Fossils from a Wisconsin roadcut show clearly that fungi and green plants moved from water onto land at about the same time, bolstering the theory that fungi helped plants successfully invade the land.
The newly discovered fossils - microscopic spores and threads in sediments dating from 460 to 455 million years ago - push the origin of land-based fungi back some 55 to 60 million years to about the same era that green plants invaded land, said Dirk Redecker, a postdoctoral researcher at the University of California, Berkeley.
"The presence of both organisms at the same time suggests that there could have been such an interaction, and that organisms even then were interdependent," Redecker said.
Redecker and his colleagues at the University of Wisconsin, Madison, report their discovery of the fossil fungi in the Sept. 15 issue of Science.
Plant/fungus interactions are widespread today, with between 80 and 90 percent of all green plants forming associations with so-called mycorrhizal fungi. These fungi grow around and into plant roots and help them absorb minerals and water, giving plants a substantial competitive advantage, said coauthor Linda K. Graham, a professor of botany and environmental studies at the University of Wisconsin, Madison.
"Plants without mycorrhizal fungi are competitively inferior," Graham said. "Though they won't die, in a highly competitive situation or places with a dearth of resources, mycorrhizal fungi function as an auxiliary root system to provide additional nutrients."
The researchers caution that the fossil fungi they discovered and report in Science show no evidence of an association with green plants. However, similar modern fungi from the genus Glomus form a simple association with modern liverworts and hornworts, relatives of the only group of land plants around 460 million years ago.
In addition, mycorrhizal fungi have been found in fossils as old as 400 million years, in company with later, more evolved vascular plants.
Together, these provide strong circumstantial evidence that fungi played a major role in helping plants move successfully onto land, Redecker said.
Graham obtained the fossil-bearing rocks from a roadcut near Madison, Wisc., not knowing what kinds of fossils they contained. It wasn't until undergraduate Robin Kodner etched the rocks with strong acid that she and Graham saw tiny structures that a colleague suggested might be fossilized fungal spores.
The two sought out Redecker, who works in the laboratory of Thomas Bruns, an associate professor of plant and microbial biology in the College of Natural Resources at UC Berkeley. Redecker is an expert in the evolution of fungi, and he was able to identify them as spores and hyphae - tiny root-like threads - of fungi similar to those in the Glomus genus. Glomus is a member of the larger group Glomales, which is more primitive than the better known fruiting fungi, the Ascomycetes and Basidiomycetes.
With the new fossil evidence, plus other previously found fossil fungi, Redecker estimated that the Ascomycetes and Basidiomycetes diverged from Glomales about 600 to 620 million years ago.
This fits well with a recent phylogenetic study of fungi by M. L. Berbee and John Taylor, UC Berkeley professor of plant and microbial biology and a curator at the University Herbarium. They compared ribosomal RNA from many species of fungi and calibrated them against geologic time to obtain an approximate time estimate for the origin of fungal lineages. They concluded in a book chapter published this year in The Mycota (vol. VII B) that the major lineages of land fungi - Basidiomycetes, Ascomycetes and Glomales - may have diverged from each other about 600 million years ago.
Colonization of the land masses by the first land plants is believed to have happened in the Ordovician (505-440 million years ago), Redecker said. However, no pieces of land plants large enough to see without using a microscope have yet been found from that epoch. This may be due to the fact that the first land plants mainly consisted of soft, quickly decaying tissue like today's hornworts and liverworts. Only fossil spores and microscopically small parts of resistant protective tissue indicating the presence of those plants have been found.
The oldest and most widespread mycorrhizal association is arbuscular mycorrhiza, and the great majority of plants form arbuscular mycorrhizae with fungal partners from the taxonomic group Glomales. Some of the earliest known land plants, which originate from the Devonian (400 million years ago), have been shown to contain structures indicative of arbuscular mycorrhizae.
The Devonian plants containing the finely branched structures called arbuscules did not have a vascular or water conduction system, though they were already relatively complex, at a stage between vascular plants and bryophytes (mosses, liverworts and hornworts), Redecker said. Nevertheless, at the same time in the same site, vascular plants already were present.
Pictures are available of a fossil spore and hypha; an evolutionary tree for the fungi; and an amusing cartoon by Redecker showing how liverworts and fungi may have interacted 460 million years ago. Contact Robert Sanders for copies, (510) 643-6998.