Zygomycete fungi were classified as a single phylum, Zygomycota, based on sexual reproduction by zygospores, frequent asexual reproduction by sporangia, absence of multicellular sporocarps, and production of coenocytic hyphae, all with some exceptions. Molecular phylogenies based on one or a few genes did not support the monophyly of the phylum, however, and the phylum was subsequently abandoned. Here we present phylogenetic analyses of a genome-scale data set for 46 taxa, including 25 zygomycetes and 192 proteins, and we demonstrate that zygomycetes comprise two major clades that form a paraphyletic grade. A formal phylogenetic classification is proposed herein and includes two phyla, six subphyla, four classes and 16 orders. On the basis of these results, the phyla Mucoromycota and Zoopagomycota are circumscribed. Zoopagomycota comprises Entomophtoromycotina, Kickxellomycotina and Zoopagomycotina; it constitutes the earliest diverging lineage of zygomycetes and contains species that are primarily parasites and pathogens of small animals (e.g. amoeba, insects, etc.) and other fungi, i.e. mycoparasites. Mucoromycota comprises Glomeromycotina, Mortierellomycotina, and Mucoromycotina and is sister to Dikarya. It is the more derived clade of zygomycetes and mainly consists of mycorrhizal fungi, root endophytes, and decomposers of plant material. Evolution of trophic modes, morphology, and analysis of genome-scale data are discussed.

The discovery of arbuscules in Aglaophyton major, an Early Devonian land plant, provides unequivocal evidence that mycorrhlizae were estabised >400 million years (Fig. 1), even though the intraradical mycelium is extensively developed within the intercellular spaces in the remaining cortical tissues. The nonseptate hyphae range up to 4.0 ,um in diameter and often show both Y-and H-shaped anastomoses. In extant VAM fungi arbuscules are formed when intercellular hyphal branches penetrate the cell wall of the host but do not rupture the plasmalemma (10). The hyphal trunk ofthe arbuscule is =1.3 Am wide (Fig. 2) and branches repeatedly to form a "bush-like" structure within the cell (Fig. 3). Secondary branches range from 0.7-2.0 pum wide, and the ultimate ones are beyond the resolving power of light microscopy (in the size range 0.2-0.5 pum). Fig. 4 shows the top of an arbuscule indicating the configuration ofthe more distal branches. In the arbuscules of extant VAMs, the walls are osmiophilic and decrease in thickness to <20 nm near the tips. In some arbuscules the tips may be slightly swollen, and this condition also occurs in the fossils.In extant VAMs, the arbuscules are ephemeral and begin to break down at the tips ofthe smallest branches in 4-6 days, ultimately forming an amorphous mass within the cell (11). We have no way of gauging the longevity of the fossil arbuscules except to note that various stages of arbuscule morphology are present in a single section, including those that have collapsed and deteriorated. Boullard (12) suggested that in some living ferns the frequent presence of clumps indicates a short functional span of an arbuscule. Several authors working with modern VAM fungi reported the formation of a subapical septum that separates the functional and nonfunctional portion of an arbuscule during deterioration. We have not observed this structure in any of the fossil arbuscules to date. The fossil arbuscules appear morphologically identical to those of many extant VAMs (13). There is far less information available about the structural and physAbbreviation: VAM, vesicular arbuscular mycorrhiza.fTo whom reprint requests should be addressed. 11841The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

The lifetime cost of stroke varies considerably by type of stroke and entails considerable costs beyond the first 2 years after a stroke.

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