(contact, water droplets, or air currents), in the
remaining zygomycete taxa (Ingold 1978 ;
Tucker 1981 ). In the latter organisms the
mature asexual reproductive structures (spor-
angia, sporangiola, merosporangia, conidia)
are either wet (forming spore drops) or dry
and are dispersed by contact (e.g., small insects,
mites), air (wind currents), or water (rain).
Asexual spores of Entomophthorales are not
produced in any sort of distinct slime but usu-
ally adhere firmly to any substrate they contact.
Taxa in Choanephoraceae and Pilobolaceae
(Mucorales) have highly specialized dispersal
methods. Species ofBlakeslea,Choanephora,
Gilbertella, and Poitrasia(Choanephoraceae;
Voigt and Olsson 2008 ) possess sporangia
with persistent, sutured walls and appendaged
sporangiospores (O’Donnell 1979 ; Voigt and
Olsson 2008 ). Many of these fungi are plant
parasites, and the appendages may aid insects
in dispersing spores from flower to flower or by
other methods such as water droplets. Species
ofPilobolusandUtharomycesare coprophilous
(Grove 1934 ; Kirk and Benny 1980 ), and all
have developed mechanisms for efficiently dis-
persing spores onto vegetative material. The
sporangia ofPilobolusare directed toward a
light source and are actively discharged by a
pressurized string of cytoplasm (Page 1964 ;
Yafetto et al. 2008 ) up to a distance of 2 m or
more, where they attach to any substrate they
contact (Page 1962 ). The larval stage of cattle
lungworm (Dictyocaulus viviparusBloch) can
be dispersed byPilobolus sporangia (Eysker
1991 ).
Utharomyces(Pilobolaceae) has phototro-
pic sporangiophores that rapidly elongate until
contacting a surface. The subsporangial vesicle
ofUtharomyces ruptures on contact with a
solid substrate, releasing the intact, stalked
sporangium (Kirk and Benny 1980 ).
Many sporangia and other aerial structures
are adorned with calcium oxalate crystals or
crystal-bearing spines. The sporangiola ofCun-
ninghamellaandHesseltinellabear long acicular
spines (O’Donnell 1979 ). The distribution and
type of calcium oxalate deposits may aid in
propagule dispersal (Birkby and Preece 1988 ).
VI. Classification
A. Phylum, Subphyla, Classes, and OrdersThe zygomycetous fungi, as treated by Benja-
min ( 1979 ), are defined as having a thallus
consisting of aseptate or regularly septate
hyphae, yeast cells, hyphal bodies, or proto-
plasts. Asexual reproduction is by the produc-
tion of sporangiospores in sporangia,
sporangiola, or merosporangia, or chlamydos-
pores, arthrospores, or conidia. Sexual repro-
duction, where known, is by zygosporogenesis,
although some taxa form azygospores.
Zygospores with opposed suspensors usu-
ally form at or above the substrate surface,
whereas those with apposed or undifferentiated
(hyphoid) suspensors occur at or below the
substrate surface. Taxa where zygospores have
never been observed were classified by their
vegetative and asexual reproductive characters
(O’Donnell 1979 ).
Zygospores form after the hormonally
mediated fusion of two gametangia that arise,
in turn, from undifferentiated vegetative cells,
hyphae, or differentiated hyphae called zygo-
phores. The majority of Mucorales are hetero-
thallic (Schipper and Stalpers 1980 )andform
zygospores only after two separate thalli (desig-
nated + and) are crossed. Most other orders
have a high percentage of homothallic species
showing no discernible mating types and,
therefore, do not require outcrossing.All zygos-
porogenesis in Entomophthorales members is
homothallic. Azygospores (sometimes referred
to as parthenospores) are zygosporelike spores
formed without a prior gametangial conjugation.
Azygospores have been reported in Mucorales
(Benjamin and Mehrotra 1963 ), and develop-
ment has been studied ultrastructurally in two
species ofMucor(Ginman and Young 1989 ).B. Ordinal DistributionBased on differences in nutrition and vegetative
and reproductive morphology, ten orders of
the zygomycotan fungi are recognized and216 G.L. Benny et al.