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July 7, 2019  |  

Identification of repetitive DNA sequences in the Chrysanthemum boreale genome

We previously revealed that the Chrysanthemum boreale genome is highly repetitive; however, the types and nucleotide sequences of repetitive DNA in this diploid wild chrysanthemum are not known. Here, we characterized repetitive DNA sequences in the C. boreale genome by analysing genomic sequences obtained by Illumina sequencing and confirmed their repetitive nature by conducting fluorescence in situ hybridization (FISH) analyses. Annotation of the obtained DNA sequences revealed that microsatellite-containing genomic sequences exhibited similarity with genomic sequences in Chrysanthemum morifolium, indicating sequence conservation of repetitive DNA sequences between the two Chrysanthemum species. Two superfamilies of repetitive DNA, Copia and Gypsy, belonging to the long-terminal repeat (LTR) class of retrotransposons, are abundant in the C. boreale genome. We propose that Copia and Gypsy retroelements contribute to the current genome architecture of C. boreale. Whole genome sequencing, which is currently in progress, will reveal the extent to which these repetitive DNA sequences contribute.

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July 7, 2019  |  

The case for not masking away repetitive DNA

In the course of analyzing whole-genome data, it is common practice to mask or filter out repetitive regions of a genome, such as transposable elements and endogenous retroviruses, in order to focus only on genes and thus simplify the results. This Commentary is a plea from one member of the Mobile DNA community to all gene-centric researchers: please do not ignore the repetitive fraction of the genome. Please stop narrowing your findings by only analyzing a minority of the genome, and instead broaden your analyses to include the rich biology of repetitive and mobile DNA. In this article, I present four arguments supporting a case for retaining repetitive DNA in your genome-wide analysis.

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July 7, 2019  |  

TriPoly: haplotype estimation for polyploids using sequencing data of related individuals.

Knowledge of haplotypes, i.e. phased and ordered marker alleles on a chromosome, is essential to answer many questions in genetics and genomics. By generating short pieces of DNA sequence, high-throughput modern sequencing technologies make estimation of haplotypes possible for single individuals. In polyploids, however, haplotype estimation methods usually require deep coverage to achieve sufficient accuracy. This often renders sequencing-based approaches too costly to be applied to large populations needed in studies of Quantitative Trait Loci.We propose a novel haplotype estimation method for polyploids, TriPoly, that combines sequencing data with Mendelian inheritance rules to infer haplotypes in parent-offspring trios. Using realistic simulations of both short and long-read sequencing data for banana (Musa acuminata) and potato (Solanum tuberosum) trios, we show that TriPoly yields more accurate progeny haplotypes at low coverages compared to existing methods that work on single individuals. We also apply TriPoly to phase Single Nucleotide Polymorphisms on chromosome 5 for a family of tetraploid potato with 2 parents and 37 offspring sequenced with an RNA capture approach. We show that TriPoly haplotype estimates differ from those of the other methods mainly in regions with imperfect sequencing or mapping difficulties, as it does not rely solely on sequence reads and aims to avoid phasings that are not likely to have been passed from the parents to the offspring.TriPoly has been implemented in Python 3.5.2 (also compatible with Python 2.7.3 and higher) and can be freely downloaded at https://github.com/EhsanMotazedi/TriPoly.Supplementary data are available at Bioinformatics online.

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July 7, 2019  |  

Genomes and transcriptomes of duckweeds.

Duckweeds (Lemnaceae family) are the smallest flowering plants that adapt to the aquatic environment. They are regarded as the promising sustainable feedstock with the characteristics of high starch storage, fast propagation, and global distribution. The duckweed genome size varies 13-fold ranging from 150 Mb in Spirodela polyrhiza to 1,881 Mb in Wolffia arrhiza. With the development of sequencing technology and bioinformatics, five duckweed genomes from Spirodela and Lemna genera are sequenced and assembled. The genome annotations discover that they share similar protein orthologs, whereas the repeat contents could mainly explain the genome size difference. The gene families responsible for cell growth and expansion, lignin biosynthesis, and flowering are greatly contracted. However, the gene family of glutamate synthase has experienced expansion, indicating their significance in ammonia assimilation and nitrogen transport. The transcriptome is comprehensively sequenced for the genera of Spirodela, Landoltia, and Lemna, including various treatments such as abscisic acid, radiation, heavy metal, and starvation. The analysis of the underlying molecular mechanism and the regulatory network would accelerate their applications in the fields of bioenergy and phytoremediation. The comparative genomics has shown that duckweed genomes contain relatively low gene numbers and more contracted gene families, which may be in parallel with their highly reduced morphology with a simple leaf and primary roots. Still, we are waiting for the advancement of the long read sequencing technology to resolve the complex genomes and transcriptomes for unsequenced Wolffiella and Wolffia due to the large genome sizes and the similarity in their polyploidy.

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July 7, 2019  |  

The recombination landscape of Drosophila virilis is robust to transposon activation in hybrid dysgenesis

DNA damage in the germline is a double-edged sword. Induced double-strand breaks establish the foundation for meiotic recombination and proper chromosome segregation but can also pose a significant challenge for genome stability. Within the germline, transposable elements are powerful agents of double-strand break formation. How different types of DNA damage are resolved within the germline is poorly understood. For example, little is known about the relationship between the frequency of double-stranded breaks, both endogenous and exogenous, and the decision to repair DNA through one of the many pathways, including crossing over and gene conversion. Here we use the Drosophila virilis hybrid dysgenesis model to determine how recombination landscapes change under transposable element activation. In this system, a cross between two strains of D. virilis with divergent transposable element profiles results in the hybrid dysgenesis phenotype, which includes the germline activation of diverse transposable elements, reduced fertility, and male recombination. However, only one direction of the cross results in hybrid dysgenesis. This allows the study of recombination in genetically identical F1 females; those with baseline levels of programmed DNA damage and those with an increased level of DNA damage resulting from transposable element proliferation. Using multiplexed shotgun genotyping to map crossover events, we compared the recombination landscapes of hybrid dysgenic and non-hybrid dysgenic individuals. The frequency and distribution of meiotic recombination appears to be robust during hybrid dysgenesis. However, hybrid dysgenesis is also associated with occasional clusters of recombination derived from single dysgenic F1 mothers. The clusters of recombination are hypothesized to be the result of mitotic crossovers during early germline development. Overall, these results show that meiotic recombination in D. virilis is robust to the damage caused by transposable elements during early development.

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July 7, 2019  |  

Evolutionary emergence of drug resistance in Candida opportunistic pathogens.

Fungal infections, such as candidiasis caused by Candida, pose a problem of growing medical concern. In developed countries, the incidence of Candida infections is increasing due to the higher survival of susceptible populations, such as immunocompromised patients or the elderly. Existing treatment options are limited to few antifungal drug families with efficacies that vary depending on the infecting species. In this context, the emergence and spread of resistant Candida isolates are being increasingly reported. Understanding how resistance can evolve within naturally susceptible species is key to developing novel, more effective treatment strategies. However, in contrast to the situation of antibiotic resistance in bacteria, few studies have focused on the evolutionary mechanisms leading to drug resistance in fungal species. In this review, we will survey and discuss current knowledge on the genetic bases of resistance to antifungal drugs in Candida opportunistic pathogens. We will do so from an evolutionary genomics perspective, focusing on the possible evolutionary paths that may lead to the emergence and selection of the resistant phenotype. Finally, we will discuss the potential of future studies enabled by current developments in sequencing technologies, in vitro evolution approaches, and the analysis of serial clinical isolates.

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July 7, 2019  |  

Measuring the mappability spectrum of reference genome assemblies

The ability to infer actionable information from genomic variation data in a resequencing experiment relies on accurately aligning the sequences to a reference genome. However, this accuracy is inherently limited by the quality of the reference assembly and the repetitive content of the subject’s genome. As long read sequencing technologies become more widespread, it is crucial to investigate the expected improvements in alignment accuracy and variant analysis over existing short read methods. The ability to quantify the read length and error rate necessary to uniquely map regions of interest in a sequence allows users to make informed decisions regarding experiment design and provides useful metrics for comparing the magnitude of repetition across different reference assemblies. To this end we have developed NEAT-Repeat, a toolkit for exhaustively identifying the minimum read length required to uniquely map each position of a reference sequence given a specified error rate. Using these tools we computed the -mappability spectrum” for ten reference sequences, including human and a range of plants and animals, quantifying the theoretical improvements in alignment accuracy that would result from sequencing with longer reads or reads with less base-calling errors. Our inclusion of read length and error rate builds upon existing methods for mappability tracks based on uniqueness or aligner-specific mapping scores, and thus enables more comprehensive analysis. We apply our mappability results to whole-genome variant call data, and demonstrate that variants called with low mapping and genotype quality scores are disproportionately found in reference regions that require long reads to be uniquely covered. We propose that our mappability metrics provide a valuable supplement to established variant filtering and annotation pipelines by supplying users with an additional metric related to read mapping quality. NEAT-Repeat can process large and repetitive genomes, such as those of corn and soybean, in a tractable amount of time by leveraging efficient methods for edit distance computation as well as running multiple jobs in parallel. NEAT-Repeat is written in Python 2.7 and C++, and is available at https://github.com/zstephens/neat-repeat.

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July 7, 2019  |  

Pilot satellitome analysis of the model plant, Physcomitrellapatens, revealed a transcribed and high-copy IGS related tandem repeat.

Satellite DNA (satDNA) constitutes a substantial part of eukaryotic genomes. In the last decade, it has been shown that satDNA is not an inert part of the genome and its function extends beyond the nuclear membrane. However, the number of model plant species suitable for studying the novel horizons of satDNA functionality is low. Here, we explored the satellitome of the model “basal” plant, Physcomitrellapatens (Hedwig, 1801) Bruch & Schimper, 1849 (moss), which has a number of advantages for deep functional and evolutionary research. Using a newly developed pyTanFinder pipeline (https://github.com/Kirovez/pyTanFinder) coupled with fluorescence in situ hybridization (FISH), we identified five high copy number tandem repeats (TRs) occupying a long DNA array in the moss genome. The nuclear organization study revealed that two TRs had distinct locations in the moss genome, concentrating in the heterochromatin and knob-rDNA like chromatin bodies. Further genomic, epigenetic and transcriptomic analysis showed that one TR, named PpNATR76, was located in the intergenic spacer (IGS) region and transcribed into long non-coding RNAs (lncRNAs). Several specific features of PpNATR76 lncRNAs make them very similar with the recently discovered human lncRNAs, raising a number of questions for future studies. This work provides new resources for functional studies of satellitome in plants using the model organism P.patens, and describes a list of tandem repeats for further analysis.

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July 7, 2019  |  

Bridging gaps in transposable element research with single-molecule and single-cell technologies

More than half of the genomic landscape in humans and many other organisms is composed of repetitive DNA, which mostly derives from transposable elements (TEs) and viruses. Recent technological advances permit improved assessment of the repetitive content across genomes and newly developed molecular assays have revealed important roles of TEs and viruses in host genome evolution and organization. To update on our current understanding of TE biology and to promote new interdisciplinary strategies for the TE research community, leading experts gathered for the 2nd Uppsala Transposon Symposium on October 4–5, 2018 in Uppsala, Sweden. Using cutting-edge single-molecule and single-cell approaches, research on TEs and other repeats has entered a new era in biological and biomedical research.

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July 7, 2019  |  

De novo genome assembly of the olive fruit fly (Bactrocera oleae) developed through a combination of linked-reads and long-read technologies

Long-read sequencing has greatly contributed to the generation of high quality assemblies, albeit at a high cost. It is also not always clear how to combine sequencing platforms. We sequenced the genome of the olive fruit fly (Bactrocera oleae), the most important pest in the olive fruits agribusiness industry, using Illumina short-reads, mate-pairs, 10x Genomics linked-reads, Pacific Biosciences (PacBio), and Oxford Nanopore Technologies (ONT). The 10x linked-reads assembly gave the most contiguous assembly with an N50 of 2.16 Mb. Scaffolding the linked-reads assembly using long-reads from ONT gave a more contiguous assembly with scaffold N50 of 4.59 Mb. We also present the most extensive transcriptome datasets of the olive fly derived from different tissues and stages of development. Finally, we used the Chromosome Quotient method to identify Y-chromosome scaffolds and show that the long-reads based assembly generates very highly contiguous Y-chromosome assembly.

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