sampling and the appropriate burn-in values were assessed
with Tracer v1.3 (Rambaut and Drummond 2003). Support
for clades was determined with parsimony bootstrap (1000
replicates with heuristic searching) as implemented in
PAUP v4.0b10 (Swofford 2002).
RESULTS AND DISCUSSION
The combined rDNA analysis represents the largest
and most comprehensive phylogenetic sampling of
the Zygomycota to date. We present an overview of
the clades resolved (F
IG
. 1) with emphasis on mono-
phyletic groups and discuss several problematic and/
or unresolved lineages.
Mucorales.—With more than 300 described species,
this is the largest order (roughly 30%) of known
Zygomycota (Hawksworth et al 1995). Morphologi-
cally members are easily distinguished from other
Fungi: Asexual reproduction is primarily via multi-
spored and/or uni- to few-spored sporangia on mostly
coenocytic, branched and rapidly growing mycelium
(F
IG
. 2E); sporangiospores are always single-celled
and nonmotile; sexual reproduction, where known, is
by the formation of zygospores between opposed or
apposed suspensors (F
IG
. 2A–D; O’Donnell 1979,
Benny et al 2001, James and O’Donnell 2004). Self-
sterility or heterothallism is much more common
than homothallism within the Mucorales, and sexual
reproduction has not been documented for a number
of species (O’Donnell et al 2001). For this reason
asexual structures are used almost exclusively to
identify species. Multilocus DNA sequence data have
not been employed to investigate species limits within
this or any other order of the Zygomycota. However,
given their simple morphology, it is reasonable to
assume that phylogenetic species recognition will
reveal cryptic speciation throughout this phylum
(Taylor et al 2000).
As saprotrophs, mucoraleans are some of the most
common microbes recovered from organic debris,
soil, air and dung. All Mucorales are easily cultured.
With the exception of the Umbelopsidaceae, which
produce restricted, low-growing, ochraceous-to-red-
dish colonies (Meyer and Gams 2003), almost all
other Mucorales (Mucoraceae sensu Benny 2005
http://www.zygomycetes.org/) produce rapidly grow-
ing colonies (distinct from the Mortierellales, see
below)
and
abundant
aerial
mycelium
fill
a 100 mm diam Petri dish in less than a week.
A few species are facultative biotrophic or necro-
trophic mycoparasites (Jeffries 1985), plant or animal
pathogens, including systemic infections of humans,
especially in immuno-compromised or suppressed
patients (de Hoog et al 2000, Ribes et al 2000, Rinaldi
1989). A notable plant disease, seedling blight of rice,
is caused by a species of Rhizopus that harbors an
endosymbiotic bacterium, Burkholderia, that produces
the phytotoxin rhizoxin (Partida-Martinez and Hert-
weck 2005). Other species cause postharvest storage
rots of fruits such as the ubiquitous Rhizopus stolonifer,
the agent of strawberry soft-rot ( James and O’Donnell
2004). In contrast beneficial species are used in
traditional fermented foods in Asia such as R.
oligosporus
in the coconut cake bongkrek and the
soybean cake tempeh, a staple in Indonesia for more
than 2000 y and now a commercial success in North
America and Europe (Nout and Kiers 2005). Actino-
mucor elegans
and Mucor dispersus are used in the
soybean cake tofu in China (Hesseltine 1991).
Two mucoralean zygomycetes have been used as
model organisms, Pilobolus kleinii for studying the
rapid nonmuscular movement associated with explo-
sive discharge of its sporangium and Phycomyces
blakesleeanus for elucidating the biology of light and
color (Cerda´-Olmedo 2001). The exquisite hydraulic
system responsible for the explosive fracture of
a Pilobolus sporangium, clocked at 0.1–10 ms, repre-
sents one of the most rapid movements ever
documented in the fungi (Skotheim and Mahadevan
2005). Whole genome sequencing projects have
targeted P. blakesleeanus (JGI DOE Joint Genome
Institute, http://www.jgi.doe.gov/sequencing/why/
CSP2006/Pblakesleeanus.html) and R. oryzae, the
most
important
etiological
agent
of
human
mucormycosis, (Broad Institute of Harvard and
MIT, http://www.broad.mit.edu/annotation/fungi/
rhizopus_oryzae/).
The available phylogenetic data support Benja-
min’s (1979) narrower circumscription of the Muco-
rales, which was based on differences in nutritional
mode, sexual and asexual reproductive morphology
and septal structure. Compared to the broad circum-
scription of Hesseltine and Ellis (1973), Benjamin
(1979) segregated the Kickxellales, Dimargaritales,
Zoopagales and Endogonales. Although the Endogo-
nales sensu Benjamin (1979) is polyphyletic with
many members now recognized in a separate phylum,
the Glomeromycota (Schu
¨ ßler et al 2001, Redecker
2005), several of Benjamin’s hypotheses have been
supported largely by molecular phylogenetic studies
(Gehrig et al 1996, Jensen et al 1998, Tanabe et al
2000, Keeling 2003, Lutzoni et al 2004, Tanabe et al
2005), including the one presented here (F
IG
. 1).
Based on our analyses of combined ribosomal RNA
loci, the Mucorales and Endogonales appear to be
strongly supported as sister taxa (F
IG
. 1); the current
report is the first to propose this novel phylogenetic
hypothesis. Previous analyses of SSU rRNA gene
sequence data have suggested the Mucorales was
either a sister group of the Entomophthorales
874
M
YCOLOGIA