Philipp H. Schiffer

959 Nematode Genomes: a semantic wiki for coordinating sequencing projects

Genome sequencing has been democratized by second-generation technologies, and even small labs can sequence metazoan genomes now. In this article, we describe ‘959 Nematode Genomes’—a community-curated semantic wiki to coordinate the sequencing efforts of individual labs to collectively sequence 959 genomes spanning the phylum Nematoda. The main goal of the wiki is to track sequencing projects that have been proposed, are in progress, or have been completed. Wiki pages for species and strains are linked to pages for people and organizations, using machine- and human-readable metadata that users can query to see the status of their favourite worm. The site is based on the same platform that runs Wikipedia, with semantic extensions that allow the underlying taxonomy and data storage models to be maintained and updated with ease compared with a conventional database-driven web site. The wiki also provides a way to track and share preliminary data if those data are not polished enough to be submitted to the official sequence repositories. In just over a year, this wiki has already fostered new international collaborations and attracted newcomers to the enthusiastic community of nematode genomicists. www.nematodegenomes.org.

Structure and evolutionary history of a large family of NLR proteins in the zebrafish.

Multicellular eukaryotes have evolved a range of mechanisms for immune recognition. A widespread family involved in innate immunity are the NACHT-domain and leucine-rich-repeat-containing (NLR) proteins. Mammals have small numbers of NLR proteins, whereas in some species, mostly those without adaptive immune systems, NLRs have expanded into very large families. We describe a family of nearly 400 NLR proteins encoded in the zebrafish genome. The proteins share a defining overall structure, which arose in fishes after a fusion of the core NLR domains with a B30.2 domain, but can be subdivided into four groups based on their NACHT domains. Gene conversion acting differentially on the NACHT and B30.2 domains has shaped the family and created the groups. Evidence of positive selection in the B30.2 domain indicates that this domain rather than the leucine-rich repeats acts as the pathogen recognition module. In an unusual chromosomal organization, the majority of the genes are located on one chromosome arm, interspersed with other large multigene families, including a new family encoding zinc-finger proteins. The NLR-B30.2 proteins represent a new family with diversity in the specific recognition module that is present in fishes in spite of the parallel existence of an adaptive immune system.

The genome of Romanomermis culicivorax: revealing fundamental changes in the core developmental genetic toolkit in Nematoda.

BackgroundThe genetics of development in the nematode Caenorhabditis elegans has been described in exquisite detail. The phylum Nematoda has two classes: Chromadorea (which includes C. elegans) and the Enoplea. While the development of many chromadorean species resembles closely that of C. elegans, enoplean nematodes show markedly different patterns of early cell division and cell fate assignment. Embryogenesis of the enoplean Romanomermis culicivorax has been studied in detail, but the genetic circuitry underpinning development in this species has not been explored.ResultsWe generated a draft genome for R. culicivorax and compared its gene content with that of C. elegans, a second enoplean, the vertebrate parasite Trichinella spiralis, and a representative arthropod, Tribolium castaneum. This comparison revealed that R. culicivorax has retained components of the conserved ecdysozoan developmental gene toolkit lost in C. elegans. T. spiralis has independently lost even more of this toolkit than has C. elegans. However, the C. elegans toolkit is not simply depauperate, as many novel genes essential for embryogenesis in C. elegans are not found in, or have only extremely divergent homologues in R. culicivorax and T. spiralis. Our data imply fundamental differences in the genetic programmes not only for early cell specification but also others such as vulva formation and sex determination.ConclusionsDespite the apparent morphological conservatism, major differences in the molecular logic of development have evolved within the phylum Nematoda. R. culicivorax serves as a tractable system to contrast C. elegans and understand how divergent genomic and thus regulatory backgrounds nevertheless generate a conserved phenotype. The R. culicivorax draft genome will promote use of this species as a research model.

Genome analysis of Diploscapter coronatus : insights into molecular peculiarities of a nematode with parthenogenetic reproduction

BackgroundSexual reproduction involving the fusion of egg and sperm is prevailing among eukaryotes. In contrast, the nematode Diploscapter coronatus, a close relative of the model Caenorhabditis elegans, reproduces parthenogenetically. Neither males nor sperm have been observed and some steps of meiosis are apparently skipped in this species. To uncover the genomic changes associated with the evolution of parthenogenesis in this nematode, we carried out a genome analysis.ResultsWe obtained a 170 Mbp draft genome in only 511 scaffolds with a N50 length of 1 Mbp. Nearly 90% of these scaffolds constitute homologous pairs with a 5.7% heterozygosity on average and inversions and translocations, meaning that the 170 Mbp sequences correspond to the diploid genome. Fluorescent staining shows that the D. coronatus genome consists of two chromosomes (2nxa0=xa02). In our genome annotation, we found orthologs of 59% of the C. elegans genes. However, a number of genes were missing or very divergent. These include genes involved in sex determination (e.g. xol-1, tra-2) and meiosis (e.g. the kleisins rec-8 and coh-3/4) giving a possible explanation for the absence of males and the second meiotic division. The high degree of heterozygosity allowed us to analyze the expression level of individual alleles. Most of the homologous pairs show very similar expression levels but others exhibit a 2–5-fold difference.ConclusionsOur high-quality draft genome of D. coronatus reveals the peculiarities of the genome of parthenogenesis and provides some clues to the genetic basis for parthenogenetic reproduction. This draft genome should be the basis to elucidate fundamental questions related to parthenogenesis such as its origin and mechanisms through comparative analyses with other nematodes. Furthermore, being the closest outgroup to the genus Caenorhabditis, the draft genome will help to disclose many idiosyncrasies of the model C. elegans and its congeners in future studies.

Signatures of the evolution of parthenogenesis and cryptobiosis in the genomes of panagrolaimid nematodes

Most animal species reproduce sexually, but parthenogenesis, asexual reproduction of various forms, has arisen repeatedly. Parthenogenetic lineages are usually short lived in evolution; though in some environments parthenogenesis may be advantageous, avoiding the cost of sex. Panagrolaimus nematodes have colonised environments ranging from arid deserts to arctic and antarctic biomes. Many are parthenogenetic, and most have cryptobiotic abilities, being able to survive repeated complete desiccation and freezing. It is not clear which genomic and molecular mechanisms led to the successful establishment of parthenogenesis and the evolution of cryptobiosis in animals in general. At the same time, model systems to study these traits in the laboratory are missing. We compared the genomes and transcriptomes of parthenogenetic and sexual Panagrolaimus able to survive crybtobiosis, as well as a non-cryptobiotic Propanogrolaimus species, to identify systems that contribute to these striking abilities. The parthenogens are most probably tripoids originating from hybridisation (allopolyploids). We identified genomic singularities like expansion of gene families, and selection on genes that could be linked to the adaptation to cryptobiosis. All Panagrolaimus have acquired genes through horizontal transfer, some of which are likely to contribute to cryptobiosis. Many genes acting in C. elegans reproduction and development were absent in distant nematode species (including the Panagrolaimids), suggesting molecular pathways cannot directly be transferred from the model system. The easily cultured Panagrolaimus nematodes offer a system to study developmental diversity in Nematoda, the molecular evolution of parthenogens, the effects of triploidy on genomes stability, and the origin and biology of cryptobiosis.

Ultra Large Gene Families: A Matter of Adaptation or Genomic Parasites?

Gene duplication is an important mechanism of molecular evolution. It offers a fast track to modification, diversification, redundancy or rescue of gene function. However, duplication may also be neutral or (slightly) deleterious, and often ends in pseudo-geneisation. Here, we investigate the phylogenetic distribution of ultra large gene families on long and short evolutionary time scales. In particular, we focus on a family of NACHT-domain and leucine-rich-repeat-containing (NLR)-genes, which we previously found in large numbers to occupy one chromosome arm of the zebrafish genome. We were interested to see whether such a tight clustering is characteristic for ultra large gene families. Our data reconfirm that most gene family inflations are lineage-specific, but we can only identify very few gene clusters. Based on our observations we hypothesise that, beyond a certain size threshold, ultra large gene families continue to proliferate in a mechanism we term “run-away evolution”. This process might ultimately lead to the failure of genomic integrity and drive species to extinction.

The mitochondrial genomes of the acoelomorph worms Paratomella rubra, Isodiametra pulchra and Archaphanostoma ylvae

Acoels are small, ubiquitous - but understudied - marine worms with a very simple body plan. Their internal phylogeny is still not fully resolved, and the position of their proposed phylum Xenacoelomorpha remains debated. Here we describe mitochondrial genome sequences from the acoels Paratomella rubra and Isodiametra pulchra, and the complete mitochondrial genome of the acoel Archaphanostoma ylvae. The P. rubra and A. ylvae sequences are typical for metazoans in size and gene content. The larger I. pulchraxa0 mitochondrial genome contains both ribosomal genes, 21 tRNAs, but only 11 protein-coding genes. We find evidence suggesting a duplicated sequence in the I. pulchra mitochondrial genome. The P. rubra, I. pulchra and A. ylvae mitochondria have a unique genome organisation in comparison to other metazoan mitochondrial genomes. We found a large degree of protein-coding gene and tRNA overlap with little non-coding sequence in the compact P. rubra genome. Conversely, the A. ylvae and I. pulchra genomes have many long non-coding sequences between genes, likely driving genome size expansion in the latter. Phylogenetic trees inferred from mitochondrial genes retrieve Xenacoelomorpha as an early branching taxon in the deuterostomes. Sequence divergence analysis between P. rubra sampled in England and Spain indicates cryptic diversity.

Evolutionary analysis indicates that DNA alkylation damage is a byproduct of cytosine DNA methyltransferase activity

Methylation at the 5 position of cytosine in DNA (5meC) is a key epigenetic mark in eukaryotes. Once introduced, 5meC can be maintained through DNA replication by the activity of ‘maintenance’ DNA methyltransferases (DNMTs). Despite their ancient origin, DNA methylation pathways differ widely across animals, such that 5meC is either confined to transcribed genes or lost altogether in several lineages. We used comparative epigenomics to investigate the evolution of DNA methylation. Although the model nematode Caenorhabditis elegans lacks DNA methylation, more basal nematodes retain cytosine DNA methylation, which is targeted to repeat loci. We found that DNA methylation coevolved with the DNA alkylation repair enzyme ALKB2 across eukaryotes. In addition, we found that DNMTs introduced the toxic lesion 3-methylcytosine into DNA both in vitro and in vivo. Alkylation damage is therefore intrinsically associated with DNMT activity, and this may promote the loss of DNA methylation in many species.The authors report that DNA methylation coevolves with the DNA alkylation repair enzyme ALKB2 across eukaryotes. They also show that DNA methyltransferases cause alkylation damage in vitro and in vivo by introducing 3-methylcytosine into DNA.

The cnidarian Hydractinia echinata employs canonical and highly adapted histones to pack its DNA

BackgroundCnidarians are a group of early branching animals including corals, jellyfish and hydroids that are renowned for their high regenerative ability, growth plasticity and longevity. Because cnidarian genomes are conventional in terms of protein-coding genes, their remarkable features are likely a consequence of epigenetic regulation. To facilitate epigenetics research in cnidarians, we analysed the histone complement of the cnidarian model organism Hydractinia echinata using phylogenomics, proteomics, transcriptomics and mRNA in situ hybridisations.ResultsWe find that the Hydractinia genome encodes 19 histones and analyse their spatial expression patterns, genomic loci and replication-dependency. Alongside core and other replication-independent histone variants, we find several histone replication-dependent variants, including a rare replication-dependent H3.3, a female germ cell-specific H2A.X and an unusual set of five H2B variants, four of which are male germ cell-specific. We further confirm the absence of protamines in Hydractinia.ConclusionsSince no protamines are found in hydroids, we suggest that the novel H2B variants are pivotal for sperm DNA packaging in this class of Cnidaria. This study adds to the limited number of full histone gene complements available in animals and sets a comprehensive framework for future studies on the role of histones and their post-translational modifications in cnidarian epigenetics. Finally, it provides insight into the evolution of spermatogenesis.

Functional studies on the role of Notch signaling in Hydractinia development

The function of Notch signaling was previously studied in two cnidarians, Hydra and Nematostella, representing the lineages Hydrozoa and Anthozoa, respectively. Using pharmacological inhibition in Hydra and a combination of pharmacological and genetic approaches in Nematostella, it was shown in both animals that Notch is required for tentacle morphogenesis and for late stages of stinging cell maturation. Surprisingly, a role for Notch in neural development, which is well documented in bilaterians, was evident in embryonic Nematostella but not in adult Hydra. Adult neurogenesis in the latter seemed to be unaffected by DAPT, a drug that inhibits Notch signaling. To address this apparent discrepancy, we studied the role of Notch in Hydractinia echinata, an additional hydrozoan, in all life stages. Using CRISPR-Cas9 mediated mutagenesis, transgenesis, and pharmacological interference we show that Notch is dispensable for Hydractinia normal neurogenesis in all life stages but is required for the maturation of stinging cells and for tentacle morphogenesis. Our results are consistent with a conserved role for Notch in morphogenesis and nematogenesis across Cnidaria, and a lineage-specific loss of Notch dependence in neurogenesis in hydrozoans.