Introduction to Fungi, Third Edition

180 ZYGOMYCOTA
Fig 7.12 Life cycle of Phycomyces blakesleeanus (diagrammatic and not to scale). From a coenocytic haploid mycelium of either
mating type (þ)or( ), sporangiophores develop. Sporangia are columellate and contain numerous sporangiospores which, in
P. blakesleeanus, are multinucleate.When hyphae of both mating types meet, sexual reproduction is initiated by the formation of
knobbly zygophores which develop into progametangia. Each progametangium divides into a gametangium and a suspensor, the
latter ornamented by black forked appendages. Plasmogamy (P) occurs by lysis of the wall separating the two multinucleate
gametangia.This is followed by mass karyogamy (K), but only one of the numerous diploid fusion nuclei seems to undergo
meiosis (M), and only one of the resulting tetrad nuclei survives in the zygospore during dormancy, so that the sporangiospores
in the germ sporangium are usually of either one or the other mating type.Open and closed circles represent haploid nuclei of
opposite mating type; diploid nuclei are larger and half-filled.
spores of germ sporangia developed from normal
zygospores. If the azygosporic strains are subcultured,
either from single sporangiospores or by
mass transfer, they show a tendency to ‘break
down’ to strains of (þ) or ( ) mating type of
normal appearance. It seems that azygosporic
strains of M. hiemalis are typically diploid and
heterozygous for mating type, i.e. the diploid
nucleus carries both (þ) and ( ) mating type
alleles. The breakdown to the normal (þ) or( )
mating type condition may be brought about
by somatic (i.e. non-meiotic) reduction leading to
aneuploid intermediates, and finally to haploids.
The germination of azygospores is unknown.
7.3 Examples of Mucorales
As mentioned before, the traditional family
classification within the Mucorales is artificial
(see Benny et al., 2001; Tanabe et al., 2004),
and we use it here solely for convenience of
presentation.
7.3.1 Mucoraceae
Mucor
About 50 species of Mucor are currently known
(Kirk et al., 2001). The genus is cosmopolitan, with