NGS Expert Blog

With this new bi-weekly series we would like to highlight some if not all genomes that have been sequenced in the last 6 to 12 months. And at this point of time I am still uncertain if the diversity of organisms and species will be the “eye-opener” or the different technologies and strategies that have been used…

We started this series off in January where we reported about the de novo sequencing of the domestic goat Capra hircus.

Today I would like to report about a plant genome, the Cicer arietinum:

According to the GenomeWeb article this de novo genome sequencing approach is only the 3rd one for crop legume plants. For me that is kind of astonishing since breeding and optimisation of crop is already done since years. Maybe this is due to the huge genomes of plants that outperform animal genomes by far. For our chickpea plant with 740 million base pairs we talk about a medium size plant genome. But let’s focus on the sequencing approach for now (Varshney et. al):

What was sequenced?

De novo sequencing of one reference chickpea plant and re-sequencing of 90 cultivated & wild chickpea lines from 10 different countries

Sequencing strategy:

1. De novo genome sequencing on HiSeq 2000 (paired-end module) of 1 genome with 11 shotgun and mate-pair libraries (insert sizes: ~ 170; 500; 800; 2,000; 5,000; 10,000; 20,000 bp) and BAC end sequencing
   Data output: 153.01 Gb; after filtering & correction steps only 87.65 Gb data were used for de novo assembly
2. Re-sequencing of genomes
   • Whole genome re-sequencing on 29 varieties using Illumina 100 bp paired-end sequencing on HiSeq 2000
   • RAD-sequencing of 61 genotypes on HiSeq 2000 (48x ApeKI; 24x HindIII)

According to D. Cook “the sequencing of the chickpea provides genetic information that will help plant breeders develop highly productive chickpea varieties that can better tolerate drought and resist disease — traits that are particularly important in light of the threat of global climate change”. (Davis Enterprise).

Read the complete publication here.

Some time ago I was introducing a new approach combining restriction site associated DNA marker genotyping (RAD) with next generation sequencing technology. Originally this method was developed for microarray platforms. However, the combination of RAD and NGS (Illumina) – resulting in RAD sequencing (RAD-Seq) – enabled the massivly parallel and multiplexed sample sequencing. RAD-Seq is becoming more and more powerful and has the potential to revolutionize agrigenomics, because one can discover and screen thousands of SNP’s and genotype large populations in a high throughput manner at the same time. The scope of the following section is to give a short technical overview how this can be accomplished:

Genomic DNA of each sample is digested in parallel with a certain restriction enzyme and a specific P1 adapter is ligated to the restriction fragments. Thereby each sample will be equipped with an individual P1-adapter containing a sample-specific molecular identifier (Barcode) and Illumina adapter sequences (forward amplification primer site and Illumina sequencing primer site, respectively). If multiplexing is desired, the adapter-ligated fragments of a number of samples can now be pooled. The level of multiplexing depends on the number of differed P1-adapters which have been used before. In a further step the RAD pool will be sheared, size-selected and ligated with a second adapter (P2). The P2 adapter comprises a divergent “Y” adapter containing the reverse amplification primer sites. However, the P2 adapter is special such that fragments lacking the P1 adapter cannot be amplified. This guarantees, that only fragments containing a P1 and a P2 adapter will be selectively and robustly enriched during amplification step following next. The overall length of RAD-tags which can be further analysed mainly depend on the size selection step and sequencing run mode (single vs. paired end), respectively.

What is RAD-Seq?

Restriction site associated DNA sequencing (RAD-Seq) using the Illumina technology was initially published in PLoS One and in PLoS Genetics. Along with other studies, these studies demonstrated that this smart technique can be very useful for the identification and further analysis of a high number of genetic markers distributed over the genome. This is especially advantageous if working with non-model organisms where no reference sequence is available. Briefly, RAD-Seq results in a reduced representation of the respective genome, because only fragments near a specific type of restriction site are sequenced to deep coverage. Such fragments are called ‘RAD tags’ and serve in subsequent analysis steps as a reference for the design of genetic markers. The overall number of possible RAD tags within an organism strongly depends on the restriction enzyme of choice and the genome of interest.