PacBio Circular Consensus Sequencing Research Articles

Chromosome-level genome provides new insight into the overwintering process of Korla pear (Pyrus sinkiangensis Yu)

Korla pear has a unique taste and aroma and is a breeding parent of numerous pear varieties. It is susceptible to Valsa mali var. pyri, which invades bark wounded by freezing injury. Its genetic relationships have not been fully defined and could offer insight into the mechanism for freezing tolerance and disease resistance. We generated a high-quality, chromosome-level genome assembly for Korla pear via the Illumina and PacBio circular consensus sequencing (CCS) platforms and high-throughput chromosome conformation capture (Hi-C). The Korla pear genome is ~ 496.63 Mb, and 99.18% of it is assembled to 17 chromosomes. Collinearity and phylogenetic analyses indicated that Korla might be derived from Pyrus pyrifolia and that it diverged ~ 3.9-4.6 Mya. During domestication, seven late embryogenesis abundant (LEA), two dehydrin (DHN), and 54 disease resistance genes were lost from Korla pear compared with P. betulifolia. Moreover, 21 LEA and 31 disease resistance genes were common to the Korla pear and P. betulifolia genomes but were upregulated under overwintering only in P. betulifolia because key cis elements were missing in Korla pear. Gene deletion and downregulation during domestication reduced freezing tolerance and disease resistance in Korla pear. These results could facilitate the breeding of novel pear varieties with high biotic and abiotic stress resistance.

Benchmarking long-read sequencing strategies for obtaining ASV-resolved rRNA operons from environmental microeukaryotes.

The use of short-read metabarcoding for classifying microeukaryotes is challenged by the lack of comprehensive 18S rRNA reference databases. While recent advances in high-throughput long-read sequencing provide the potential to greatly increase the phylogenetic coverage of these databases, the performance of different sequencing technologies and subsequent bioinformatics processing remain to be evaluated, primarily because of the absence of well-defined eukaryotic mock communities. To address this challenge, we created a eukaryotic rRNA operon clone-library and turned it into a precisely defined synthetic eukaryotic mock community. This mock community was then used to evaluate the performance of three long-read sequencing strategies (PacBio circular consensus sequencing and two Nanopore approaches using unique molecular identifiers) and three tools for resolving amplicons sequence variants (ASVs) (USEARCH, VSEARCH, and DADA2). We investigated the sensitivity of the sequencing techniques based on the number of detected mock taxa, and the accuracy of the different ASV-calling tools with a specific focus on the presence of chimera among the final rRNA operon ASVs. Based on our findings, we provide recommendations and best practice protocols for how to cost-effectively obtain essentially error-free rRNA operons in high-throughput. An agricultural soil sample was used to demonstrate that the sequencing and bioinformatic results from the mock community also translates to highly diverse natural samples, which enables us to identify previously undescribed microeukaryotic lineages.

Abstract 1763: Long-read sequencing of pediatric leukemia identifies clinically relevant genomic rearrangements

Abstract Clinical detection of genomic rearrangements currently depends on a range of complementary and overlapping methods including fluorescence in situ hybridization (FISH), karyotyping, and DNA and/or RNA sequencing. This requirement for multiple tests results in higher total costs, extended analysis time, and increased usage of sometimes limited tumor material. In our laboratory, we apply long high-fidelity reads to human samples using PacBio circular consensus sequencing (HiFi-GS) of 12-18kb DNA fragments, which offers better structural variant detection relative to short-read sequencing. Here, we assess the ability of HiFi-GS to detect clinically relevant genomic rearrangements (CRGRs) in pediatric leukemia. We selected 11 pediatric leukemia cases with corresponding normal (remission) samples, including 7 cases of B-cell acute lymphoblastic leukemia (B-ALL) and 4 cases of acute myeloid leukemia (AML); 5 of these 11 leukemia cases had known CRGRs as below. The remaining 6 leukemia cases had also undergone clinical genetic profiling via karyotyping, FISH, microarray, and sequencing yet a clinical grade genomic driver was not identified. DNA from each sample was sequenced on a Revio instrument (PacBio, Menlo Park, CA) to a target depth of 30x. The PacBio Human Whole Genome Sequencing (WGS) workflow was used to process HiFi reads for haplotagged alignment and phasing, followed by somatic structural variant calling using Severus. Subsequent analysis focused on the presence of break ends in known cancer-relevant genes. We found that HiFi-GS detected the known CRGRs in all 5 cases with prior findings, providing precise break ends and clarifying sometimes unclear cytogenetic observations (such as exact partner genes), in these cases. These included detection of an ETV6-RUNX1 fusion, NUP98-NSD1 fusion, KMT2A-AFF1 fusion, KMT2A-AFDN fusion, and RUNX1-RUNX1T1 fusion. HiFi sequencing also detected CRGRs in 2 of the 6 “undiagnosed cases”; namely, one case of ZNF384-TCF3 fusion and one case of KMT2A-MLLT10 fusion. The former can be cytogenetically cryptic, while the latter can often be missed due to its potentially complex nature. Our results demonstrate that HiFi-GS can reliably detect CRGRs, bringing us closer to the promise of a single comprehensive genetic test for cancer characterization. Additional work examining other somatic structural and copy number variants (i.e., inversions, deletions, and duplications) in these cases, as well as oncogenic sequence variants, is ongoing. Citation Format: Byunggil Yoo, Ayse Keskus, Chengpeng Bi, Lisa Lansdon, Tanveer Ahmad, Irina Pushel, Adam Walter, Margaret Gibson, Erin Guest, Tomi Pastinen, Mikhail Kolmogorov, Midhat S. Farooqi. Long-read sequencing of pediatric leukemia identifies clinically relevant genomic rearrangements [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 1763.

Bacterial pathogens associated with the plastisphere of surgical face masks and their dispersion potential in the coastal marine environment

Huge numbers of face masks (FMs) were discharged into the ocean during the coronavirus pandemic. These polymer-based artificial surfaces can support the growth of specific bacterial assemblages, pathogens being of particular concern. However, the potential risks from FM-associated pathogens in the marine environment remain poorly understood. Here, FMs were deployed in coastal seawater for two months. PacBio circular consensus sequencing of the full-length 16S rRNA was used for pathogen identification, providing enhanced taxonomic resolution. Selective enrichment of putative pathogens (e.g., Ralstonia pickettii) was found on FMs, which provided a new niche for these pathogens rarely detected in the surrounding seawater or the stone controls. The total relative abundance of the putative pathogens in FMs was higher than in seawater but lower than in the stone controls. FM exposure during the two months resulted in 3% weight loss and the release of considerable amounts of microfibers. The ecological assembly process of the putative FM-associated pathogens was less impacted by the dispersal limitation, indicating that FM-derived microplastics can serve as vectors of most pathogens for their regional transport. Our results indicate a possible ecological risk of FMs for marine organisms or humans in the coastal and potentially in the open ocean.

DNA 5-methylcytosine detection and methylation phasing using PacBio circular consensus sequencing

Long single-molecular sequencing technologies, such as PacBio circular consensus sequencing (CCS) and nanopore sequencing, are advantageous in detecting DNA 5-methylcytosine in CpGs (5mCpGs), especially in repetitive genomic regions. However, existing methods for detecting 5mCpGs using PacBio CCS are less accurate and robust. Here, we present ccsmeth, a deep-learning method to detect DNA 5mCpGs using CCS reads. We sequence polymerase-chain-reaction treated and M.SssI-methyltransferase treated DNA of one human sample using PacBio CCS for training ccsmeth. Using long (≥10 Kb) CCS reads, ccsmeth achieves 0.90 accuracy and 0.97 Area Under the Curve on 5mCpG detection at single-molecule resolution. At the genome-wide site level, ccsmeth achieves >0.90 correlations with bisulfite sequencing and nanopore sequencing using only 10× reads. Furthermore, we develop a Nextflow pipeline, ccsmethphase, to detect haplotype-aware methylation using CCS reads, and then sequence a Chinese family trio to validate it. ccsmeth and ccsmethphase can be robust and accurate tools for detecting DNA 5-methylcytosines.

High-throughput and high-accuracy single-cell RNA isoform analysis using PacBio circular consensus sequencing

Although long-read single-cell RNA isoform sequencing (scISO-Seq) can reveal alternative RNA splicing in individual cells, it suffers from a low read throughput. Here, we introduce HIT-scISOseq, a method that removes most artifact cDNAs and concatenates multiple cDNAs for PacBio circular consensus sequencing (CCS) to achieve high-throughput and high-accuracy single-cell RNA isoform sequencing. HIT-scISOseq can yield >10 million high-accuracy long-reads in a single PacBio Sequel II SMRT Cell 8M. We also report the development of scISA-Tools that demultiplex HIT-scISOseq concatenated reads into single-cell cDNA reads with >99.99% accuracy and specificity. We apply HIT-scISOseq to characterize the transcriptomes of 3375 corneal limbus cells and reveal cell-type-specific isoform expression in them. HIT-scISOseq is a high-throughput, high-accuracy, technically accessible method and it can accelerate the burgeoning field of long-read single-cell transcriptomics.

Clostridioides difficile infection with isolates of cryptic clade C-II: a genomic analysis of polymerase chain reaction ribotype 151

ObjectivesWe report a patient case of pseudomembranous colitis associated with a monotoxin-producing Clostridioides difficile belonging to the very rarely diagnosed polymerase chain reaction (PCR) ribotype (RT) 151. To understand why this isolate was not identified using a routine commercial test, we performed a genomic analysis of RT151. MethodsIllumina short-read sequencing was performed on n = 11 RT151s from various geographical regions to study their genomic characteristics and relatedness. Subsequently, we used PacBio circular consensus sequencing to determine the complete genome sequence of isolates belonging to cryptic clades C–I and C-II, which includes the patient isolate. ResultsWe found that 1) RT151s are polyphyletic with isolates falling into clades 1 and cryptic clades C–I and C-II; 2) RT151 contains both nontoxigenic and toxigenic isolates and 3) RT151 C-II isolates contained monotoxin pathogenicity loci. The isolate from our patient case report contains a novel-pathogenicity loci insertion site, lacked tcdA and had a divergent tcdB sequence that might explain the failure of the diagnostic test. DiscussionThis study shows that RT151 encompasses both typical and cryptic clades and provides conclusive evidence for C. difficile infection due to clade C-II isolates that was hitherto lacking. Vigilance towards C. difficile infection as a result of cryptic clade isolates is warranted.

High-quality Japanese flounder genome aids in identifying stress-related genes using gene coexpression network

The Japanese flounder is one of the most economically important marine flatfish. However, due to the increased frequency of extreme weather events and high-density industrial farming, an increasing number of environmental stresses have become severe threats to the healthy development of the Japanese flounder culture industry. Herein, we produced a high-quality chromosome-scale Japanese flounder genome using PacBio Circular Consensus Sequencing technologies. The assembled Japanese flounder genome spanned 588.22 Mb with a contig N50 size of 24.35 Mb. In total, 105.89 Mb of repetitive sequences and 22,565 protein-coding genes were identified by genome annotation. In addition, 67 candidate genes responding to distinct stresses were identified by gene coexpression network analysis based on 16 published stress-related RNA-seq datasets encompassing 198 samples. A high-quality chromosome-scale Japanese flounder genome and candidate stress-related gene set will not only serve as key resources for genomics studies and further research on the underlying stress responsive molecular mechanisms in Japanese flounder but will also advance the progress of genetic improvement and comprehensive stress-resistant molecular breeding of Japanese flounder.

The Capparis spinosa var. herbacea genome provides the first genomic instrument for a diversity and evolution study of the Capparaceae family.

The caper bush Capparis spinosa L., one of the most economically important species of Capparaceae, is a xerophytic shrub that is well adapted to drought and harsh environments. However, genetic studies on this species are limited because of the lack of its reference genome. We sequenced and assembled the Capparis spinosa var. herbacea (Willd.) genome using data obtained from the combination of PacBio circular consensus sequencing and high-throughput chromosome conformation capture. The final genome assembly was approximately 274.53 Mb (contig N50 length of 9.36 Mb, scaffold N50 of 15.15 Mb), 99.23% of which was assigned to 21 chromosomes. In the whole-genome sequence, tandem repeats accounted for 19.28%, and transposable element sequences accounted for 43.98%. The proportion of tandem repeats in the C. spinosa var. herbacea genome was much higher than the average of 8.55% in plant genomes. A total of 21,577 protein-coding genes were predicted, with 98.82% being functionally annotated. The result of species divergence times showed that C. spinosa var. herbacea and Tarenaya hassleriana separated from a common ancestor 43.31 million years ago. This study reported a high-quality reference genome assembly and genome features for the Capparaceae family. The assembled C. spinosa var. herbacea genome might provide a system for studying the diversity, speciation, and evolution of this family and serve as an important resource for understanding the mechanism of drought and high-temperature resistance.

Study on sex-linked region and sex determination candidate gene using a high-quality genome assembly in yellow drum

Yellow drum is marine fish species distributed in East Asia, exhibiting a sex-dependent dimorphic growth pattern where females grow much faster than males. The sex determination candidate gene and molecular mechanism were still unclear in yellow drum. The interpretation of sex chromosome sequences could shed light on sex determination. Herein, using high-fidelity (HiFi) reads generated with the PacBio circular consensus sequencing mode, we assembled and annotated a male genome of the yellow drum. The genome sequence assembly has a contig N50 size of 22.37 Mb and 98.17% of sequence length of contigs can be assigned to 24 chromosomes via Hi-C interaction data. Fst analysis demonstrated that the genetic divergence regions on sex chromosome occupied 2/3 of sex chromosome. The site with genotypes concordance (the male is heterozygous and female is homozygous) reached 100% in both males and females spanning from 14.9 – 24.9 Mb on sex chromosome, this 10 Mb region was designed as sex determination region (SDR). We integrated the RNAseq data generated with gonads of early development larvae and analyzed the expression of the genes located in SDR, suggested that dmrt1 might play as sex determination key gene in males of yellow drum. Subsequently, we screened out a Y chromosome contig (∼1.4 Mb) using a sex-linked marker. We found that there are two tandem dmrt1 genes in yellow drum, designated as dmrt1a and dmrt1b, and dmrt1b harbors a premature stop codon. A male-specific 438-bp deletion in the promoter of dmrt1a was successfully developed as a sex specific marker. Our study laid an important foundation for clarifying the sex determination mechanism in yellow drum and accelerating the production of all-female population.

Comparison of ONT and CCS sequencing technologies on the polyploid genome of a medicinal plant showed that high error rate of ONT reads are not suitable for self-correction

BackgroundMany medicinal plants are known for their complex genomes with high ploidy, heterozygosity, and repetitive content which pose severe challenges for genome sequencing of those species. Long reads from Oxford nanopore sequencing technology (ONT) or Pacific Biosciences Single Molecule, Real-Time (SMRT) sequencing offer great advantages in de novo genome assembly, especially for complex genomes with high heterozygosity and repetitive content. Currently, multiple allotetraploid species have sequenced their genomes by long-read sequencing. However, we found that a considerable proportion of these genomes (7.9% on average, maximum 23.7%) could not be covered by NGS (Next Generation Sequencing) reads (uncovered region by NGS reads, UCR) suggesting the questionable and low-quality of those area or genomic areas that can’t be sequenced by NGS due to sequencing bias. The underlying causes of those UCR in the genome assembly and solutions to this problem have never been studied.MethodsIn the study, we sequenced the tetraploid genome of Veratrum dahuricum (Turcz.) O. Loes (VDL), a Chinese medicinal plant, with ONT platform and assembled the genome with three strategies in parallel. We compared the qualities, coverage, and heterozygosity of the three ONT assemblies with another released assembly of the same individual using reads from PacBio circular consensus sequencing (CCS) technology, to explore the cause of the UCR.ResultsBy mapping the NGS reads against the three ONT assemblies and the CCS assembly, we found that the coverage of those ONT assemblies by NGS reads ranged from 49.15 to 76.31%, much smaller than that of the CCS assembly (99.53%). And alignment between ONT assemblies and CCS assembly showed that most UCR can be aligned with CCS assembly. So, we conclude that the UCRs in ONT assembly are low-quality sequences with a high error rate that can’t be aligned with short reads, rather than genomic regions that can’t be sequenced by NGS. Further comparison among the intermediate versions of ONT assemblies showed that the most probable origin of those errors is a combination of artificial errors introduced by “self-correction” and initial sequencing error in long reads. We also found that polishing the ONT assembly with CCS reads can correct those errors efficiently.ConclusionsThrough analyzing genome features and reads alignment, we have found the causes for the high proportion of UCR in ONT assembly of VDL are sequencing errors and additional errors introduced by self-correction. The high error rates of ONT-raw reads make them not suitable for self-correction prior to allotetraploid genome assembly, as the self-correction will introduce artificial errors to > 5% of the UCR sequences. We suggest high-precision CCS reads be used to polish the assembly to correct those errors effectively for polyploid genomes.

Initial Analysis of Structural Variation Detections in Cattle Using Long-Read Sequencing Methods.

Structural variations (SVs), as a great source of genetic variation, are widely distributed in the genome. SVs involve longer genomic sequences and potentially have stronger effects than SNPs, but they are not well captured by short-read sequencing owing to their size and relevance to repeats. Improved characterization of SVs can provide more advanced insight into complex traits. With the availability of long-read sequencing, it has become feasible to uncover the full range of SVs. Here, we sequenced one cattle individual using 10× Genomics (10 × G) linked read, Pacific Biosciences (PacBio) continuous long reads (CLR) and circular consensus sequencing (CCS), as well as Oxford Nanopore Technologies (ONT) PromethION. We evaluated the ability of various methods for SV detection. We identified 21,164 SVs, which amount to 186 Mb covering 7.07% of the whole genome. The number of SVs inferred from long-read-based inferences was greater than that from short reads. The PacBio CLR identified the most of large SVs and covered the most genomes. SVs called with PacBio CCS and ONT data showed high uniformity. The one with the most overlap with the results obtained by short-read data was PB CCS. Together, we found that long reads outperformed short reads in terms of SV detections.

Chromosome-scale Cerasus humilis genome assembly reveals gene family evolution and possible genomic basis of calcium accumulation in fruits

Chinese dwarf cherry (Cerasus humilis), which is well known as “calcium fruit” for its high calcium content, is widely utilized in North China for its ecological and economical value. Understanding the evolutionary history and genetic basis underlying the calcium accumulation in this fruit is a fundamental goal in comparative genomics. In this study, we obtained a full phased chromosome-scale C. humilis genome for the cultivar “zhisha” using PacBio circular consensus sequencing reads and Phase genomics Proximo Hi-C technologies. The final assembled genome lengths were 235.41 and 237.35 Mb for primary assembly (p0) and alternate haplotig (p1), respectively, with an N50 contig length of 3.23 Mb. The genome contained 26,800 protein-coding genes, while 41.33% of the genome length consisted of TEs. Phylogenetic analysis revealed that C. humilis displayed a close relationship with Prunus and phylogenetically diverged from its common ancestor ∼23.2 million years ago (MYA). Functional enrichment of expanded gene families in C. humilis genome highlighted genes involved in signal transduction, antiporter activity, polygalacturonase activity, auxin efflux transmembrane transporter activity, and auxin efflux, which is related to calcium transport and distribution. Phylogenetic analysis coupled with tissue-specific expression patterns indicated that PSGs WAK2 and CBL10, may also be responsible for the high Ca2+accumulation ability in C. humilis. These results glean tremendous insight into the fundamental genetic basis of the high calcium content in C. humilis fruit. In addition, the reference C. humilis genome we present in this study will be valuable resources for molecular breeding and genetic research of this fruit species.

High-Quality Genome of the Medicinal Plant Strobilanthes cusia Provides Insights Into the Biosynthesis of Indole Alkaloids.

Strobilanthes cusia (Nees) Kuntze is an important plant used to process the traditional Chinese herbal medicines “Qingdai” and “Nanbanlangen”. The key active ingredients are indole alkaloids (IAs) that exert antibacterial, antiviral, and antitumor pharmacological activities and serve as natural dyes. We assembled the S. cusia genome at the chromosome level through combined PacBio circular consensus sequencing (CCS) and Hi-C sequencing data. Hi-C data revealed a draft genome size of 913.74 Mb, with 904.18 Mb contigs anchored into 16 pseudo-chromosomes. Contig N50 and scaffold N50 were 35.59 and 68.44 Mb, respectively. Of the 32,974 predicted protein-coding genes, 96.52% were functionally annotated in public databases. We predicted 675.66 Mb repetitive sequences, 47.08% of sequences were long terminal repeat (LTR) retrotransposons. Moreover, 983 Strobilanthes-specific genes (SSGs) were identified for the first time, accounting for ~2.98% of all protein-coding genes. Further, 245 putative centromeric and 29 putative telomeric fragments were identified. The transcriptome analysis identified 2,975 differentially expressed genes (DEGs) enriched in phenylpropanoid, flavonoid, and triterpenoid biosynthesis. This systematic characterization of key enzyme-coding genes associated with the IA pathway and basic helix-loop-helix (bHLH) transcription factor family formed a network from the shikimate pathway to the indole alkaloid synthesis pathway in S. cusia. The high-quality S. cusia genome presented herein is an essential resource for the traditional Chinese medicine genomics studies and understanding the genetic underpinning of IA biosynthesis.

Performance assessment of DNA sequencing platforms in the ABRF Next-Generation Sequencing Study.

Assessing the reproducibility, accuracy and utility of massively parallel DNA sequencing platforms remains an ongoing challenge. Here the Association of Biomolecular Resource Facilities (ABRF) Next-Generation Sequencing Study benchmarks the performance of a set of sequencing instruments (HiSeq/NovaSeq/paired-end 2 × 250-bp chemistry, Ion S5/Proton, PacBio circular consensus sequencing (CCS), Oxford Nanopore Technologies PromethION/MinION, BGISEQ-500/MGISEQ-2000 and GS111) on human and bacterial reference DNA samples. Among short-read instruments, HiSeq 4000 and X10 provided the most consistent, highest genome coverage, while BGI/MGISEQ provided the lowest sequencing error rates. The long-read instrument PacBio CCS had the highest reference-based mapping rate and lowest non-mapping rate. The two long-read platforms PacBio CCS and PromethION/MinION showed the best sequence mapping in repeat-rich areas and across homopolymers. NovaSeq 6000 using 2 × 250-bp read chemistry was the most robust instrument for capturing known insertion/deletion events. This study serves as a benchmark for current genomics technologies, as well as a resource to inform experimental design and next-generation sequencing variant calling.

HELLO: improved neural network architectures and methodologies for small variant calling

BackgroundModern Next Generation- and Third Generation- Sequencing methods such as Illumina and PacBio Circular Consensus Sequencing platforms provide accurate sequencing data. Parallel developments in Deep Learning have enabled the application of Deep Neural Networks to variant calling, surpassing the accuracy of classical approaches in many settings. DeepVariant, arguably the most popular among such methods, transforms the problem of variant calling into one of image recognition where a Deep Neural Network analyzes sequencing data that is formatted as images, achieving high accuracy. In this paper, we explore an alternative approach to designing Deep Neural Networks for variant calling, where we use meticulously designed Deep Neural Network architectures and customized variant inference functions that account for the underlying nature of sequencing data instead of converting the problem to one of image recognition.ResultsResults from 27 whole-genome variant calling experiments spanning Illumina, PacBio and hybrid Illumina-PacBio settings suggest that our method allows vastly smaller Deep Neural Networks to outperform the Inception-v3 architecture used in DeepVariant for indel and substitution-type variant calls. For example, our method reduces the number of indel call errors by up to 18%, 55% and 65% for Illumina, PacBio and hybrid Illumina-PacBio variant calling respectively, compared to a similarly trained DeepVariant pipeline. In these cases, our models are between 7 and 14 times smaller.ConclusionsWe believe that the improved accuracy and problem-specific customization of our models will enable more accurate pipelines and further method development in the field. HELLO is available at https://github.com/anands-repo/hello

Chromosome-scale genome assembly of the transformation-amenable common wheat cultivar 'Fielder'.

We have established a high-quality, chromosome-level genome assembly for the hexaploid common wheat cultivar ‘Fielder’, an American, soft, white, pastry-type wheat released in 1974 and known for its amenability to Agrobacterium tumefaciens-mediated transformation and genome editing. Accurate, long-read sequences were obtained using PacBio circular consensus sequencing with the HiFi approach. Sequence reads from 16 SMRT cells assembled using the hifiasm assembler produced assemblies with N50 greater than 20 Mb. We used the Omni-C chromosome conformation capture technique to order contigs into chromosome-level assemblies, resulting in 21 pseudomolecules with a cumulative size of 14.7 and 0.3 Gb of unanchored contigs. Mapping of published short reads from a transgenic wheat plant with an edited seed-dormancy gene, TaQsd1, identified four positions of transgene insertion into wheat chromosomes. Detection of guide RNA sequences in pseudomolecules provided candidates for off-target mutation induction. These results demonstrate the efficiency of chromosome-scale assembly using PacBio HiFi reads and their application in wheat genome-editing studies.

Immunogenetic variation at MHC class I loci in desert bighorn sheep (Ovis canadensis nelsoni) reflects natural and anthropogenic fragmentation

Abstract Desert bighorn sheep (Ovis canadensis nelsoni) are a charismatic ungulate native to western North America that live in mountainous regions. In the desert of the southwest United States, populations of bighorn sheep can be isolated by both natural and anthropogenic barriers. Isolation limits gene flow, exacerbates loss of genetic diversity to drift, and thus could affect immune responses to infectious agents. We sought to develop a relatively rapid method for determining MHC class I gene diversity in individual sheep. Both DNA and RNA were isolated from leukocytes collected from 154 adult bighorn sheep from across different mountain ranges in Southern California. Previously identified primers from domestic sheep were used to amplify exons 2 and 3 from OMHC I genes while appending adapters for PacBio circular consensus sequencing. We successfully identified over 40 unique MHC class I sequences expressed by bighorn sheep. Using a combination of phylogenic tree analysis and mapping MHC sequences to the domestic sheep genome, we detect at least 4 unique classical MHC class I regions. Comparison of results using either genomic DNA or mRNA (cDNA) as a template for analysis suggest that certain MHC alleles are expressed at different levels within leukocytes, while many potential pseudogenes were identified when genomic DNA was used as a template. Our data suggests using messenger RNA (cDNA) as a template to identify MHC class I diversity is superior to genomic DNA. Finally, we found clear patterns of MHC class I genotypes differentiated by population, which suggests that both natural and anthropogenic-induced population fragmentation can limit the diversity of MHC genes within a given population.

Robust Benchmark Structural Variant Calls of An Asian Using State-of-the-art Long-read Sequencing Technologies

The importance of structural variants (SVs) for human phenotypes and diseases is now recognized. Although a variety of SV detection platforms and strategies that vary in sensitivity and specificity have been developed, few benchmarking procedures are available to confidently assess their performances in biological and clinical research. To facilitate the validation and application of these SV detection approaches, we established an Asian reference material by characterizing the genome of an Epstein-Barr virus (EBV)-immortalized B lymphocyte line along with identified benchmark regions and high-confidence SV calls. We established a high-confidence SV callset with 8938 SVs by integrating four alignment-based SV callers, including 109× Pacific Biosciences (PacBio) continuous long reads (CLRs), 22× PacBio circular consensus sequencing (CCS) reads, 104× Oxford Nanopore Technologies (ONT) long reads, and 114× Bionano optical mapping platform, and one de novo assembly-based SV caller using CCS reads. A total of 544 randomly selected SVs were validated by PCR amplification and Sanger sequencing, demonstrating the robustness of our SV calls. Combining trio-binning-based haplotype assemblies, we established an SV benchmark for identifying false negatives and false positives by constructing the continuous high-confidence regions (CHCRs), which covered 1.46 gigabase pairs (Gb) and 6882 SVs supported by at least one diploid haplotype assembly. Establishing high-confidence SV calls for a benchmark sample that has been characterized by multiple technologies provides a valuable resource for investigating SVs in human biology, disease, and clinical research.

A comprehensive investigation of metagenome assembly by linked-read sequencing

BackgroundThe human microbiota are complex systems with important roles in our physiological activities and diseases. Sequencing the microbial genomes in the microbiota can help in our interpretation of their activities. The vast majority of the microbes in the microbiota cannot be isolated for individual sequencing. Current metagenomics practices use short-read sequencing to simultaneously sequence a mixture of microbial genomes. However, these results are in ambiguity during genome assembly, leading to unsatisfactory microbial genome completeness and contig continuity. Linked-read sequencing is able to remove some of these ambiguities by attaching the same barcode to the reads from a long DNA fragment (10–100 kb), thus improving metagenome assembly. However, it is not clear how the choices for several parameters in the use of linked-read sequencing affect the assembly quality.ResultsWe first examined the effects of read depth (C) on metagenome assembly from linked-reads in simulated data and a mock community. The results showed that C positively correlated with the length of assembled sequences but had little effect on their qualities. The latter observation was corroborated by tests using real data from the human gut microbiome, where C demonstrated minor impact on the sequence quality as well as on the proportion of bins annotated as draft genomes. On the other hand, metagenome assembly quality was susceptible to read depth per fragment (CR) and DNA fragment physical depth (CF). For the same C, deeper CR resulted in more draft genomes while deeper CF improved the quality of the draft genomes. We also found that average fragment length (μFL) had marginal effect on assemblies, while fragments per partition (NF/P) impacted the off-target reads involved in local assembly, namely, lower NF/P values would lead to better assemblies by reducing the ambiguities of the off-target reads. In general, the use of linked-reads improved the assembly for contig N50 when compared to Illumina short-reads, but not when compared to PacBio CCS (circular consensus sequencing) long-reads.ConclusionsWe investigated the influence of linked-read sequencing parameters on metagenome assembly comprehensively. While the quality of genome assembly from linked-reads cannot rival that from PacBio CCS long-reads, the case for using linked-read sequencing remains persuasive due to its low cost and high base-quality. Our study revealed that the probable best practice in using linked-reads for metagenome assembly was to merge the linked-reads from multiple libraries, where each had sufficient CR but a smaller amount of input DNA.7PaeKL-RUCqQkRQg5_3dd9Video