola contain one or a few
spores, also with brown striate walls and with
(Blakeslea) or without (Choanephora) polar appendages.
The dark sporangium walls and the dark
walls of the sporangiospores (due to melanin
and carotenoid pigments), both unusual features
in the Mucorales, may have evolved as a protection
against the mutagenic and oxidizing UV
light and may help to explain the tropical