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Endogonaceae (Zygomycetes) has attracted considerable attention because
five of its six genera form beneficial mycorrhizal associations with plants.
Yet very little is known about evolutionary relationships among the fungi
in this family. A cladistic analysis was performed using 27 morphological
characters of asexual spores and the endophytic phase of the mycorrhizal
fungi. All of these species comprised a monophyletic group defined by two
synapomorphies: mutualistic symbiosis with terrestrial plants and specialized
dichotomously-branched intraradical arbuscules interfacing host and fungal
symbiont. Members of the type genus, Endogone, did not share these
characters and thus were a polyphyletic group related to some arbuscular
fungi in Glomus and Sclerocystis by convergence of spores
arranged in compact sporocarps. Two main branches of symbiotic fungi were
hypothesized to evolved from a common arbuscular ancestor. One branch consisted
of Gigaspora and Scutellospora, as defined by extraradical
auxiliary cells and spores formed within a thin unit wall on a sporogenous
cell. Gigaspora possessed no unique autapomorphies or showed divergence,
suggesting that only ancestral traits were retained in this group. Scutellospora
evolved from Gigaspora with development of a membranous inner
wall, separable wall groups, a germination shield, and less ornamented auxiliary
cells. The other branch, as defined by intraradical vesicles in mycorrhizal
roots, consisted of Glomus, Sclerocystis, Acaulospora,
and Entrophospora. Glomus and Sclerocystis were
closely related by a unique transformation series from spores formed singly
to radially in compact highly organized sporocarps. Patristic distance among
members of this group was low. Acaulospora and Entrophospora
were sister monophyletic groups whose common ancestor formed spores
on the stalk of a sporiferous saccule. Entrophospora diverged from
Acaulospora with evolution of spores within the stalk. Parallelisms
were numerous within all descendant monophyletic groups of symbiotic fungi,
lowering resolution of branch patterns. The cladogram and phylogenetic tree
reconstructed from this analysis establishes a theoretical framework to
incorporate other morphological, biochemical, and genetic characters. It
is aimed at stimulating new hypotheses of biological and ecological relationships
to advance our knowledge of systematics. |
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