Phylogeny of the Zygomycota based on nuclear ribosomal sequence data
Merlin M. White
1,2
Department of Ecology & Evolutionary Biology,
University of Kansas, Lawrence, Kansas 66045-7534
Timothy Y. James
Department of Biology, Duke University, Durham,
North Carolina 27708
Kerry O’Donnell
NCAUR, ARS, USDA, Peoria, Illinois 61604
Matı´as J. Cafaro
Department of Biology, University of Puerto Rico at
Mayagu
¨ ez, Mayagu
¨ ez, Puerto Rico 00681
Yuuhiko Tanabe
Laboratory of Intellectual Fundamentals for
Environmental Studies, National Institute for
Environmental Studies, Ibaraki 305-8506, Japan
Junta Sugiyama
Tokyo Office, TechnoSuruga Co. Ltd., 1-8-3, Kanda
Ogawamachi, Chiyoda-ku, Tokyo 101-0052, Japan
Abstract:
The Zygomycota is an ecologically heter-
ogenous assemblage of nonzoosporic fungi compris-
ing two classes, Zygomycetes and Trichomycetes.
Phylogenetic analyses have suggested that the phylum
is polyphyletic; two of four orders of Trichomycetes
are related to the Mesomycetozoa (protists) that
diverged near the fungal/animal split. Current
circumscription of the Zygomycota includes only
orders with representatives that produce zygospores.
We present a molecular-based phylogeny including
recognized representatives of the Zygomycetes and
Trichomycetes with a combined dataset for nuclear
rRNA 18S (SSU), 5.8S and 28S (LSU) genes. Tree
reconstruction by Bayesian analyses suggests the
Zygomycota is paraphyletic. Although 12 clades were
identified only some of these correspond to the nine
orders of Zygomycota currently recognized. A large
superordinal clade, comprising the Dimargaritales,
Harpellales, Kickxellales and Zoopagales, grouping
together many symbiotic fungi, also is identified in
part by a unique septal structure. Although Harpel-
lales and Kickxellales are not monophyletic, these
lineages are distinct from the Mucorales, Endogo-
nales and Mortierellales, which appear more closely
related to the Ascomycota + Basidiomycota +
Glomeromycota. The final major group, the insect-
associated Entomophthorales, appears to be poly-
phyletic. In the present analyses Basidiobolus and
Neozygites group within Zygomycota but not with the
Entomophthorales. Clades are discussed with special
reference to traditional classifications, mapping mor-
phological characters and ecology, where possible, as
a snapshot of our current phylogenetic perspective of
the Zygomycota.
Key words:
Asellariales, basal lineages, Chytridio-
mycota, Fungi, molecular systematics, opisthokont
INTRODUCTION
Most studies suggest that the phylum Zygomycota is
not monophyletic and the classification of the entire
phylum is in flux. The Zygomycota currently is
divided into two classes, the Zygomycetes and
Trichomycetes. However molecular phylogenies sug-
gest that neither group is natural (i.e. monophyletic).
Two orders previously classified as Trichomycetes, the
Amoebidiales and Eccrinales, are not fungi (Benny
and O’Donnell 2000, Cafaro 2005). Furthermore
Microsporidia, animal pathogens previously consid-
ered to be protists, might have evolved within the
Zygomycota (but see Tanabe et al 2005) (Keeling et al
2000, Forget et al 2002, Keeling 2003). One signifi-
cant recent change has been the removal of the
glomeralean arbuscular mycorrhizal (AM) fungi from
Zygomycota and their reclassification as a separate
phylum, the Glomeromycota (Schu
¨ ßler et al 2001).
Members of the Zygomycota have been considered
a primitive and early diverging lineage of the Fungi
because they lack complex fruiting structures and
most representatives have coenocytic aseptate hyphae
during all or part of their life cycle. Zygomycota often
might be overlooked even by mycologists, but they
can be encountered as near as the kitchen, disguised
as moldy bread or fruit (James and O’Donnell 2004).
Some are beneficial, such as Rhizopus, used in several
traditional fermented foods, including tempeh and
cassava. Most species of the Zygomycota are saprobic,
but some species are facultative pathogens of plants,
animals (including humans) or even other fungi. The
Trichomycetes are obligate endosymbionts of arthro-
pods. Indeed members of the Zygomycota are
ubiquitous in nature, in a seemingly endless variety
of interactions.
2
Current Address: Department of Biology, Boise State University,
Boise, Idaho 83725-1515 (as of Jan 2007)
1
Corresponding Author. E-mail: MerlinWhite@boisestate.edu
This paper is dedicated to the great many students of the ‘‘lower
Fungi’’—past and present—who helped introduce us to this
wonderfully eclectic and diverse group that we can only hope to
highlight herein.
Accepted for publication 10 October 2006.
Mycologia, 98(6), 2006, pp. 872–884.
#
2006 by The Mycological Society of America, Lawrence, KS 66044-8897
872
For this paper the Zygomycota is divided into nine
orders: Asellariales, Dimargaritales, Endogonales,
Entomophthorales, Harpellales, Kickxellales, Mortier-
ellales, Mucorales and Zoopagales. The clades de-
fined by this ordinal classification correspond in large
part with niche and habitat, as well as mode of asexual
and sexual reproduction. Many species are saprophyt-
ic in soil and dung (Mucorales, Mortierellales and
Kickxellales). Some (Mucorales) are extremely fast
growing in agar culture while others (Zoopagales and
Entomophthorales) may be slower growing reflecting
their parasitic strategy on various small animals,
including insects, rotifers or even amoebae. Members
of Zoopagales, Dimargaritales and Mucorales may be
parasitic on other fungi (including mushrooms, as
well as other zygomycetes). Trichomycetes (Harpel-
lales and Asellariales) have specialized for arthropod
gut wall attachment. Some Harpellales are remark-
able in their ability to shift from a commensalistic to
parasitic mode for dispersal (White et al 2006). Other
parasitic groups have invasive haustoria (Dimargar-
itales and Zoopagales) or grow as naked protoplasm
inside the host cell wall where they are in direct
contact with the invaginated host cytoplasm (Ento-
mophthorales). The Endogonales are either sapro-
phytic or ectomycorrhizal with plants.
Not only do these fungi display a variety of life
histories but also the morphology of the Zygomycota
is diverse with regard to reproduction and dispersal.
Asexual spores are single-celled and include mito-
sporic sporangiospores and true conidia (for review
see Benny et al 2001). Most zygomycetes reproduce by
sporangiospores, which are derived from the internal
cleavage of sporangial cytoplasm. These spores are
wind or animal dispersed after rupture of the
sporangium wall (Ingold 1978). Asexual spores of
the Entomophthorales appear to have a different
ontogeny and are referred to as conidia. They are
produced at the end of differentiated hyphae and
lack a sporangial wall. Most are forcibly discharged
and some can germinate and produce a secondary
spore. Meiospores called zygospores, where known,
are produced with or without the conjugation of
compatible hyphae and are usually thick-walled and
globose; however they are biconical in the Harpellales
(Benny et al 2001, Lichtwardt et al 2001).
Whereas morphology formed the traditional foun-
dation for taxonomy and classification, phylogenetic
analyses of DNA sequence data were necessary to
reveal major evolutionary trends among the basal
fungi ( Jensen et al 1998, O’Donnell et al 1998,
Sugiyama 1998, James et al 2000, Tanabe et al 2000,
Gottlieb and Lichtwardt 2001, O’Donnell et al 2001,
White et al 2001, White 2002, 2006, Tanabe et al 2004,
Cafaro 2005, Seif et al 2005, Tanabe et al 2005). Most
molecular phylogenetic studies have highlighted the
nonmonophyly of the previously defined Zygomycota
and recognized the need for additional contributions,
especially with understudied and unculturable taxa.
For example analyses to date have shown that the
Entomophthorales is monophyletic with the excep-
tion of the inconsistent placement of Basidiobolus.
Aside from the protistan Trichomycetes (Amoebi-
diales and Eccrinales) a third order (Harpellales)
appears nested within the Zygomycetes (O’Donnell et
al 1998, Lutzoni et al 2004, Taylor et al 2004, Cafaro
2005) whereas the remaining order (Asellariales) has
not been included in phylogenetic studies to date.
This is the first phylogeny to incorporate nearly full
length rDNA sequences (18S, 5.8S and 28S rRNA
genes) for the Zygomycota. Taxa were selected to
include representatives of as many recognized orders
of this phylum as possible. Based on the results of this
three-locus phylogeny, the monophyly of the Zygo-
mycota, its two classes, the Zygomycetes and Tricho-
mycetes, and eight orders are assessed. The results
presented herein serve to highlight our current
understanding of their evolutionary history and to
punctuate areas where our knowledge is woefully
inadequate.
MATERIALS AND METHODS
Sequences of 104 taxa representing all fungal phyla and
including metazoan and Mesomycetozoa outgroups were
obtained from the Assembling the Fungal Tree of Life
(http://ocid.nacse.org/research/aftol/) and GenBank
(http://www.ncbi.nlm.nih.gov/) databases and combined
with sequences generated during this study. Many taxa (n 5
46) contain nearly full length sequences for the rRNA
operon (nearly complete 18S [SSU], 28S [LSU] and in most
cases 5.8S). Both 18S and partial 28S sequences were
available for 38 taxa; 20 are represented only by 18S
sequences (see S
UPPLEMENTARY
T
ABLE
I).
Data for the 18S, 28S and 5.8S ribosomal coding regions
were manually aligned with GeneDoc v2.6 (Nicholas and
Nicholas 1997) and combined into a single supermatrix
(10 053 characters) in which ambiguous regions (exsets)
were excluded from further analyses with MacClade v4.05
(Maddison and Maddison 2002). The final dataset con-
tained 3832 aligned characters used for analyses (1395
parsimony informative and 1996 invariant). We used
Modeltest v3.7 (Posada and Crandall 1998) to determine
the most appropriate model of evolution for use in
a maximum likelihood framework. This model (GTR+I+G)
was used to estimate phylogenetic relationships and clade
support with MrBayes v3.1 (Huelsenbeck and Ronquist
2001). Two independent runs were conducted, each with
four chains for 1 3 10
7
generations, in which trees were
sampled every 500 generations. Stationarity of MCMC
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