All posts by Jason Stajich

Assistant Professor at UC Riverside

Genome resources for Candida species

The Candida clade of Hemiascomycete fungi have received much attention from funding bodies so that many genomic and experimental resources are available address questions of pathogenecity and industrial applications of these species.

The Candida genus

Traditionally, species of yeasts that were thought to be asexual were given the genus name Candida. This has lead to Candida being a sort of taxonomic rubbish bin as this system of classification breaks down when asexuality arises more than once (creating homoplasy). For example, the asexual Candida glabrata is found within the Saccharomyces clade when molecular phylogenetics is applied. The problem lies in that many of these species appear very similar visually and microscopically and so there had not been enough phylogenetically informative phenotypic characters to easily classify them further. With the use of molecular phylogenetics the classifications have been improved as shown in several studies, however we retain the historical nature of the genus and species names for these organisms for the time being even though the phylogenetic diversity of species in the “genus” is much broader than other genus-level classifications. It will be interesting to see whether taxonomic proposals like PhyloCode or traditional revisions of the species names will provide new names for the group.

The Candida Genome Database (CGD) sister to the Saccharomyces Genome Database (SGD) provides resources for phenotype and sequences related to human commensal and dimorphic fungus Candida albicans. A recent paper by Arnaud et al describes the resources that are available through their website. An essentially completed C. albicans diploid genome with curated gene models and annotations provides an essential resource for this model pathogenic system. In addition to the SC5314 strain of C. albicans the white-opaque (WO) strain can switch between different colony morphologies – white and smooth or gray and rod shaped.

6 additional species have had their genomes in the Candida clade have had their genomes sequenced including Pichia stipis, Debaryomyces hansenii, Candida lusitaniae, Candida tropicalis, Candida guilliermondii, and Lodderomyces elongisporus. These resources will hopefully shed some light on the importance and mechanisms for dimorphic switching in the pathogen C. albicans, the importance and evolution of alternative codon usage in the clade, and better usage of the industrial yeasts like P. stipitis and D. hansenii.

Yeast keeps to itself

Cliff Zeyl and Sally Otto present a nice review on research from the Kruglyak lab regarding evidence that Saccharomyces is primarily a selfer in nature as it outbreeds very infrequently (once in 50,000 generations). The implications of this work has relevance on the importance of sexual reproduction and recombination in natural populations.

Approaching 100% coverage for GO assignments in S.pombe

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.

S. pombe taken from Paul Young’s site

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.

FGI Chytrid annotations available

The public release of the Batrachochytrium dendrobatidis automated annotation from the Broad/FGI has been made available.

“This project is part of the Fungal Genome Initiative at the Broad Institute and was funded by NHGRI. This release contains a set of 8,794 predicted genes, BLAST databases, precomputed BlastX and HMMer analyses, alternative gene predictions, tRNA predictions, and RFAM features.

The annotation can be accessed through the project website:
http://www.broad.mit.edu/annotation/genome/batrachochytrium_dendrobatidis

We would like to thank Franz Lang and Mary Berbee for sharing their B. dendrobatidis EST sequences and contributing a cDNA library for end-sequencing.”

Mystery in the mechanism of yeast speciation

A paper in PLoS Genetics studied what happens when individual chromosomes of S. cerevisiae are replaced with a homologous copy its sister species, S. paradoxus. Previous work from Ken Wolfe’s lab interpreted the differential loss of genes after the whole genome duplication in the Saccharomyces lineage played a role in speciation among the yeast species. Surprisingly (or not, depending on how you interpret the previous work) Greig did not find any lethality in haploid F1 offspring from a diploid synthetically constructed individuals. Certainly this is not the last word but it represents a nice experimental screen to identify interacting genotypes. What would be interesting in followup work would be more subtle dissection of epistatic interactions among the genes on the different chromosomes to score phenotypes other than complete inviability. This might help understand what pathways are operating differently.

Continue reading Mystery in the mechanism of yeast speciation

Fungi for bioremediation

Saprophytic fungi degrade organic matter to release carbon, nitrogen, and other elements locked up in complexes. There is interest in better degradation of recalictrant ligin and cellulose plant matter as part of a bioenergy program. Some fungi are able to break down these plant molecules that would otherwise remain behind when left to digestion by bacteria.

Continue reading Fungi for bioremediation

Full(er) length methods sections in Nature papers

Nature is reporting that it is now going to expand the methods section in print and online versions of its papers. This will also include a 300 word summary of the methods in the print version as well as a full length methods section in the online version which is not a supplemental methods document.

Nature also uses the news piece to remind us that the author formated version of the paper can be submitted to pubmed central (6 months after publication) (well only for NIH supported pubs though – see comments exchange on Jonathan’s blog) and that can include the full length methods.

This seems to be all around a GOOD THING. I’ve always heard complaining about how the glossy publications skimp on actually providing enough evidence to reproduce the results (“telegraphic tradition” in Naturespeak). The best thing is if this means methods are actually peer-reviewed. I don’t really know that they are. You can download the supplemental materials but it isn’t clear to me that someone has actually reviewed it and made sure that a) methods are clearly explained and indicates a reproduceable protocol, b) is typographically proofread.