Reference Genomes

When analyzing data generated by high-throughput genome sequencing instruments, one common task is the comparison of those small genome fragments against a reference genome of a known organism. Because multiple groups may study the same organism, a collection of commonly used reference files have been made available for use in high-performance computing on the on-campus SLURM cluster.

iGenomes

The iGenomes collection of reference genomes was developed by Illumina to provide a canonical source of reference genome information. While that project is no longer actively updated, the available reference files have been copied to a folder which is accessible at /shared/biodata/reference/iGenomes/. The structure of those files is described below, and moving forward additional reference genome indices may be generated which follow the same format and structure.

File Structure

All files described below can be found within the directory /shared/biodata/reference/iGenomes/:

• The top-level folder in the iGenomes directory is named for the source organism: e.g. /shared/biodata/reference/iGenomes/Homo_sapiens;

• The second-level folder is named for the source of the annotation used for the reference genome: e.g. /shared/biodata/reference/iGenomes/Homo_sapiens/UCSC;

• The third-level folder is named for the version of the genome: e.g. /shared/biodata/reference/iGenomes/Homo_sapiens/UCSC/hg19;

• The fourth-level folders are Annotation and Sequence;

  • The Annotation folder contains a folder Genes and may also contain SmallRNA and Variation. The exact files provided for each genome may vary, but Genes may contain genes.bed and genes.gtf to describe exon structure of genes, while SmallRNA may contain hairpin.fa and mature.fa to describe miRNAs;

  • The Sequence folder contains a folder WholeGenomeFasta and Chromosomes which contain the FASTA sequence of the reference genome, either in full or broken out by chromosome, respectively;

  • The Sequence folder also contains a set of references which have been pre-compiled for various alignment / analysis algorithms, which may include:

    • BismarkIndex
    • BlastDB
    • Bowtie2Index
    • BowtieIndex
    • BWAIndex
    • MDSBowtieIndex
    • STARIndex: For STAR < 2.7.6a
    • STAR2Index: For STAR ≥ 2.7.6a

Example

Below is an example of the folders available for the UCSC human genome GRCh37/hg19 reference:

├── Homo_sapiens                        # Organism 
│   └── UCSC                            # Source 
│       ├── hg19                        # Version 
│       │   ├── Annotation
│       │   │   ├── Genes               # Gene annotations 
│       │   │   ├── README.txt 
│       │   │   ├── SmallRNA            # Small RNA annotations 
│       │   │   └── Variation           # Variant annotations 
│       │   └── Sequence 
│       │       ├── AbundantSequences	 
│       │       ├── BismarkIndex        # Index for Bismark 
│       │       ├── BlastDB             # Index for BLAST 
│       │       ├── Bowtie2Index        # Index for Bowtie2 
│       │       ├── BowtieIndex         # Index for Bowtie 
│       │       ├── BWAIndex            # Index for BWA 
│       │       ├── Chromosomes         # Genome sequence by chromosome 
│       │       ├── MDSBowtieIndex      # Index for MDS Bowtie 
│       │       ├── STARIndex           # Index for STAR 
│       │       ├── STAR2Index          # Index for STAR2 
│       │       └── WholeGenomeFasta    # Genome sequence 

Ongoing Support

To maintain the utility of this data resource, the Bioinformatics Core and the Data Core will incrementally add reference genomes for additional organisms and alignment algorithms. If your research group is interested in using an organism or alignment algorithm which is not currently available, please get in touch.