A new paper from the Zilberman lab at UC Berkeley shows the application of high throughput sequencing to the study of DNA methylation in eukaryotes. They generate an huge data set of whole genome methylation patterns in several plants, animals, and five fungi including early diverging Zygomycete.
The work was performed using Bisulfite sequencing (Illumina) to capture methylated DNA, RNA-Seq of mRNA. The also performed some ChIP-Seq of H2A.Z on pufferfish to look at the nucleosome positioning in that species. For aligning the reads, they used BowTie to align the bisulfite sequences (though I’d be curious how a new aligner, BRAT, designed for Bisulfite seq reads would perform) to the genome. They also sequenced mRNA via RNA-Seq to assay gene expression for some of the species.
They find several interesting patterns in animal and fungal genomes. I’ll highlight one in the fungi. They find an unexpected pattern in U. reesii of reduced CGs in repeats, which shows signatures of a RIP-like process, are also methylated. This finding is also consistent with observations in Coccidioides (Sharpton et al, Genome Res 2009) that showed depleted CGs pairs in repeats. Since the phenomenon is also found in Coccidioides genomes this methylation of some repeats is likely not unique to U. reesii but may be important in recent evolution of the Onygenales fungi or the larger Eurotiales fungi. There are several other interesting findings with the first such study that shows methylation data for Zygomycete fungi and a basidiomycete close to my heart, Coprinopsis. It will be interesting is to dig deeper into this data and see how the patterns of methylation compare to other genomic features and the mechanisms regulating methylation process.
Zemach, A., McDaniel, I., Silva, P., & Zilberman, D. (2010). Genome-Wide Evolutionary Analysis of Eukaryotic DNA Methylation Science DOI: 10.1126/science.1186366
In followup to the Aspergillus RIP paper discussion, Jo Anne posted in the comments that her paper published in FGB about RIP in another asexual species of fungi also found that evidence for the meiosis-specific process of Repeat Induced Point-mutations (RIP).
Continue reading More RIP without sex?
Interesting blog post that mentions N. crassa and RIP.
A paper in Current Genetics describes the discovery of Repeat Induced Polymorphism (RIP) in two Euriotiales fungi. RIP has been extensively studied in Neurospora crassa and has been identified in other Sordariomycete fungi Magnaporthe, Fusiarium. This is not the first Aspergillus species to have RIP described as it was demonstrated in the biotech workhorse Aspergillus oryzae. However, I think this study is the first to describe RIP in a putatively asexual fungus. The evidence for RIP is only found in transposon sequences in the Aspergillus and Penicillium. A really interesting aspect of this discovery is RIP is thought to only occur during sexual stage, but a sexual state has never been observed for these fungi. Continue reading RIPing in an asexual fungus
The genome of the wheat and cereal pathogen Fusarium graminearum was published in Science this week in an article entitled “The Fusarium graminearum Genome Reveals a Link Between Localized Polymorphism and Pathogen Specializationtion”. The project was a collaboration of many different Fusarium research groups. The genome sequencing was spearheaded by the Broad Institute at Harvard and MIT and is part of a larger project to sequence several different species of Fusarium. The group sequenced a second strain in order to identify polymorphisms.
Some of the key findings
- The presence of Repeat Induced point-mutation (RIP) has likely limited the amount of repetitive and duplicated sequences in the genome
- Most of the genes unique to F. graminearum (and thus not present in 4 other Fusarium spp genomes) are found in the telomeres
- Between the sequenced strains SNP density ranged from 0 to 17.5 polymorphisms per kb.
- Some of the genes expressed uniquely during plant infection (408 total) include known virulence factors and many plant cell-wall degrading enzymes.
- The genes showing some of the highest SNP diversity tended to be unique to Fusarium and often unique to F. graminearum