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Until one and a half centuries ago, the colorful lichens that colonize soil, rocks and trees from the tundra to the hot and dry deserts were believed to be organisms separate from plants and fungi. Their similarity with certain fungi and algae was partly explained by the idea that lichens occupy a taxonomic position intermediate between those two groups. With the discovery (in 1867) that lichens are in fact a symbiosis between fungi and algae, and more particularly involve a mode of nutrition by specialized fungi, the lichen-forming fungi came gradually to be integrated into the fungal system of classification. The first large-scale molecular, DNA-based phylogeny of lichen-forming fungi (1995) seemed to confirm that lichenization evolved many times independently within the fungi, but a recent study (2001) has proposed an alternative, that there was only one major lichenization event within Ascomycota, from which secondarily non-lichenized fungi evolved along several lineages, including such important groups as Aspergillus and Penicillium. Additional studies on the molecular phylogeny of Ascomycota suggest that the situation is more complex, with one or two major lichenization events, several delichenization events, and possibly even relichenization events. One of the biologically and morphologically most diverse fungal groups are the Ostropales, which include lichenized and lichenicolous (fungi parasitizing lichens) groups, as well as non-lichenized saprotrophs and plant parasites.
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