June 1, 2021  |  

Near perfect de novo assemblies of eukaryotic genomes using PacBio long read sequencing.

Third generation single molecule sequencing technology from Pacific Biosciences, Moleculo, Oxford Nanopore, and other companies are revolutionizing genomics by enabling the sequencing of long, individual molecules of DNA and RNA. One major advantage of these technologies over current short read sequencing is the ability to sequence much longer molecules, thousands or tens of thousands of nucleotides instead of mere hundreds. This capacity gives researchers substantially greater power to probe into microbial, plant, and animal genomes, but it remains unknown on how to best use these data. To answer this, we systematically evaluated the human genome and 25 other important genomes across the tree of life ranging in size from 1Mbp to 3Gbp in an attempt to answer how long the reads need to be and how much coverage is necessary to completely assemble their chromosomes with single molecule sequencing. We also present a novel error correction and assembly algorithm using a combination of PacBio and pre-assembled Illumina sequencing. This new algorithm greatly outperforms other published hybrid algorithms.

June 1, 2021  |  

The “Art” of shotgun sequencing

2015 SMRT Informatics Developers Conference Presentation Slides: Jason Chin of PacBio highlighted some of the challenges for shotgun assembly while suggesting some potential solutions to obtain diploid assemblies, including the FALCON method.

June 1, 2021  |  

Long read sequencing technology to solve complex genomic regions assembly in plants

Numerous whole genome sequencing projects already achieved or ongoing have highlighted the fact that obtaining a high quality genome sequence is necessary to address comparative genomics questions such as structural variations among genotypes and gain or loss of specific function. Despite the spectacular progress that has been done regarding sequencing technologies, accurate and reliable data are still challenging, at the whole genome scale but also when targeting specific genomic regions. These issues are even more noticeable for complex plant genomes. Most plant genomes are known to be particularly challenging due to their size, high density of repetitive elements and various levels of ploidy. To overcome these issues, we have developed a strategy in order to reduce the genome complexity by using the large insert BAC libraries combined with next generation sequencing technologies. We have compared two different technologies (Roche-454 and Pacific Biosciences PacBio RS II) to sequence pools of BAC clones in order to obtain the best quality sequence. We targeted nine BAC clones from different species (maize, wheat, strawberry, barley, sugarcane and sunflower) known to be complex in terms of sequence assembly. We sequenced the pools of the nine BAC clones with both technologies. We have compared results of assembly and highlighted differences due to the sequencing technologies used. We demonstrated that the long reads obtained with the PacBio RS II technology enables to obtain a better and more reliable assembly notably by preventing errors due to duplicated or repetitive sequences in the same region.

June 1, 2021  |  

Single chromosomal genome assemblies on the Sequel System with Circulomics high molecular weight DNA extraction for microbes

Background: The Nanobind technology from Circulomics provides an elegant HMW DNA extraction solution for genome sequencing of Gram-positive and -negative microbes. Nanobind is a nanostructured magnetic disk that can be used for rapid extraction of high molecular weight (HMW) DNA from diverse sample types including cultured cells, blood, plant nuclei, and bacteria. Processing can be completed in <1 hour for most sample types and can be performed manually or automated with common instruments. Methods:We have validated several critical steps for generating high-quality microbial genome assemblies in a streamlined microbial multiplexing workflow. This new workflow enables high-volume, cost-effective sequencing of up to 16 microbes totaling 30 Mb in genome size on a single SMRT Cell 1M using a target shear size of 10 kb. We also evaluated this method on a pool of four “class 3” microbes that contain >7 kb repeats. Fragment size was increased to ~14 kb, with some fragments >30 kb. Results: Here we present a demonstration of these capabilities using isolates relevant to high-throughput sequencing applications, including common foodborne pathogens (Shigella, Listeria, Salmonella), and species often seen in hospital settings (Klebsiella, Staphylococcus). For nearly all microbes, including difficult-to-assemble class III microbes, we achieved complete de novo microbial assemblies of =5 chromosomal contigs with minimum quality scores of 40 (99.99% accuracy) using data from multiplexed SMRTbell libraries. Each library was sequenced on a single SMRT Cell 1M with the PacBio Sequel System and analyzed with streamlined SMRT Analysis assembly methods. Conclusions: We achieved high-quality, closed microbial genomes using a combination of Circulomics Nanobind extraction and PacBio SMRT Sequencing, along with a newly streamlined workflow that includes automated demultiplexing and push-button assembly.

June 1, 2021  |  

Beyond Contiguity: Evaluating the accuracy of de novo genome assemblies

HiFi reads (>99% accurate, 15-20 kb) from the PacBio Sequel II System consistently provide complete and contiguous genome assemblies. In addition to completeness and contiguity, accuracy is of critical importance, as assembly errors complicate downstream analysis, particularly by disrupting gene frames. Metrics used to assess assembly accuracy include: 1) in-frame gene count, 2) kmer consistency, and 3) concordance to a benchmark, where discordances are interpreted as assembly errors. Genome in a Bottle (GIAB) provides a benchmark for the human genome with estimated accuracy of 99.9999% (Q60). Concordance for human HiFi assemblies exceeds Q50, which provides excellent genomes for downstream analysis, but presents a challenge that any new benchmark must significantly exceed Q50 or the discordance will represent the error rate of the benchmark. To establish benchmarks for Oryza sativa and Drosophila melanogaster, we collected draft references, Illumina short reads, and PacBio HiFi reads. By species, the benchmark was defined as regions of normal coverage that are not within 5 bp of a small variant or 50 bp of a structural variant. For both species, the benchmark regions span around 60% of the genome and HiFi assemblies achieve Q50 accuracy, which is notably more accurate than assemblies with other technologies and meets typical standards for a finished, reference-grade assembly. Here we present a protocol to generate benchmarks for any sample that rival the GIAB benchmark in accuracy. These benchmarks allow the comparison and improvement of genome assemblies and highlight the superior accuracy of assemblies generated with PacBio HiFi reads.

June 1, 2021  |  

A complete solution for high-quality genome annotation using the PacBio Iso-Seq method

The PacBio Iso-Seq method produces high-quality, full-length transcripts of up to 10 kb and longer and has been used to annotate many important plant and animal genomes. We describe here the full Iso-Seq ecosystem that enables researchers to achieve high-quality genome annotations. The Iso-Seq Express workflow is a 1-day protocol that requires only 60-300 ng of total RNA and supports multiplexing of different tissues. Sequencing on a single SMRT Cell 8M on the Sequel II System produces up to 4 million full-length reads, sufficient to exhaustively characterize a whole transcriptome on the order of 15,000-17,000 genes with 100,000 or more transcripts. Most importantly, the method is supported by a maturing suite of official and community-developed tools. The SMRT Link Iso-Seq application outputs high-quality (>99% accurate), full-length transcript sequences that can optionally be mapped to a reference genome for a single SMRT Cell worth of data in 6-9 hours. For example, the SQANTI2 tool classifies Iso-Seq transcripts against a reference annotation, filters potential library artifacts, and processes information from both long read-only and short read-based quantification. IsoPhase is a tool for identifying allele-specific isoform expression. Cogent has been used to process Iso-Seq transcripts in a genome-independent manner to assess genome assemblies. Finally, IsoAnnot is an up-and-coming tool for identifying differential isoform expression across different samples. We describe how these tools complement each other and provide guidelines to make the best use out of Iso-Seq data for understanding transcriptomes.

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