Targeted engineering of the Caenorhabditis elegans genome following Mos1-triggered chromosomal breaks

Silencing of a NRDE-3 transgene in C. elegans germ cells and early embryos is mediated by the RNAi pathway

Transgenes are highly susceptible to gene silencing in the C. elegans germline. Here, we examine the expression of the nuclear Argonaute protein NRDE-3 , comparing two GFP::NRDE-3 strains, one constructed by bombardment and one by CRISPR. We found that the GFP::NRDE-3 strain constructed by bombardment displays transgene silencing in germline and early embryos and that NRDE-3 expression can be restored in a rde-3 mutant, which disrupts the RNAi pathway. This finding reveals that NRDE-3 is not a soma-specific Argonaute protein and is, in fact, expressed in the proximal germline and early embryos.

BRCA1/BRC-1 and SMC-5/6 regulate DNA repair pathway engagement during Caenorhabditis elegans meiosis

The preservation of genome integrity during sperm and egg development is vital for reproductive success. During meiosis, the tumor suppressor BRCA1/BRC-1 and structural maintenance of chromosomes 5/6 (SMC-5/6) complex genetically interact to promote high fidelity DNA double strand break (DSB) repair, but the specific DSB repair outcomes these proteins regulate remain unknown. Using genetic and cytological methods to monitor resolution of DSBs with different repair partners in Caenorhabditis elegans, we demonstrate that both BRC-1 and SMC-5 repress intersister crossover recombination events. Sequencing analysis of conversion tracts from homolog-independent DSB repair events further indicates that BRC-1 regulates intersister/intrachromatid noncrossover conversion tract length. Moreover, we find that BRC-1 specifically inhibits error prone repair of DSBs induced at mid-pachytene. Finally, we reveal functional interactions of BRC-1 and SMC-5/6 in regulating repair pathway engagement: BRC-1 is required for localization of recombinase proteins to DSBs in smc-5 mutants and enhances DSB repair defects in smc-5 mutants by repressing theta-mediated end joining (TMEJ). These results are consistent with a model in which some functions of BRC-1 act upstream of SMC-5/6 to promote recombination and inhibit error-prone DSB repair, while SMC-5/6 acts downstream of BRC-1 to regulate the formation or resolution of recombination intermediates. Taken together, our study illuminates the coordinated interplay of BRC-1 and SMC-5/6 to regulate DSB repair outcomes in the germline.

Modular safe-harbor transgene insertion for targeted single-copy and extrachromosomal array integration in Caenorhabditis elegans

Efficient and reproducible transgenesis facilitates and accelerates research using genetic model organisms. Here, we describe a modular safe-harbor transgene insertion (MosTI) for use in Caenorhabditis elegans which improves targeted insertion of single-copy transgenes by homology directed repair and targeted integration of extrachromosomal arrays by nonhomologous end-joining. MosTI allows easy conversion between selection markers at insertion site and a collection of universal targeting vectors with commonly used promoters and fluorophores. Insertions are targeted at three permissive safe-harbor intergenic locations and transgenes are reproducibly expressed in somatic and germ cells. Chromosomal integration is mediated by CRISPR/Cas9, and positive selection is based on a set of split markers (unc-119, hygroR, and gfp) where only animals with chromosomal insertions are rescued, resistant to antibiotics, or fluorescent, respectively. Single-copy insertion is efficient using either constitutive or heat-shock inducible Cas9 expression (25-75%) and insertions can be generated from a multiplexed injection mix. Extrachromosomal array integration is also efficient (7-44%) at modular safe-harbor transgene insertion landing sites or at the endogenous unc-119 locus. We use short-read sequencing to estimate the plasmid copy numbers for 8 integrated arrays (6-37 copies) and long-read Nanopore sequencing to determine the structure and size (5.4 Mb) of 1 array. Using universal targeting vectors, standardized insertion strains, and optimized protocols, it is possible to construct complex transgenic strains which should facilitate the study of increasingly complex biological problems in C. elegans.

Targeted and Random Transposon-Assisted Single-Copy Transgene Insertion in C. elegans

Transgenesis in model organisms is an essential tool for determining the function of protein-coding genes and non-coding regulatory regions. In Caenorhabditis elegans, injected DNA can be propagated as multicopy extra-chromosomal arrays, but transgenes in arrays are frequently mosaic, over-expressed in some tissues, and silenced in the germline. Here, we describe methods to insert single-copy transgenes into specific genomic locations (MosSCI) or random locations (miniMos) using Mos1 transposons. Single-copy insertions allow expression at endogenous levels, expression in the germline, and identification of active and repressed regions of the genome.

Detection of homolog-independent meiotic DNA repair events in C. elegans with the intersister/intrachromatid repair assay

Accurate repair of DNA double-strand breaks (DSBs) in developing germ cells is critical to promote proper chromosome segregation and to maintain genome integrity. To directly detect homolog-independent (intersister/intrachromatid) meiotic DSB repair, we exploited the genetics and germline physiology of C. elegans to (1) induce a single DSB in nuclei across discrete stages of meiotic prophase I; (2) detect repair of that DSB as a homolog-independent crossover or noncrossover; and (3) sequence the resultant product to assess mechanisms of recombination. For complete details on the use and execution of this protocol, please refer to Toraason et al. (2021).

Harnessing model organism genomics to underpin the machine learning-based prediction of essential genes in eukaryotes - Biotechnological implications

The availability of high-quality genomes and advances in functional genomics have enabled large-scale studies of essential genes in model eukaryotes, including the 'elegant worm' (Caenorhabditis elegans; Nematoda) and the 'vinegar fly' (Drosophila melanogaster; Arthropoda). However, this is not the case for other, much less-studied organisms, such as socioeconomically important parasites, for which functional genomic platforms usually do not exist. Thus, there is a need to develop innovative techniques or approaches for the prediction, identification and investigation of essential genes. A key approach that could enable the prediction of such genes is machine learning (ML). Here, we undertake an historical review of experimental and computational approaches employed for the characterisation of essential genes in eukaryotes, with a particular focus on model ecdysozoans (C. elegans and D. melanogaster), and discuss the possible applicability of ML-approaches to organisms such as socioeconomically important parasites. We highlight some recent results showing that high-performance ML, combined with feature engineering, allows a reliable prediction of essential genes from extensive, publicly available 'omic data sets, with major potential to prioritise such genes (with statistical confidence) for subsequent functional genomic validation. These findings could 'open the door' to fundamental and applied research areas. Evidence of some commonality in the essential gene-complement between these two organisms indicates that an ML-engineering approach could find broader applicability to ecdysozoans such as parasitic nematodes or arthropods, provided that suitably large and informative data sets become/are available for proper feature engineering, and for the robust training and validation of algorithms. This area warrants detailed exploration to, for example, facilitate the identification and characterisation of essential molecules as novel targets for drugs and vaccines against parasitic diseases. This focus is particularly important, given the substantial impact that such diseases have worldwide, and the current challenges associated with their prevention and control and with drug resistance in parasite populations.

Meiotic DNA break repair can utilize homolog-independent chromatid templates in C. elegans

During meiosis, the maintenance of genome integrity is critical for generating viable haploid gametes.¹ In meiotic prophase I, double-strand DNA breaks (DSBs) are induced and a subset of these DSBs are repaired as interhomolog crossovers to ensure proper chromosome segregation. DSBs not resolved as crossovers with the homolog must be repaired by other pathways to ensure genome integrity.² To determine if alternative repair templates can be engaged for meiotic DSB repair during oogenesis, we developed an assay to detect sister and/or intra-chromatid repair events at a defined DSB site during Caenorhabditis elegans meiosis. Using this assay, we directly demonstrate that the sister chromatid or the same DNA molecule can be engaged as a meiotic repair template for both crossover and noncrossover recombination, with noncrossover events being the predominant recombination outcome. We additionally find that the sister or intra-chromatid substrate is available as a recombination partner for DSBs induced throughout meiotic prophase I, including late prophase when the homolog is unavailable. Analysis of noncrossover conversion tract sequences reveals that DSBs are processed similarly throughout prophase I. We further present data indicating that the XPF-1 nuclease functions in late prophase to promote sister or intra-chromatid repair at steps of recombination following joint molecule processing. Despite its function in sister or intra-chromatid repair, we find that xpf-1 mutants do not exhibit severe defects in progeny viability following exposure to ionizing radiation. Overall, we propose that C. elegans XPF-1 may assist as an intersister or intrachromatid resolvase only in late prophase I.

Peel-1 negative selection promotes screening-free CRISPR-Cas9 genome editing in Caenorhabditis elegans

Improved genome engineering methods that enable automation of large and precise edits are essential for systematic investigations of genome function. We adapted peel-1 negative selection to an optimized Dual-Marker Selection (DMS) cassette protocol for CRISPR-Cas9 genome engineering in Caenorhabditis elegans and observed robust increases in multiple measures of efficiency that were consistent across injectors and four genomic loci. The use of Peel-1-DMS selection killed animals harboring transgenes as extrachromosomal arrays and spared genome-edited integrants, often circumventing the need for visual screening to identify genome-edited animals. To demonstrate the applicability of the approach, we created deletion alleles in the putative proteasomal subunit pbs-1 and the uncharacterized gene K04F10.3 and used machine vision to automatically characterize their phenotypic profiles, revealing homozygous essential and heterozygous behavioral phenotypes. These results provide a robust and scalable approach to rapidly generate and phenotype genome-edited animals without the need for screening or scoring by eye.

Crossover Position Drives Chromosome Remodeling for Accurate Meiotic Chromosome Segregation

Interhomolog crossovers (COs) are a prerequisite for achieving accurate chromosome segregation during meiosis [1, 2]. COs are not randomly positioned, occurring at distinct genomic intervals during meiosis in all species examined [3-10]. The role of CO position as a major determinant of accurate chromosome segregation has not been previously directly analyzed in a metazoan. Here, we use spo-11 mutants, which lack endogenous DNA double-strand breaks (DSBs), to induce a single DSB by Mos1 transposon excision at defined chromosomal locations in the C. elegans germline and show that the position of the resulting CO directly affects the formation of distinct chromosome subdomains during meiotic chromosome remodeling. CO formation in the typically CO-deprived center region of autosomes leads to premature loss of sister chromatid cohesion and chromosome missegregation, whereas COs at an off-centered position, as in wild type, can result in normal remodeling and accurate segregation. Ionizing radiation (IR)-induced DSBs lead to the same outcomes, and modeling of IR dose-response reveals that the CO-unfavorable center region encompasses up to 6% of the total chromosome length. DSBs proximal to telomeres rarely form COs, likely because of formation of unstable recombination intermediates that cannot be sustained as chiasmata until late prophase. Our work supports a model in which regulation of CO position early in meiotic prophase is required for proper designation of chromosome subdomains and normal chromosome remodeling in late meiotic prophase I, resulting in accurate chromosome segregation and providing a mechanism to prevent aneuploid gamete formation.

Visualization and Quantification of Transposon Activity in Caenorhabditis elegans RNAi Pathway Mutants

RNA silencing pathways play critical roles in maintaining quiescence of transposons in germ cells to promote genome integrity. However the precise mechanism by which different types of transposons are recognized by these pathways is not fully understood. Furthermore, the location in the germline where this transposition occurs after disruption of transposon silencing was previously unknown. Here we utilize the spatial and temporal organization of the Caenorhabditis elegans germline to demonstrate that transposition of DNA transposons in RNA silencing pathway mutants occur in all stages of adult germ cells. We further demonstrate that the double-strand breaks generated by transposons can restore homologous recombination in a mutant defective for the generation of meiosis-specific double-strand breaks. Finally, we detected clear differences in transposase expression and transposon excision between distinct branches of the RNA silencing pathway, emphasizing that there are multiple mechanisms by which transposons can be recognized and routed for small-RNA-mediated silencing.

The Claudin-like Protein HPO-30 Is Required to Maintain LAChRs at the C. elegans Neuromuscular Junction

Communications across chemical synapses are primarily mediated by neurotransmitters and their postsynaptic receptors. There are diverse molecular systems to localize and regulate the receptors at the synapse. Here, we identify HPO-30, a member of the claudin superfamily of membrane proteins, as a positive regulator for synaptic localization of levamisole-dependent AChRs (LAChRs) at the Caenorhabditis elegans neuromuscular junction (NMJ). The HPO-30 protein localizes at the NMJ and shows genetic and physical association with the LAChR subunits LEV-8, UNC-29, and UNC-38. Using genetic and electrophysiological assays in the hermaphrodite C. elegans, we demonstrate that HPO-30 functions through Neuroligin at the NMJ to maintain postsynaptic LAChR levels at the synapse. Together, this work suggests a novel function for a tight junction protein in maintaining normal receptor levels at the NMJ.

SIGNIFICANCE STATEMENT Claudins are a large superfamily of membrane proteins. Their role in maintaining the functional integrity of tight junctions has been widely explored. Our experiments suggest a critical role for the claudin-like protein, HPO-30, in maintaining synaptic levamisole-dependent AChR (LAChR) levels. LAChRs contribute to <20% of the acetylcholine-mediated conductance in adult Caenorhabditis elegans; however, they play a significant functional role in worm locomotion. This study provides a new perspective in the study of LAChR physiology.

Preventing Illegitimate Extrasynaptic Acetylcholine Receptor Clustering Requires the RSU-1 Protein

Diffuse extrasynaptic neurotransmitter receptors constitute an abundant pool of receptors that can be recruited to modulate synaptic strength. Whether the diffuse distribution of receptors in extrasynaptic membranes is a default state or is actively controlled remains essentially unknown. Here we show that RSU-1 (Ras Suppressor-1) is required for the proper distribution of extrasynaptic acetylcholine receptors (AChRs) in Caenorhabditis elegans muscle cells. RSU-1 is an evolutionary conserved cytoplasmic protein that contains multiple leucine-rich repeats (LRRs) and interacts with integrin-dependent adhesion complexes. In rsu-1 mutants, neuromuscular junctions differentiate as in the wild type, but AChRs assemble into ectopic clusters that progressively enlarge during development. As a consequence, the synaptic content of AChRs is reduced. Our study provides the first evidence that an RSU-1-dependent active mechanism maintains extrasynaptic receptors dispersed and indirectly regulates synapse maturation.

SIGNIFICANCE STATEMENT

Using Caenorhabditis elegans neuromuscular junction as a model synapse, we uncovered a novel mechanism that regulates the distribution of acetylcholine receptors (AChRs). In an unbiased visual screen for mutants with abnormal AChR distribution, we isolated the ras suppressor 1 (rsu-1) mutant based on the presence of large extrasynaptic clusters. We show that disrupting rsu-1 causes spontaneous clustering of extrasynaptic receptors that are normally dispersed, independently of synaptic cues. These clusters outcompete synaptic domains and cause a decrease of synaptic receptor content. These results indicate that the diffuse state of extrasynaptic receptors is not a default state that is simply explained by the lack of synaptic cues but necessitates additional proteins to prevent spontaneous clustering, a concept that is relevant for developmental and pathological situations.

CRISPR Technology Reveals RAD(51)-ical Mechanisms of Repair in Roundworms: An Educational Primer for Use with "Promotion of Homologous Recombination by SWS-1 in Complex with RAD-51 Paralogs in Caenorhabditis elegans"

The mechanisms cells use to maintain genetic fidelity via DNA repair and the accuracy of these processes have garnered interest from scientists engaged in basic research to clinicians seeking improved treatment for cancer patients. Despite the continued advances, many details of DNA repair are still incompletely understood. In addition, the inherent complexity of DNA repair processes, even at the most fundamental level, makes it a challenging topic. This primer is meant to assist both educators and students in using a recent paper, "Promotion of homologous recombination by SWS-1 in complex with RAD-51 paralogs in Caenorhabditis elegans," to understand mechanisms of DNA repair. The goals of this primer are to highlight and clarify several key techniques utilized, with special emphasis on the clustered, regularly interspaced, short palindromic repeats technique and the ways in which it has revolutionized genetics research, as well as to provide questions for deeper in-class discussion.

Reliable CRISPR/Cas9 Genome Engineering in Caenorhabditis elegans Using a Single Efficient sgRNA and an Easily Recognizable Phenotype

CRISPR/Cas9 genome engineering strategies allow the directed modification of the Caenorhabditis elegans genome to introduce point mutations, generate knock-out mutants, and insert coding sequences for epitope or fluorescent tags. Three practical aspects, however, complicate such experiments. First, the efficiency and specificity of single-guide RNAs (sgRNA) cannot be reliably predicted. Second, the detection of animals carrying genome edits can be challenging in the absence of clearly visible or selectable phenotypes. Third, the sgRNA target site must be inactivated after editing to avoid further double-strand break events. We describe here a strategy that addresses these complications by transplanting the protospacer of a highly efficient sgRNA into a gene of interest to render it amenable to genome engineering. This sgRNA targeting the dpy-10 gene generates genome edits at comparatively high frequency. We demonstrate that the transplanted protospacer is cleaved at the same time as the dpy-10 gene. Our strategy generates scarless genome edits because it no longer requires the introduction of mutations in endogenous sgRNA target sites. Modified progeny can be easily identified in the F1 generation, which drastically reduces the number of animals to be tested by PCR or phenotypic analysis. Using this strategy, we reliably generated precise deletion mutants, transcriptional reporters, and translational fusions with epitope tags and fluorescent reporter genes. In particular, we report here the first use of the new red fluorescent protein mScarlet in a multicellular organism. wrmScarlet, a C. elegans-optimized version, dramatically surpassed TagRFP-T by showing an eightfold increase in fluorescence in a direct comparison.

CRISPR-Based Methods for Caenorhabditis elegans Genome Engineering

The advent of genome editing techniques based on the clustered regularly interspersed short palindromic repeats (CRISPR)-Cas9 system has revolutionized research in the biological sciences. CRISPR is quickly becoming an indispensible experimental tool for researchers using genetic model organisms, including the nematode Caenorhabditis elegans. Here, we provide an overview of CRISPR-based strategies for genome editing in C. elegans. We focus on practical considerations for successful genome editing, including a discussion of which strategies are best suited to producing different kinds of targeted genome modifications.

Targeted genome engineering in Caenorhabditis elegans

The generation of mutants and transgenes are indispensible for biomedical research. In the nematode Caenorhabditis elegans, a series of methods have been developed to introduce genome modifications, including random mutagenesis by chemical reagents, ionizing radiation and transposon insertion. In addition, foreign DNA can be integrated into the genome through microparticle bombardment approach or by irradiation of animals carrying microinjected extrachromosomal arrays. Recent research has revolutionized the genome engineering technologies by using customized DNA nucleases to manipulate particular genes and genomic sequences. Many streamlined editing strategies are developed to simplify the experimental procedure and minimize the cost. In this review, we will summarize the recent progress of the site-specific genome editing methods in C. elegans, including the Cre/LoxP, FLP/FRT, MosTIC system, zinc-finger nucleases (ZFNs), transcriptional activator-like nucleases (TALENs), and the clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 nuclease. Particularly, the recent studies of CRISPR/Cas9-mediated genome editing method in C. elegans will be emphatically discussed.

Polq-Mediated End Joining Is Essential for Surviving DNA Double-Strand Breaks during Early Zebrafish Development

Error-prone repair of DNA double-strand breaks (DSBs) has been postulated to occur through classical non-homologous end joining (NHEJ) in systems ranging from nematode somatic tissues to zebrafish embryos. Contrary to this model, we show that zebrafish embryos mutant for DNA polymerase theta (Polq), a critical component of alternative end joining (alt-EJ), cannot repair DSBs induced by CRISPR/Cas9 or ionizing radiation. In the absence of DSBs, polq mutants are phenotypically normal, but they do not survive mutagenesis and display dramatic differences in the mutation profiles compared with the wild-type. These results show that alt-EJ repair is essential and dominant during the early development of a vertebrate.

Genome Editing in C. elegans and Other Nematode Species

Caenorhabditis elegans, a 1 mm long free-living nematode, is a popular model animal that has been widely utilized for genetic investigations of various biological processes. Characteristic features that make C. elegans a powerful model of choice for eukaryotic genetic studies include its rapid life cycle (development from egg to adult in 3.5 days at 20 °C), well-annotated genome, simple morphology (comprising only 959 somatic cells in the hermaphrodite), and transparency (which facilitates non-invasive fluorescence observations). However, early approaches to introducing mutations in the C. elegans genome, such as chemical mutagenesis and imprecise excision of transposons, have required large-scale mutagenesis screens. To avoid this laborious and time-consuming procedure, genome editing technologies have been increasingly used in nematodes including C. briggsae and Pristionchus pacificus, thereby facilitating their genetic analyses. Here, I review the recent progress in genome editing technologies using zinc-finger nucleases (ZFNs), transcriptional activator-like nucleases (TALENs), and clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 in nematodes and offer perspectives on their use in the future.

The application of CRISPR-Cas9 genome editing in Caenorhabditis elegans

Genome editing using the Cas9 endonuclease of Streptococcus pyogenes has demonstrated unparalleled efficacy and facility for modifying genomes in a wide variety of organisms. Caenorhabditis elegans is one of the most convenient multicellular organisms for genetic analysis, and the application of this novel genome editing technique to this organism promises to revolutionize analysis of gene function in the future. CRISPR-Cas9 has been successfully used to generate imprecise insertions and deletions via non-homologous end-joining mechanisms and to create precise mutations by homology-directed repair from donor templates. Key variables are the methods used to deliver the Cas9 endonuclease and the efficiency of the single guide RNAs. CRISPR-Cas9-mediated editing appears to be highly specific in C. elegans, with no reported off-target effects. In this review, I briefly summarize recent progress in CRISPR-Cas9-based genome editing in C. elegans, highlighting technical improvements in mutagenesis and mutation detection, and discuss potential future applications of this technique.

20 Years of unc-119 as a transgene marker

This fall marks 20 years since the cloning of unc-119 was reported. Despite having a strong phenotype that makes animals somewhat difficult to grow and handle, unc-119 mutant rescue has become one of the most frequently-used markers for C. elegans transformation. In this Commentary, I describe the history of how unc-119 rescue traveled through the worm community, contributing to the development of transgene methods in C. elegans.

Polymerase Θ is a key driver of genome evolution and of CRISPR/Cas9-mediated mutagenesis

Cells are protected from toxic DNA double-stranded breaks (DSBs) by a number of DNA repair mechanisms, including some that are intrinsically error prone, thus resulting in mutations. To what extent these mechanisms contribute to evolutionary diversification remains unknown. Here, we demonstrate that the A-family polymerase theta (POLQ) is a major driver of inheritable genomic alterations in Caenorhabditis elegans. Unlike somatic cells, which use non-homologous end joining (NHEJ) to repair DNA transposon-induced DSBs, germ cells use polymerase theta-mediated end joining, a conceptually simple repair mechanism requiring only one nucleotide as a template for repair. Also CRISPR/Cas9-induced genomic changes are exclusively generated through polymerase theta-mediated end joining, refuting a previously assumed requirement for NHEJ in their formation. Finally, through whole-genome sequencing of propagated populations, we show that only POLQ-proficient animals accumulate genomic scars that are abundantly present in genomes of wild C. elegans, pointing towards POLQ as a major driver of genome diversification.

Initiation of meiotic homologous recombination: flexibility, impact of histone modifications, and chromatin remodeling

Meiotic recombination is initiated by the formation of DNA double-strand breaks (DSBs) catalyzed by the evolutionary conserved Spo11 protein and accessory factors. DSBs are nonrandomly distributed along the chromosomes displaying a significant (~400-fold) variation of frequencies, which ultimately establishes local and long-range "hot" and "cold" domains for recombination initiation. This remarkable patterning is set up within the chromatin context, involving multiple layers of biochemical activity. Predisposed chromatin accessibility, but also a range of transcription factors, chromatin remodelers, and histone modifiers likely promote local recruitment of DSB proteins, as well as mobilization, sliding, and eviction of nucleosomes before and after the occurrence of meiotic DSBs. Here, we assess our understanding of meiotic DSB formation and methods to change its patterning. We also synthesize current heterogeneous knowledge on how histone modifications and chromatin remodeling may impact this decisive step in meiotic recombination.

Genome-wide comparison of Asian and African rice reveals high recent activity of DNA transposons

Background

DNA (Class II) transposons are ubiquitous in plant genomes. However, unlike for (Class I) retrotransposons, only little is known about their proliferation mechanisms, activity, and impact on genomes. Asian and African rice (Oryza sativa and O. glaberrima) diverged approximately 600,000 years ago. Their fully sequenced genomes therefore provide an excellent opportunity to study polymorphisms introduced from recent transposon activity.

Results

We manually analyzed 1,821 transposon related polymorphisms among which we identified 487 loci which clearly resulted from DNA transposon insertions and excisions. In total, we estimate about 4,000 (3.5% of all DNA transposons) to be polymorphic between the two species, indicating a high level of transposable element (TE) activity. The vast majority of the recently active elements are non-autonomous. Nevertheless, we identified multiple potentially functional autonomous elements. Furthermore, we quantified the impacts of insertions and excisions on the adjacent sequences. Transposon insertions were found to be generally precise, creating simple target site duplications. In contrast, excisions almost always go along with the deletion of flanking sequences and/or the insertion of foreign ‘filler’ segments. Some of the excision-triggered deletions ranged from hundreds to thousands of bp flanking the excision site. Furthermore, we found in some superfamilies unexpectedly low numbers of excisions. This suggests that some excisions might cause such large-scale rearrangements so that they cannot be detected anymore.

Conclusions

We conclude that the activity of DNA transposons (particularly the excision process) is a major evolutionary force driving the generation of genetic diversity.

Electronic supplementary material

The online version of this article (doi:10.1186/s13100-015-0040-x) contains supplementary material, which is available to authorized users.

Transposon-Assisted Genetic Engineering with Mos1-Mediated Single-Copy Insertion (MosSCI)

Transgenesis in model organisms is necessary to determine the function, expression, and subcellular localization of gene products. In Caenorhabditis elegans, injected DNA can be propagated as multicopy extrachromosomal arrays but transgenes in arrays are mosaic, over-expressed in some tissues and silenced in the germline. Here, a method to insert a transgene into a specific genomic location called Mos1-mediated single-copy insertion (MosSCI) is described. Single-copy insertion allows transgene expression at levels that approximate endogenous gene expression as well as expression in the germline.

Rapid and precise engineering of the Caenorhabditis elegans genome with lethal mutation co-conversion and inactivation of NHEJ repair

As in other organisms, CRISPR/Cas9 methods provide a powerful approach for genome editing in the nematode Caenorhabditis elegans. Oligonucleotides are excellent repair templates for introducing substitutions and short insertions, as they are cost effective, require no cloning, and appear in other organisms to target changes by homologous recombination at DNA double-strand breaks (DSBs). Here, I describe a methodology in C. elegans to efficiently knock in epitope tags in 8–9 days, using a temperature-sensitive lethal mutation in the pha-1 gene as a co-conversion marker. I demonstrate that 60mer oligos with 29 bp of homology drive efficient knock-in of point mutations, and that disabling nonhomologous end joining by RNAi inactivation of the cku-80 gene significantly improves knock-in efficiency. Homology arms of 35–80 bp are sufficient for efficient editing and DSBs up to 54 bp away from the insertion site produced knock-ins. These findings will likely be applicable for a range of genome editing approaches in C. elegans, which will improve editing efficiency and minimize screening efforts.

Functional genetics for all: engineered nucleases, CRISPR and the gene editing revolution

Developmental biology, as all experimental science, is empowered by technological advances. The availability of genetic tools in some species - designated as model organisms - has driven their use as major platforms for understanding development, physiology and behavior. Extending these tools to a wider range of species determines whether (and how) we can experimentally approach developmental diversity and evolution. During the last two decades, comparative developmental biology (evo-devo) was marked by the introduction of gene knockdown and deep sequencing technologies that are applicable to a wide range of species. These approaches allowed us to test the developmental role of specific genes in diverse species, to study biological processes that are not accessible in established models and, in some cases, to conduct genome-wide screens that overcome the limitations of the candidate gene approach. The recent discovery of CRISPR/Cas as a means of precise alterations into the genome promises to revolutionize developmental genetics. In this review we describe the development of gene editing tools, from zinc-finger nucleases to TALENs and CRISPR, and examine their application in gene targeting, their limitations and the opportunities they present for evo-devo. We outline their use in gene knock-out and knock-in approaches, and in manipulating gene functions by directing molecular effectors to specific sites in the genome. The ease-of-use and efficiency of CRISPR in diverse species provide an opportunity to close the technology gap that exists between established model organisms and emerging genetically-tractable species.

piggyBac: A vehicle for integrative DNA transformation of parasitic nematodes

In addition to their natural role in eukaryotic genome evolution, transposons can be powerful tools for functional genomics in diverse taxa. The piggyBac transposon has been applied as such in eukaryotic parasites, both protozoa and helminths, and in several important vector mosquitoes. piggyBac is advantageous for functional genomics because of its ability to transduce a wide range of taxa, its capacity to integrate large DNA ‘cargoes’ relative to other mobile genetic elements, its propensity to target transcriptional units and its ability to re-mobilize without leaving a pattern of non-excised sequences or ‘footprint’ in the genome. We recently demonstrated that piggyBac can integrate transgenes into the genome of the parasitic nematode Strongyloides ratti, an important model for parasitic nematode biology and a close relative of the significant human pathogen S. stercoralis. Unlike transgenes encoded in conventional plasmid vectors, which we assume are assembled into multi-copy episomal arrays as they are in Caenorhabditis elegans, transgenes integrated via piggyBac are not only stably inherited in S. ratti, they are also continuously expressed. This has allowed derivation of the first stable transgene expressing lines in any parasitic nematode, a significant advance in the development of functional genomic tools for these important pathogens.

An assay to monitor the activity of DNA transposition complexes yields a general quality control measure for transpositional recombination reactions

Transposon-based technologies have many applications in molecular biology and can be used for gene delivery into prokaryotic and eukaryotic cells. Common transpositional activity measurement assays suitable for many types of transposons would be beneficial, as diverse transposon systems could be compared for their performance attributes. Therefore, we developed a general-purpose assay to enable and standardize the activity measurement for DNA transposition complexes (transpososomes), using phage Mu transposition as a test platform. This assay quantifies transpositional recombination efficiency and is based on an in vitro transposition reaction with a target plasmid carrying a lethal ccdB gene. If transposition targets ccdB, this gene becomes inactivated, enabling plasmid-receiving Escherichia coli cells to survive and to be scored as colonies on selection plates. The assay was validated with 3 mini-Mu transposons varying in size and differing in their marker gene constitution. Tests with different amounts of transposon DNA provided a linear response and yielded a 10-fold operational range for the assay. The colony formation capacity was linearly correlated with the competence status of the E.coli cells, enabling normalization of experimental data obtained with different batches of recipient cells. The developed assay can now be used to directly compare transpososome activities with all types of mini-Mu transposons, regardless of their aimed use. Furthermore, the assay should be directly applicable to other transposition-based systems with a functional in vitro reaction, and it provides a dependable quality control measure that previously has been lacking but is highly important for the evaluation of current and emerging transposon-based applications.

Exciting prospects for precise engineering of Caenorhabditis elegans genomes with CRISPR/Cas9

With remarkable speed, the CRISPR-Cas9 nuclease has become the genome-editing tool of choice for essentially all genetically tractable organisms. Targeting specific DNA sequences is conceptually simple because the Cas9 nuclease can be guided by a single, short RNA (sgRNA) to introduce double-strand DNA breaks (DSBs) at precise locations. Here I contrast and highlight protocols recently developed by eight different research groups, six of which are published in GENETICS, to modify the Caenorhabditis elegans genome using CRISPR/Cas9. This reverse engineering tool levels the playing field for experimental geneticists.

Random and targeted transgene insertion in Caenorhabditis elegans using a modified Mos1 transposon

We have generated a recombinant Mos1 transposon that can insert up to 45-kb transgenes into the Caenorhabditis elegans genome. The minimal Mos1 transposon (miniMos) is 550 bp long and inserts DNA into the genome at high frequency (~60% of injected animals). Genetic and antibiotic markers can be used for selection, and the transposon is active in C. elegans isolates and Caenorhabditis briggsae. We used the miniMos transposon to generate six universal Mos1-mediated single-copy insertion (mosSCI) landing sites that allow targeted transgene insertion with a single targeting vector into permissive expression sites on all autosomes. We also generated two collections of strains: a set of bright fluorescent insertions that are useful as dominant, genetic balancers and a set of lacO insertions to track genome position.

Tools for gene-regulatory analyses in the marine annelid Platynereis dumerilii

The advent of high-throughput sequencing technology facilitates the exploration of a variety of reference species outside the few established molecular genetic model systems. Bioinformatic and gene expression analyses provide new ways for comparative analyses between species, for instance, in the field of evolution and development. Despite these advances, a critical bottleneck for the exploration of new model species remains the establishment of functional tools, such as the ability to experimentally express genes in specific cells of an organism. We recently established a first transgenic strain of the annelid Platynereis, using a Tc1/mariner-type Mos1 transposon vector. Here, we compare Mos1 with Tol2, a member of the hAT family of transposons. In Platynereis, Tol2-based constructs showed a higher frequency of nuclear genome insertion and sustained gene expression in the G0 generation. However, in contrast to Mos1-mediated transgenes, Tol2-mediated insertions failed to retain fluorescence in the G1 generation, suggesting a germ line-based silencing mechanism. Furthermore, we present three novel expression constructs that were generated by a simple fusion-PCR approach and allow either ubiquitous or cell-specific expression of a reporter gene. Our study indicates the versatility of Tol2 for transient transgenesis, and provides a template for transgenesis work in other emerging reference species.

Engineering the Caenorhabditis elegans genome with CRISPR/Cas9

The development in early 2013 of CRISPR/Cas9-based genome engineering promises to dramatically advance our ability to alter the genomes of model systems at will. A single, easily produced targeting RNA guides the Cas9 endonuclease to a specific DNA sequence where it creates a double strand break. Imprecise repair of the break can yield mutations, while homologous recombination with a repair template can be used to effect specific changes to the genome. The tremendous potential of this system led several groups to independently adapt it for use in Caenorhabditiselegans, where it was successfully used to generate mutations and to create tailored genome changes through homologous recombination. Here, we review the different approaches taken to adapt CRISPR/Cas9 for C. elegans, and provide practical guidelines for CRISPR/Cas9-based genome engineering.

Forward and reverse mutagenesis in C. elegans

Mutagenesis drives natural selection. In the lab, mutations allow gene function to be deciphered. C. elegans is highly amendable to functional genetics because of its short generation time, ease of use, and wealth of available gene-alteration techniques. Here we provide an overview of historical and contemporary methods for mutagenesis in C. elegans, and discuss principles and strategies for forward (genome-wide mutagenesis) and reverse (target-selected and gene-specific mutagenesis) genetic studies in this animal.

Is non-homologous end-joining really an inherently error-prone process?

DNA double-strand breaks (DSBs) are harmful lesions leading to genomic instability or diversity. Non-homologous end-joining (NHEJ) is a prominent DSB repair pathway, which has long been considered to be error-prone. However, recent data have pointed to the intrinsic precision of NHEJ. Three reasons can account for the apparent fallibility of NHEJ: 1) the existence of a highly error-prone alternative end-joining process; 2) the adaptability of canonical C-NHEJ (Ku- and Xrcc4/ligase IV-dependent) to imperfect complementary ends; and 3) the requirement to first process chemically incompatible DNA ends that cannot be ligated directly. Thus, C-NHEJ is conservative but adaptable, and the accuracy of the repair is dictated by the structure of the DNA ends rather than by the C-NHEJ machinery. We present data from different organisms that describe the conservative/versatile properties of C-NHEJ. The advantages of the adaptability/versatility of C-NHEJ are discussed for the development of the immune repertoire and the resistance to ionizing radiation, especially at low doses, and for targeted genome manipulation.

Microhomology-mediated intron loss during metazoan evolution

How introns are lost from eukaryotic genomes during evolution remains an enigmatic question in biology. By comparative genome analysis of five Caenorhabditis and eight Drosophila species, we found that the likelihood of intron loss is highly influenced by the degree of sequence homology at exon–intron junctions: a significant elevated degree of microhomology was observed for sequences immediately flanking those introns that were eliminated from the genome of one or more subspecies. This determinant was significant even at individual nucleotides. We propose that microhomology-mediated DNA repair underlies this phenomenon, which we termed microhomology-mediated intron loss. This hypothesis is further supported by the observations that in both species 1) smaller introns are preferentially lost over longer ones and 2) genes that are highly transcribed in germ cells, and are thus more prone to DNA double strand breaks, display elevated frequencies of intron loss. Our data also testify against a prominent role for reverse transcriptase-mediated intron loss in metazoans.

Efficient genome editing in Caenorhabditis elegans by CRISPR-targeted homologous recombination

Cas9 is an RNA-guided double-stranded DNA nuclease that participates in clustered regularly interspaced short palindromic repeats (CRISPR)-mediated adaptive immunity in prokaryotes. CRISPR-Cas9 has recently been used to generate insertion and deletion mutations in Caenorhabditis elegans, but not to create tailored changes (knock-ins). We show that the CRISPR-CRISPR-associated (Cas) system can be adapted for efficient and precise editing of the C. elegans genome. The targeted double-strand breaks generated by CRISPR are substrates for transgene-instructed gene conversion. This allows customized changes in the C. elegans genome by homologous recombination: sequences contained in the repair template (the transgene) are copied by gene conversion into the genome. The possibility to edit the C. elegans genome at selected locations will facilitate the systematic study of gene function in this widely used model organism.

Efficient and rapid C. elegans transgenesis by bombardment and hygromycin B selection

We report a simple, cost-effective, scalable and efficient method for creating transgenic Caenorhabditis elegans that requires minimal hands-on time. The method combines biolistic bombardment with selection for transgenics that bear a hygromycin B resistance gene on agar plates supplemented with hygromycin B, taking advantage of our observation that hygromycin B is sufficient to kill wild-type C. elegans at very low concentrations. Crucially, the method provides substantial improvements in the success of bombardments for isolating transmitting strains, the isolation of multiple independent strains, and the isolation of integrated strains: 100% of bombardments in a large data set yielded transgenics; 10 or more independent strains were isolated from 84% of bombardments, and up to 28 independent strains were isolated from a single bombardment; 82% of bombardments yielded stably transmitting integrated lines with most yielding multiple integrated lines. We anticipate that the selection will be widely adopted for C. elegans transgenesis via bombardment, and that hygromycin B resistance will be adopted as a marker in other approaches for manipulating, introducing or deleting DNA in C. elegans.

Engineering the Caenorhabditis elegans genome using Cas9-triggered homologous recombination

Study of the nematode Caenorhabditis elegans has provided important insights in a wide range of fields in biology. The ability to precisely modify genomes is critical to fully realize the utility of model organisms. Here, we report a method to edit the C. elegans genome using the Clustered Regularly Interspersed Short Palindromic Repeats (CRISPR) RNA-guided Cas9 nuclease followed by homologous recombination. We demonstrate that Cas9 is able to induce DNA double-strand breaks with specificity for targeted sites, and that these breaks can be efficiently repaired by homologous recombination. By supplying engineered homologous repair templates, we generated GFP knock-ins and targeted mutations. Together, our results outline a flexible methodology to produce essentially any desired modification in the C. elegans genome quickly and at low cost. This technology is an important addition to the array of genetic techniques already available in this experimentally tractable model organism.

Targeted heritable mutation and gene conversion by Cas9-CRISPR in Caenorhabditis elegans

We have achieved targeted heritable genome modification in Caenorhabditis elegans by injecting mRNA of the nuclease Cas9 and Cas9 guide RNAs. This system rapidly creates precise genomic changes, including knockouts and transgene-instructed gene conversion.

Transgene-free genome editing in Caenorhabditis elegans using CRISPR-Cas

CRISPR-Cas is an efficient method for genome editing in organisms from bacteria to human cells. We describe a transgene-free method for CRISPR-Cas-mediated cleavage in nematodes, enabling RNA-homology-targeted deletions that cause loss of gene function; analysis of whole-genome sequencing indicates that the nuclease activity is highly specific.

Heritable custom genomic modifications in Caenorhabditis elegans via a CRISPR-Cas9 system

We adapted the CRISPR-Cas9 system for template-mediated repair of targeted double-strand breaks via homologous recombination in Caenorhabditis elegans, enabling customized and efficient genome editing. This system can be used to create specific insertions, deletions, and base pair changes in the germline of C. elegans.

The million mutation project: a new approach to genetics in Caenorhabditis elegans

We have created a library of 2007 mutagenized Caenorhabditis elegans strains, each sequenced to a target depth of 15-fold coverage, to provide the research community with mutant alleles for each of the worm's more than 20,000 genes. The library contains over 800,000 unique single nucleotide variants (SNVs) with an average of eight nonsynonymous changes per gene and more than 16,000 insertion/deletion (indel) and copy number changes, providing an unprecedented genetic resource for this multicellular organism. To supplement this collection, we also sequenced 40 wild isolates, identifying more than 630,000 unique SNVs and 220,000 indels. Comparison of the two sets demonstrates that the mutant collection has a much richer array of both nonsense and missense mutations than the wild isolate set. We also find a wide range of rDNA and telomere repeat copy number in both sets. Scanning the mutant collection for molecular phenotypes reveals a nonsense suppressor as well as strains with higher levels of indels that harbor mutations in DNA repair genes and strains with abundant males associated with him mutations. All the strains are available through the Caenorhabditis Genetics Center and all the sequence changes have been deposited in WormBase and are available through an interactive website.

Biosynthesis of ionotropic acetylcholine receptors requires the evolutionarily conserved ER membrane complex

The number of nicotinic acetylcholine receptors (AChRs) present in the plasma membrane of muscle and neuronal cells is limited by the assembly of individual subunits into mature pentameric receptors. This process is usually inefficient, and a large number of the synthesized subunits are degraded by endoplasmic reticulum (ER)-associated degradation. To identify cellular factors required for the synthesis of AChRs, we performed a genetic screen in the nematode Caenorhabditis elegans for mutants with decreased sensitivity to the cholinergic agonist levamisole. We isolated a partial loss-of-function allele of ER membrane protein complex-6 (emc-6), a previously uncharacterized gene in C. elegans. emc-6 encodes an evolutionarily conserved 111-aa protein with two predicted transmembrane domains. EMC-6 is ubiquitously expressed and localizes to the ER. Partial inhibition of EMC-6 caused decreased expression of heteromeric levamisole-sensitive AChRs by destabilizing unassembled subunits in the ER. Inhibition of emc-6 also reduced the expression of homomeric nicotine-sensitive AChRs and GABAA receptors in C. elegans muscle cells. emc-6 is orthologous to the yeast and human EMC6 genes that code for a component of the recently identified ER membrane complex (EMC). Our data suggest this complex is required for protein folding and is connected to ER-associated degradation. We demonstrated that inactivation of additional EMC members in C. elegans also impaired AChR synthesis and induced the unfolded protein response. These results suggest that the EMC is a component of the ER folding machinery. AChRs might provide a valuable proxy to decipher the function of the EMC further.

Microscopic analysis of chromatin localization and dynamics in C. elegans

During development, the genome undergoes drastic reorganization within the nuclear space. To determine tridimensional genome folding, genome-wide techniques (damID/Hi-C) can be applied using cell populations, but these have to be calibrated using microscopy and single-cell analysis of gene positioning. Moreover, the dynamic behavior of chromatin has to be assessed on living samples. Combining fast stereotypic development with easy genetics and microscopy, the nematode C. elegans has become a model of choice in recent years to study changes in nuclear organization during cell fate acquisition. Here we present two complementary techniques to evaluate nuclear positioning of genes either by fluorescence in situ hybridization in fixed samples or in living worm embryos using the GFP-lacI/lacO chromatin-tagging system.

Dclk1 distinguishes between tumor and normal stem cells in the intestine

There is great interest in tumor stem cells (TSCs) as potential therapeutic targets; however, cancer therapies targeting TSCs are limited. A drawback is that TSC markers are often shared by normal stem cells (NSCs); thus, therapies that target these markers may cause severe injury to normal tissues. To identify a potential TSC-specific marker, we focused on doublecortin-like kinase 1 (Dclk1). Dclk1 was reported as a candidate NSC marker in the gut, but recent reports have implicated it as a marker of differentiated cells (for example, Tuft cells). Using lineage-tracing experiments, we show here that Dclk1 does not mark NSCs in the intestine but instead marks TSCs that continuously produce tumor progeny in the polyps of Apc(Min/+) mice. Specific ablation of Dclk1-positive TSCs resulted in a marked regression of polyps without apparent damage to the normal intestine. Our data suggest the potential for developing a therapy for colorectal cancer based on targeting Dclk1-positive TSCs.