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Phylogeny of the Ascomycota
Introduction
Core Alignments
Publications
Introduction
A lichen is essentially an obligate, stable, self-supporting mutualism between a fungus (mycobiont) and an alga or cyanobacterium (photobiont). In most cases, the life form and behavior resulting from this symbiotic interaction differ so considerably from the isolated components, that lichens were believed to be single organisms until 1867. In the early 1930's, lichens were still recognized as a taxonomic unit. It is now well established that lichens are symbiotic organisms and that approximately 98% of all lichen-forming fungi are ascomycetes, also named "ascolichens". The phylum Ascomycota (one of the four phyla of fungi) is the largest group of fungi with 264 families, 3,266 genera and more than 32,000 described species, accounting for more than 57% of all known fungi (according to The Dictionary of the Fungi, eighth edition). A substantial move towards an integral classification of both lichenized and non-lichenized ascomycetes started to take shape with Luttrell's work in 1951. In 1971, lichen-forming fungi were included for the first time in both the "Index of Fungi" and "Dictionary of the Fungi", and it was only in 1981 that lichens were no more recognized as a "group" in the International Code of Botanical Nomenclature.
Concurrent to this relatively recent but essential integration of lichenized and non-lichenized ascomycetes within one system of classification, there was more and more reluctance to use supraordinal classification (classes) within the phylum Ascomycota. So much so, that the classification of Ascomycota in the seventh edition (1983) of the "Dictionary of the Fungi" consisted no more than a list of 39 orders, i.e., without any classes. Twelve years later the situation did not improve significantly since the classification of Ascomycota is reported in the eighth edition (1995) of the "Dictionary of the Fungi" as a list of 46 orders still without any higher taxonomic categories, with the recommendation that for general purposes it remains pragmatic to use only ordinal names. Moreover, the delimitation of most of these orders is unclear, and their phylogenetic relationships are unknown for the most part. This unfortunate situation in the phylum Ascomycota is impeding taxonomic work at all levels. An exhaustive and solid phylogenetic study at the ordinal level would provide a structure upon which mycologists could build a stable and evolutionary meaningful classification.
Many factors are responsible for this undesirable taxonomical situation. One of the major reasons is that lichenologists and mycologists are very exclusive in the group of organisms they study, each focussing on lichenized or non-lichenized taxa, respectively. However, the biological reality is different, with 14 of the 46 orders of ascomycetes including lichenized species, nine of which include a mixture of lichen- and non lichen-forming species. Compared to the diversity of ascolichens (42% of all Ascomycota are lichenized, representing 1/5 of all known fungi) lichen-forming ascomycetes are phylogenetically understudied. It is clear that the phylogeny of the ascomycetes and the impact of mutualism (lichenization) on the evolution of this phylum will not be attained without an extensive sampling of lichen-forming ascomycetes, at least comparable to the sampling that has been achieved for non lichen-forming ascomycetes.
In 1996, François Lutzoni was awarded an NSF grant (DEB-9615542) to resolve the phylogenetic relationships among the main groups of lichenized and non-lichenized Ascomycota. The following core Ascomycota data sets and publications resulted from this research.
Core small and large subunit (SSU & LSU) nuclear rDNA alignments for lichenized and non-lichenized Ascomycota
54 species data sets: Our sample includes representatives from 24 of 46 orders (according to The Dictionary of the Fungi, eighth edition, 1995), representing = 75% of the Ascomycota species diversity. Taxa were selected to represent all main Ascomycete orders known to include lichenized species (13 out of 15 orders,) and nearly all main orders of Ascomycota known to include only non-lichenized species (11 out of 31 orders). The results of the phylogenetic and comparative study were published in Nature (2001, see below), and Voucher/GenBank information is accessible as “Supplementary information” on Nature’s web site.
5’ nuclear SSU & LSU rDNA sequence alignments for 52 Ascomycota species and 2 Basidiomycota outgroup species: Downloadable nexus file available soon.
Publications and data sets
Lutzoni, F., Pagel, M., Reeb, V. 2001. Major fungal lineages are derived from lichen symbiotic ancestors. Nature (in press).
Bhattacharya, D., Lutzoni, F., Reeb, V., Simon, D., Nason, J., Fernandez, F. 2000. Widespread occurrence of spliceosomal introns in the rDNA genes of ascomycetes. Molecular Biology and Evolution 17:1971-1984. Downloadable data set (Stuffit file)
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Department of Biology, Duke University, Box 90338, Durham, NC 27708
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Last update June 2001
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