Phylogeny Friday - 03 November 2006

When Phylogeny Friday last made an appearance on this blog, we were exploring the vertebrates. This was part of a larger series in which we were working our way through the eukaryotes, focusing on animals. I've come to realize that weekly phylogenies are too much, so we're scaling Phylogeny Friday back to appear on the first Friday of the month.

Today, we will take a look at another group of eukaryotes: the fungi. (We'll return to the animals in a future edition.) Both fungi and animals are unikonts, and the clade containing fungi is sister to the clade containing the animals:


This is one fifth of the eukaryotic tree. The animals are circled in red, and the fungi are in the green box. But how accurate is this representation of fungal evolution? A recent paper in Nature suggests that it's not so hot. It turns out a lot of the fungal taxa in the tree above are paraphyletic. Here is a summary of what the new study found:


This figure was taken from a review of the fungal phylogeny paper, and it oversimplifies the actual tree. This tree was constructed using six genes, whereas the previous analyses usually consisted of a single gene.

The Dikarya are a previously supported clade made up of yeasts, mushrooms, and their close relatives. They share a common life history trait where haploid cells fuse but the nuclei do not. That results in a cell that contains two haploid nuclei. Eventually, the nuclei fuse giving rise to a true diploid.

Aside from the Dikarya, however, most of the previous fungal phylogeny was riddled with paraphyletic taxa. The phyla Zygomycota and Chytridiomycota are far from monophyletic. In fact, the new evidence splits the two phyla into six.

The chytrids are an interesting case, as they are the only fungi with a flagellum -- a trait thought to be shared with the most recent common ancestor of all fungi. This led taxonomists to classify chytrids as the basal fungal phylum. But the new analysis indicates that chytrids are paraphyletic, suggesting that the loss of a flagellum occurred multiple times throughout fungal evolution. Because the possession of a flagellum is coupled with an aquatic lifestyle, it appears that individual fungal phyla have colonized terrestrial habits independently.

The fact that this study used more genes than previous ones gives support to the new fungal phylogeny, but there is still work to be done. Less than 5% of fungal species have been described, and it's unclear where the unknown species would fall in the tree. They may belong to known phyla, or they could be members of lineages that are currently missing from this phylogeny.

Bruns, T. 2006. Evolutionary biology: A kingdom revised. Nature 443: 758-761. doi: 10.1038/443758a

James, TY et al. 2006. Reconstructing the early evolution of Fungi using a six-gene phylogeny. Nature 443: 818-822. doi: 10.1038/nature05110


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