A recent paper in MBE presents evidence that the Taphrinomycota (containing S. pombe and Pneumocystis) are in fact a monophyletic group. This is considered an early branch in the Ascomycota with the Pezizomycotina (filamentous ascomycete fungi like Neurospora and Aspergillus) and Saccharomycotina (fungi mainly with yeast forms including Candida and Saccharomyces). The monophyly of Taphrinomyoctina fungi is something that has been fairly accepted but there are a few publications reporting conflicting evidence in some sets gene trees. This conflict is most likely due to Long Branch Attraction (LBA) and the Philippe lab has long worked on this problem of LBA working to develop tools like PhyloBayes that attempt to correct for LBA with a parameter rich model and using lots of data (like whole genomes). These authors are employing phylogenomics in the sense that multiple genes are used to reconstruct the phylogeny. This use is different from the J.Eisen/Sjölander sense which is to infer gene function from a phylogeny.
This paper presents evidence using proteins of 113 mitochondrial and nuclear genes and finds strong statistical support for this monophyly. They also note that it was necessary to remove fast evolving sites from a dataset of only mitochondrial genes in order to overcome LBA artifacts that lead to Saccharomyces and S. pombe sister relationship in previous analyses.
This paper also presents work using the Pneumocystis genome sequence helps resolve its placement and eventually understanding the evolution of this pathogen. In this tree the sister group to Pneumocystis is Schizosaccharomyces but both lineages have very long branches. The Saitoella lineage is basal in this paper which is different from what was found with a 4 gene (AFTOL) dataset (see Figure 2). Further work sampling more genes from these Taphrina lineages will likely help resolve the intra-clade relationships.
Y. Liu, J. W. Leigh, H. Brinkmann, M. T. Cushion, N. Rodriguez-Ezpeleta, H. Philippe, B. F. Lang (2008). Phylogenomic Analyses Support the Monophyly of Taphrinomycotina, including Schizosaccharomyces Fission Yeasts Molecular Biology and Evolution, 26 (1), 27-34 DOI: 10.1093/molbev/msn221
The Broad Institute has made available the Schizosaccharomyces octosporus genome sequence producing another model system (S.pombe) with several related species for comparative genomics. I believeS. octosporus genome was entirely sequenced with 454 technology. The other genome sequences in the Taphrina clade include the S. japonicus genome. S. octosporus is pretty interesting as it grows filamentously and is 8-spored unlike S. pombe. The origin of this filamentous growth would be quite important to understand how reversions to simpler fission yeast forms form and whether this is loss of whole gene families or remodeling of gene networks.
There is also some preliminary (old) sequence from Pneumocystis (although it is hard to track down that sequence, a paper from 2006 says there is draft sequence but none shows up in GenBank).
Webb, C.J., Zakian, V.A. (2008). Identification and characterization of the Schizosaccharomyces pombe TER1 telomerase RNA. Nature Structural & Molecular Biology, 15(1), 34-42. DOI: 10.1038/nsmb1354
Leonardi, J., Box, J.A., Bunch, J.T., Baumann, P. (2008). TER1, the RNA subunit of fission yeast telomerase. Nature Structural & Molecular Biology, 15(1), 26-33. DOI: 10.1038/nsmb1343
Two papers in Nature Structural & Molecular Biology identify the telomerase RNA in Schizosaccharomyces pombe. Telomerase is a multi-unit enzyme that has both protein and RNA components. While the protein subunit is highly conserved and identifiable through sequence comparisons of eukaryotes, the RNA subunit has a variable size and sequence making identification through comparative means more difficult. The S. pombe telomerase RNA subunit, or TER1, was discovered by two labs applying similar biochemical approaches to identify the locus.
Lots of papers in Mycologia (subscription required) this month of different groups analyzing the fine-scale relationships of many different fungal clades using the loads of sequences that were generated as part of the Fungal Tree of Life project.
Some highlights – there are just too many papers in the issue to cover them all. As usual with more detailed studies of clades with molecular sequences we find that morphologically defined groupings aren’t always truly monophyletic and some species even end up being reclassified. Not that molecular sequence approaches are infallable, but for many fungi the morphological characters are not always stable and can revert (See Hibbet 2004 for a nice treatment of this in mushrooms; subscription required).
The early diverging Chytridiomycota, Glomeromycota, and Zygomycota are each described. Tim James and others present updated Chytridiomycota relationships so of which were only briefly introducted in the kingdom-wide analysis paper published last year.
There is a nice overview paper of the major Agaricales clades (mushrooms for the non-initiated) from Brandon Matheny as well as as individual treatment of many of the sub-clades like the cantharelloid clade (mmm chanterelles…) .
Relationships of the Puccinia clade are also presented – we bloggedabout the wheat pathogen P. graminis before.
A new Saccharomycetales phylogeny is presented by Sung-Oui Suh and others.
The validity of the Archiascomycete group is also tested (containing the fission yeast Schizosaccharomyces pombe and the mammalian pathogen Pneumocystis) and they confirm that it is basal to the two sister clades the euascomycete (containing Neurospora) and hemiascomycete (containing Saccharomyces) clades. However it doesn’t appear there are enough sampled species/genes to confirm monophyly of the group. There are/will be soon three genome sequences of Schizosaccharomyces plus one or two Pneumocystis genomes – it will be interesting to see how this story turns out if more species can be identified.
This was a monster effort by a lot of people who it is really nice to see it all have come together in what looks like some really nice papers.
A paper by Martin Aslett and Val Wood indicate that the fission yeast community is approaching 100% coverage of a GO annotation for every gene in the S. pombe genome. Only Ashbya gossypii has a smaller genome in the fungi (see a recent paper on Ashbya annotation database) and doesn’t yet have complete GO coverage. This is quite remarkable and a great dataset for studies in S. pombe and all fungi.
My quick predictions of genes a closely related species, S. japonicus, has more than twice as many genes as S. pombe (but be over-prediction by ab initio predictors). Taken in comparison to many other fungi, S. pombe represents a streamlined and reduced genome which probably occured indepdently from reduction in the Hemiascomycetes.