Adam English, lead bioinformatics programmer at Baylor College of Medicine, discusses challenges with resolving gaps in high-quality draft genome assemblies. Sequencing biases, repetitive genomic features, genomic polymorphism, and other complicating factors all come together to make some regions difficult or impossible to assemble. For more facile assembly and automated finishing of draft genomes, he presents an automated approach to finishing using long reads from the PacBio System. The tool PBJelly automates the finishing process using long sequence reads in a reference-guided assembly process. Using PBJelly and SMRT Sequencing, they upgraded the draft genome sequences of a simulated Drosophila melanogaster, the version…
PacBio Sequencing is characterized by very long sequence reads (averaging > 10,000 bases), lack of GC-bias, and high consensus accuracy. These features have allowed the method to provide a new gold standard in de novo genome assemblies, producing highly contiguous (contig N50 > 1 Mb) and accurate (> QV 50) genome assemblies. We will briefly describe the technology and then highlight the full workflow, from sample preparation through sequencing to data analysis, on examples of insect genome assemblies, and illustrate the difference these high-quality genomes represent with regard to biological insights, compared to fragmented draft assemblies generated by short-read sequencing.
In this PacBio User Group Meeting presentation, PacBio scientist Kristin Mars speaks about recent updates, such as the single-day library prep that’s now possible with the Iso-Seq Express workflow. She also notes that one SMRT Cell 8M is sufficient for most Iso-Seq experiments for whole transcriptome sequencing at an affordable price.
The release of the PacBio Sequel II System in 2019 brought dramatic throughput improvements and protocols for producing a new data type, highly accurate long reads or HiFi reads. PacBio is the only sequencing technology to offer highly accurate long reads (HiFi reads) that provide Sanger-quality accuracy (>99%) with the read lengths needed for assembly of complex genomes. The long length and high accuracy of HiFi reads makes them the ideal starting point for many applications, and one area of major interest is genome assembly. HiFi assembly is faster, cheaper, more accurate, and easier to phase than standard long-read assembly.…
Understanding interactions among plants and the complex communities of organisms living on, in and around them requires more than one experimental approach. A new method for de novo metagenome assembly, PacBio HiFi sequencing, has unique strengths for determining the functional capacity of metagenomes. With HiFi sequencing, the accuracy and median read length of unassembled data outperforms the quality metrics for many existing assemblies generated with other technologies, enabling cost-competitive recovery of full-length genes and operons even from rare species. When paired with the ability to close the genomes of even challenging isolates like Xanthomonas, the PacBio Sequel II System is…
As the foundation for scientific discoveries in genetic diversity, sequencing data must be accurate and complete. With highly accurate long-read sequencing, or HiFi sequencing, there is no longer a compromise between read length and accuracy. HiFi sequencing enables some of the highest quality de novo genome assemblies available today as well as comprehensive variant detection in human samples. PacBio HiFi libraries constructed using our standard library workflows require at least 3 µg of DNA input per 1 Gb of genome length, or ~10 µg for a human sample. For some samples it is not possible to extract this amount of…
Domestication of clonally propagated crops such as pineapple from South America was hypothesized to be a ‘one-step operation’. We sequenced the genome of Ananas comosus var. bracteatus CB5 and assembled 513?Mb into 25 chromosomes with 29,412 genes. Comparison of the genomes of CB5, F153 and MD2 elucidated the genomic basis of fiber production, color formation, sugar accumulation and fruit maturation. We also resequenced 89 Ananas genomes. Cultivars ‘Smooth Cayenne’ and ‘Queen’ exhibited ancient and recent admixture, while ‘Singapore Spanish’ supported a one-step operation of domestication. We identified 25 selective sweeps, including a strong sweep containing a pair of tandemly duplicated…
The emergence of third generation sequencing (3GS; long-reads) is making closer the goal of chromosome-size fragments in de novo genome assemblies. This allows the exploration of new and broader questions on genome evolution for a number of non-model organisms. However, long-read technologies result in higher sequencing error rates and therefore impose an elevated cost of sufficient coverage to achieve high enough quality. In this context, hybrid assemblies, combining short-reads and long-reads provide an alternative efficient and cost-effective approach to generate de novo, chromosome-level genome assemblies. The array of available software programs for hybrid genome assembly, sequence correction and manipulation is…
Sequencing technology and assembly algorithms have matured to the point that high-quality de novo assembly is possible for large, repetitive genomes. Current assemblies traverse transposable elements (TEs) and allow for annotation of TEs. There are numerous methods for each class of elements with unknown relative performance metrics. We benchmarked existing programs based on a curated library of rice TEs. Using the most robust programs, we created a comprehensive pipeline called Extensive de-novo TE Annotator (EDTA) that produces a condensed TE library for annotations of structurally intact and fragmented elements. EDTA is open-source and freely available: https://github.com/oushujun/EDTA.List of abbreviationsTETransposable ElementsLTRLong Terminal…
Haplotype phasing of genetic variants is important for interpretation of the maize genome, population genetic analysis, and functional genomic analysis of allelic activity. Accordingly, accurate methods for phasing full-length isoforms are essential for functional genomics study. In this study, we performed an isoform-level phasing study in maize, using two inbred lines and their reciprocal crosses, based on single-molecule full-length cDNA sequencing. To phase and analyze full-length transcripts between hybrids and parents, we developed a tool called IsoPhase. Using this tool, we validated the majority of SNPs called against matching short read data and identified cases of allele-specific, gene-level, and isoform-level…
In the wake of constant improvements in sequencing technologies, numerous insect genomes have been sequenced. Currently, 1219 insect genome-sequencing projects have been registered with the National Center for Biotechnology Information, including 401 that have genome assemblies and 155 with an official gene set of annotated protein-coding genes. Comparative genomics analysis showed that the expansion or contraction of gene families was associated with well-studied physiological traits such as immune system, metabolic detoxification, parasitism and polyphagy in insects. Here, we summarize the progress of insect genome sequencing, with an emphasis on how this impacts research on pest control. We begin with a…
Long-read RNA sequencing (RNA-seq) is promising to transcriptomics studies, however, the alignment of the reads is still a fundamental but non-trivial task due to the sequencing errors and complicated gene structures. We propose deSALT, a tailored two-pass long RNA-seq read alignment approach, which constructs graph-based alignment skeletons to sensitively infer exons, and use them to generate spliced reference sequence to produce refined alignments. deSALT addresses several difficult issues, such as small exons, serious sequencing errors and consensus spliced alignment. Benchmarks demonstrate that this approach has a better ability to produce high-quality full-length alignments, which has enormous potentials to transcriptomics studies.
Satellite repeats are a structural component of centromeres and telomeres, and in some instances their divergence is known to drive speciation. Due to their highly repetitive nature, satellite sequences have been understudied and underrepresented in genome assemblies. To investigate their turnover in great apes, we studied satellite repeats of unit sizes up to 50?bp in human, chimpanzee, bonobo, gorilla, and Sumatran and Bornean orangutans, using unassembled short and long sequencing reads. The density of satellite repeats, as identified from accurate short reads (Illumina), varied greatly among great ape genomes. These were dominated by a handful of abundant repeated motifs, frequently…
Motivation: Third-generation sequencing technologies can sequence long reads, which is advancing the frontiers of genomics research. However, their high error rates prohibit accurate and efficient downstream analysis. This difficulty has motivated the development of many long read error correction tools, which tackle this problem through sampling redundancy and/or leveraging accurate short reads of the same biological samples. Existing studies to asses these tools use simulated data sets, and are not sufficiently comprehensive in the range of software covered or diversity of evaluation measures used. Results: In this paper, we present a categorization and review of long read error correction methods,…
Brassica napus (AACC, 2n = 38) is an important oilseed crop grown worldwide. However, little is known about the population evolution of this species, the genomic difference between its major genetic groups, such as European and Asian rapeseed, and the impacts of historical large-scale introgression events on this young tetraploid. In this study, we reported the de novo assembly of the genome sequences of an Asian rapeseed (B. napus), Ningyou 7, and its four progenitors and compared these genomes with other available genomic data from diverse European and Asian cultivars. Our results showed that Asian rapeseed originally derived from European rapeseed but subsequently…