Pestiferous slugs and their associated nematodes in agricultural fields, greenhouses, and nurseries in Alberta, Canada

Some slug species are considered a nuisance in agriculture and horticulture worldwide, causing economic losses to growers. Phasmarhabditis is a genus of bacteria-feeding nematodes that can parasitize slugs and snails and thus potentially serve as a biological control agent. Canada had no record of Phasmarhabditis until a survey conducted in 2019 reported a Canadian strain of Phasmarhabditis californica from a single Arion rufus slug. To build on this discovery, we surveyed three major agricultural sites, ten greenhouses, and nurseries in Alberta from June to September 2021 to collect pest slug species and investigate their associated nematodes, specifically P. californica. Slugs were collected from the field and returned to the laboratory to check for emerging nematodes on White traps. We collected 1331 slugs belonging to nine species, with Deroceras reticulatum being the most common. Only 45 (3.38%) slug samples were positive for nematodes, and the majority were identified to species level: Alloionema appendiculatum, Caenorhabditis briggsae, Caenorhabditis elegans, Panagrolaimus subelongatus, and Mesorhabditis spiculigera. We did not isolate P. californica from any of the slugs collected from these survey sites, which included the original site where P. californica was discovered. However, four D. reticulatum slugs retrieved from a residential garden sample were infected with P. californica. These findings suggest the possibility of a fragmented distribution of P. californica across Alberta. Future research should focus on extensively surveying agriculture and horticulture sites and residential gardens in different provinces across Canada.

First Report of the Gastropod-Killing Nematode, Phasmarhabditis californica, in Washington State, U.S.A

Phasmarhabditis californica, a commercially available biological control agent in England, Scotland, and Wales (Nemaslug 2.0 ®), was discovered for the first time in Washington State during 2022. Nematodes were recovered from the invasive gastropods Arion hortensis, Deroceras reticulatum, and Oxychilus sp. in garden centers in both Vancouver and Spokane. The 18S ribosomal RNA gene was used to identify the nematodes. This discovery builds upon previous reports of P. californica in California and Oregon and suggests that the species is widespread throughout the west coast of the U.S. Future research directions with P. californica are proposed.

Phylogenetic evidence for the invasion of a commercialized European Phasmarhabditis hermaphrodita lineage into North America and New Zealand

Biological control (biocontrol) as a component of pest management strategies reduces reliance on synthetic chemicals, and seemingly offers a natural approach that minimizes environmental impact. However, introducing a new organism to new environments as a classical biocontrol agent can have broad and unanticipated biodiversity effects and conservation consequences. Nematodes are currently used in a variety of commercial biocontrol applications, including the use of Phasmarhabditis hermaphrodita as an agent targeting pest slug and snail species. This species was originally discovered in Germany, and is generally thought to have European origins. P. hermaphrodita is sold under the trade name Nemaslug®, and is available only in European markets. However, this nematode species was discovered in New Zealand and the western United States, though its specific origins remained unclear. In this study, we analyzed 45 nematode strains representing eight different Phasmarhabditis species, collected from nine countries around the world. A segment of nematode mitochondrial DNA (mtDNA) was sequenced and subjected to phylogenetic analyses. Our mtDNA phylogenies were overall consistent with previous analyses based on nuclear ribosomal RNA (rRNA) loci. The recently discovered P. hermaphrodita strains in New Zealand and the United States had mtDNA haplotypes nearly identical to that of Nemaslug®, and these were placed together in an intraspecific monophyletic clade with high support in maximum likelihood and Bayesian analyses. We also examined bacteria that co-cultured with the nematode strains isolated in Oregon, USA, by analyzing 16S rRNA sequences. Eight different bacterial genera were found to associate with these nematodes, though Moraxella osloensis, the bacteria species used in the Nemaslug® formulation, was not detected. This study provided evidence that nematodes deriving from the Nemaslug® biocontrol product have invaded countries where its use is prohibited by regulatory agencies and not commercially available.

Mitochondrial DNA Variation and Selfish Propagation Following Experimental Bottlenecking in Two Distantly Related Caenorhabditis briggsae Isolates

Understanding mitochondrial DNA (mtDNA) evolution and inheritance has broad implications for animal speciation and human disease models. However, few natural models exist that can simultaneously represent mtDNA transmission bias, mutation, and copy number variation. Certain isolates of the nematode Caenorhabditis briggsae harbor large, naturally-occurring mtDNA deletions of several hundred basepairs affecting the NADH dehydrogenase subunit 5 (nduo-5) gene that can be functionally detrimental. These deletion variants can behave as selfish DNA elements under genetic drift conditions, but whether all of these large deletion variants are transmitted in the same preferential manner remains unclear. In addition, the degree to which transgenerational mtDNA evolution profiles are shared between isolates that differ in their propensity to accumulate the nduo-5 deletion is also unclear. We address these knowledge gaps by experimentally bottlenecking two isolates of C. briggsae with different nduo-5 deletion frequencies for up to 50 generations and performing total DNA sequencing to identify mtDNA variation. We observed multiple mutation profile differences and similarities between C. briggsae isolates, a potentially species-specific pattern of copy number dysregulation, and some evidence for genetic hitchhiking in the deletion-bearing isolate. Our results further support C. briggsae as a practical model for characterizing naturally-occurring mtgenome variation and contribute to the understanding of how mtgenome variation persists in animal populations and how it presents in mitochondrial disease states.

First report of molecular characterization and phylogeny of Trichuris fossor Hall, 1916 (Nematoda: Trichuridae)

Because species of Trichuris are morphologically similar and ranges of host preference are variable, using molecular data to evaluate species delineations is essential for properly quantifying biodiversity of and relationships within Trichuridae. Trichuris fossor has been reported from Thomomys spp. (Rodentia: Geomyidae, ‘pocket gophers’) hosts based on morphological features alone. Partial 18S rRNA sequences for specimens identified as T. fossor based on morphology, along with sequences from 26 additional taxa, were used for a phylogenetic analysis. Evolutionary histories were constructed using maximum likelihood and Bayesian inference. In both analyses, the specimens fell within the Trichuris clade with 100% support and formed a distinct subclade with 100% support. These results confirm that T. fossor is a distinct species and represent the first molecular report for it. Relatedness among species within the family were well resolved in the BI tree. This study represents an initial effort to obtain a more comprehensive view of Trichuridae by including a new clade member, T. fossor. A better understanding of Trichuridae phylogeny could contribute to further characterization of host-associations, including species that infect livestock and humans.

Complex Transmission Patterns and Age-Related Dynamics of a Selfish mtDNA Deletion

Despite wide-ranging implications of selfish mitochondrial DNA (mtDNA) elements for human disease and topics in evolutionary biology (e.g., speciation), the forces controlling their formation, age-related accumulation, and offspring transmission remain largely unknown. Selfish mtDNA poses a significant challenge to genome integrity, mitochondrial function, and organismal fitness. For instance, numerous human diseases are associated with mtDNA mutations; however, few genetic systems can simultaneously represent pathogenic mitochondrial genome evolution and inheritance. The nematode Caenorhabditis briggsae is one such system. Natural C. briggsae isolates harbor varying levels of a large-scale deletion affecting the mitochondrial nduo-5 gene, termed nad5Δ. A subset of these isolates contains putative compensatory mutations that may reduce the risk of deletion formation. We studied the dynamics of nad5Δ heteroplasmy levels during animal development and transmission from mothers to offspring in genetically diverse C. briggsae natural isolates. Results support previous work demonstrating that nad5Δ is a selfish element and that heteroplasmy levels of this deletion can be quite plastic, exhibiting high degrees of inter-family variability and divergence between generations. The latter is consistent with a mitochondrial bottleneck effect, and contrasts with previous findings from a laboratory-derived model uaDf5 mtDNA deletion in C. elegans. However, we also found evidence for among-isolate differences in the ability to limit nad5Δ accumulation, the pattern of which suggested that forces other than the compensatory mutations are important in protecting individuals and populations from rampant mtDNA deletion expansion over short time scales.

Genome Announcement: The Draft Genomes of Two Radopholus similis populations from Costa Rica

Radopholus similis is an economically important pest of both banana and citrus in tropical regions. Here we present draft genomes from two populations of R. similis from Costa Rica that were created and assembled using short read libraries from Illumina HiSeq technology.

Variable Abundance and Distribution of Wolbachia and Cardinium Endosymbionts in Plant-Parasitic Nematode Field Populations

The bacterial endosymbiont Wolbachia interacts with different invertebrate hosts, engaging in diverse symbiotic relationships. Wolbachia is often a reproductive parasite in arthropods, but an obligate mutualist in filarial nematodes. Wolbachia was recently discovered in plant-parasitic nematodes, and, is thus far known in just two genera Pratylenchus and Radopholus, yet the symbiont's function remains unknown. The occurrence of Wolbachia in these economically important plant pests offers an unexplored biocontrol strategy. However, development of Wolbachia-based biocontrol requires an improved understanding of symbiont-host functional interactions and the symbiont's prevalence among nematode field populations. This study used a molecular-genetic approach to assess the prevalence of a Wolbachia lineage (wPpe) in 32 field populations of Pratylenchus penetrans. Populations were examined from eight different plant species in Washington, Oregon, and California. Nematodes were also screened for the endosymbiotic bacterium Cardinium (cPpe) that was recently shown to co-infect P. penetrans. Results identified wPpe in 9/32 and cPpe in 1/32 of P. penetrans field populations analyzed. No co-infection was observed in field populations. Wolbachia was detected in nematodes from 4/8 plant-hosts examined (raspberry, strawberry, clover, and lily), and in all three states surveyed. Cardinium was detected in nematodes from mint in Washington. In the wPpe-infected P. penetrans populations collected from raspberry, the prevalence of wPpe infection ranged from 11 to 58%. This pattern is unlike that in filarial nematodes where Wolbachia is an obligate mutualist and occurs in 100% of the host. Further analysis of wPpe-infected populations revealed female-skewed sex ratios (up to 96%), with the degree of skew positively correlating with wPpe prevalence. Uninfected nematode populations had approximately equal numbers of males and females. Comparisons of 54 wPpe 16S ribosomal RNA sequences revealed high similarity across the geographic isolates, with 45 of 54 isolates being identical at this locus. The complete absence of wPpe among some populations and low prevalence in others suggest that this endosymbiont is not an obligate mutualist of P. penetrans. The observed sex ratio bias in wPpe-infected nematode populations is similar to that observed in arthropods where Wolbachia acts as a reproductive manipulator, raising the question of a similar role in plant-parasitic nematodes.

Analysis of nematode-endosymbiont coevolution in the Xiphinema americanum species complex using molecular markers of variable evolutionary rates

Bacterial symbioses play important roles in shaping diverse biological processes in nematodes, and serve as targets in nematode biocontrol strategies. Focusing on the Xiphinema americanum species complex, we expanded upon recent research investigating patterns of coevolution between Xiphinema spp. and Xiphinematobacter spp., utilising two symbiont genetic markers of varying evolutionary rates. Phylogenetic analysis of nematode mitochondrial DNA (mtDNA) revealed five strongly supported major clades. Analysis of slow-evolving 16S rDNA in bacterial symbionts resulted in a phylogenetic topology composed of four major clades that grouped taxa highly congruent with the nematode mtDNA topology. A faster evolving protein-coding symbiont gene (nad) provided more phylogenetic resolution with seven well-supported clades, also congruent with the nematode mtDNA tree topology. Our results reinforce recent studies suggesting extensive coevolution between Xiphinema spp. and their vertically transmitted endosymbionts Xiphinematobacter spp. and illustrate the advantages of including genetic markers of varying evolutionary rates in coevolutionary and phylogenetic studies.

Adaptive Evolution under Extreme Genetic Drift in Oxidatively Stressed Caenorhabditis elegans

A mutation-accumulation (MA) experiment with Caenorhabditis elegans nematodes was conducted in which replicate, independently evolving lines were initiated from a low-fitness mitochondrial electron transport chain mutant, gas-1. The original intent of the study was to assess the effect of electron transport chain dysfunction involving elevated reactive oxygen species production on patterns of spontaneous germline mutation. In contrast to results of standard MA experiments, gas-1 MA lines evolved slightly higher mean fitness alongside reduced among-line genetic variance compared with their ancestor. Likewise, the gas-1 MA lines experienced partial recovery to wildtype reactive oxygen species levels. Whole-genome sequencing and analysis revealed that the molecular spectrum but not the overall rate of nuclear DNA mutation differed from wildtype patterns. Further analysis revealed an enrichment of mutations in loci that occur in a gas-1-centric region of the C. elegans interactome, and could be classified into a small number of functional-genomic categories. Characterization of a backcrossed four-mutation set isolated from one gas-1 MA line revealed this combination to be beneficial on both gas-1 mutant and wildtype genetic backgrounds. Our combined results suggest that selection favoring beneficial mutations can be powerful even under unfavorable population genetic conditions, and agree with fitness landscape theory predicting an inverse relationship between population fitness and the likelihood of adaptation.

Comparative Genomics of Wolbachia-Cardinium Dual Endosymbiosis in a Plant-Parasitic Nematode

Wolbachia and Cardinium are among the most important and widespread of all endosymbionts, occurring in nematodes and more than half of insect and arachnid species, sometimes as coinfections. These symbionts are of significant interest as potential biocontrol agents due to their abilities to cause major effects on host biology and reproduction through cytoplasmic incompatibility, sex ratio distortion, or obligate mutualism. The ecological and metabolic effects of coinfections are not well understood. This study examined a Wolbachia-Cardinium coinfection in the plant-parasitic nematode (PPN), Pratylenchus penetrans, producing the first detailed study of such a coinfection using fluorescence in situ hybridization (FISH), polymerase chain reaction (PCR), and comparative genomic analysis. Results from FISH and single-nematode PCR showed 123/127 individuals in a focal population carried Cardinium (denoted strain cPpe), and 48% were coinfected with Wolbachia strain wPpe. Both endosymbionts showed dispersed tissue distribution with highest densities in the anterior intestinal walls and gonads. Phylogenomic analyses confirmed an early place of cPpe and long distance from a sister strain in another PPN, Heterodera glycines, supporting a long history of both Cardinium and Wolbachia in PPNs. The genome of cPpe was 1.36 Mbp with 35.8% GC content, 1,131 predicted genes, 41% having no known function, and missing biotin and lipoate synthetic capacity and a plasmid present in other strains, despite having a slightly larger genome compared to other sequenced Cardinium. The larger genome revealed expansions of gene families likely involved in host-cellular interactions. More than 2% of the genes of cPpe and wPpe were identified as candidate horizontally transferred genes, with some of these from eukaryotes, including nematodes. A model of the possible Wolbachia-Cardinium interaction is proposed with possible complementation in function for pathways such as methionine and fatty acid biosynthesis and biotin transport.

Comparative Genomic Analysis of 130 Bacteriophages Infecting Bacteria in the Genus Pseudomonas

Bacteria of the genus Pseudomonas are genetically diverse and ubiquitous in the environment. Like other bacteria, those of the genus Pseudomonas are susceptible to bacteriophages which can significantly affect their host in many ways, ranging from cell lysis to major changes in morphology and virulence. Insights into phage genomes, evolution, and functional relationships with their hosts have the potential to contribute to a broader understanding of Pseudomonas biology, and the development of novel phage therapy strategies. Here we provide a broad-based comparative and evolutionary analysis of 130 complete Pseudomonas phage genome sequences available in online databases. We discovered extensive variation in genome size (ranging from 3 to 316 kb), G + C percentage (ranging from 37 to 66%), and overall gene content (ranging from 81–96% of genome space). Based on overall nucleotide similarity and the numbers of shared gene products, 100 out of 130 genome sequences were grouped into 12 different clusters; 30 were characterized as singletons, which do not have close relationships with other phage genomes. For 5/12 clusters, constituent phage members originated from two or more different Pseudomonas host species, suggesting that phage in these clusters can traverse bacterial species boundaries. An analysis of CRISPR spacers in Pseudomonas bacterial genome sequences supported this finding. Substantial diversity was revealed in analyses of phage gene families; out of 4,462 total families, the largest had only 39 members and there were 2,992 families with only one member. An evolutionary analysis of 72 phage gene families, based on patterns of nucleotide diversity at non-synonymous and synonymous sites, revealed strong and consistent signals for purifying selection. Our study revealed highly diverse and dynamic Pseudomonas phage genomes, and evidence for a dominant role of purifying selection in shaping the evolution of genes encoded in them.

Molecular signatures of host specificity linked to habitat specialization in Exaiptasia sea anemones

Rising ocean temperatures associated with global climate change induce breakdown of the symbiosis between coelenterates and photosynthetic microalgae of the genus Symbiodinium. Association with more thermotolerant partners could contribute to resilience, but the genetic mechanisms controlling specificity of hosts for particular Symbiodinium types are poorly known. Here, we characterize wild populations of a sea anemone laboratory model system for anthozoan symbiosis, from contrasting environments in Caribbean Panama. Patterns of anemone abundance and symbiont diversity were consistent with specialization of holobionts for particular habitats, with Exaiptasia pallida/S. minutum (ITS2 type B1) abundant on vertical substrate in thermally stable, shaded environments but E. brasiliensis/Symbiodinium sp. (ITS2 clade A) more common in shallow areas subject to high temperature and irradiance. Population genomic sequencing revealed a novel E. pallida population from the Bocas del Toro Archipelago that only harbors S. minutum. Loci most strongly associated with divergence of the Bocas-specific population were enriched in genes with putative roles in cnidarian symbiosis, including activators of the complement pathway of the innate immune system, thrombospondin-type-1 repeat domain proteins, and coordinators of endocytic recycling. Our findings underscore the importance of unmasking cryptic diversity in natural populations and the role of long-term evolutionary history in mediating interactions with Symbiodinium.

Natural Variation in Responses to Acute Heat and Cold Stress in a Sea Anemone Model System for Coral Bleaching

Rising ocean temperatures disrupt the symbiosis between corals and their microalgae, accelerating global decline of coral reef ecosystems. Because of the difficulty of performing laboratory experiments with corals, the sea anemone Aiptasia has emerged as an important model system for molecular studies of coral bleaching and symbiosis. Here, we investigate natural variation in bleaching responses among different genetic lineages of Aiptasia. Both heat- and cold-induced paths to symbiosis breakdown were analyzed. Significant genetic variation in response to acute heat stress was observed, with severe bleaching of two Aiptasia strains from Hawaii but minimal bleaching of strains from the U.S. South Atlantic, including the strain used to generate the Aiptasia reference genome. Both strains from Hawaii hosted Symbiodinium type B1, whereas strains from the U.S. South Atlantic hosted type A4 or B2. In contrast to the results from exposures to acute heat stress, negligible variation was observed in response to a pulsed cold shock despite moderate bleaching across all strains. These results support our hypothesis that bleaching responses to distinct stressors are independent. Our findings emphasize the role of stress regime when predicting adaptive responses of symbiotic cnidarians to changing climates, because genetic variation may exist for some forms of stress-induced bleaching but not others.

Transcription through the eye of a needle: daily and annual cyclic gene expression variation in Douglas-fir needles

BACKGROUND

Perennial growth in plants is the product of interdependent cycles of daily and annual stimuli that induce cycles of growth and dormancy. In conifers, needles are the key perennial organ that integrates daily and seasonal signals from light, temperature, and water availability. To understand the relationship between seasonal cycles and seasonal gene expression responses in conifers, we examined diurnal and circannual needle mRNA accumulation in Douglas-fir (Pseudotsuga menziesii) needles at diurnal and circannual scales. Using mRNA sequencing, we sampled 6.1 × 10⁹ reads from 19 trees and constructed a de novo pan-transcriptome reference that includes 173,882 tree-derived transcripts. Using this reference, we mapped RNA-Seq reads from 179 samples that capture daily and annual variation.

RESULTS

We identified 12,042 diurnally-cyclic transcripts, 9299 of which showed homology to annotated genes from other plant genomes, including angiosperm core clock genes. Annual analysis revealed 21,225 circannual transcripts, 17,335 of which showed homology to annotated genes from other plant genomes. The timing of maximum gene expression is associated with light intensity at diurnal scales and photoperiod at annual scales, with approximately half of transcripts reaching maximum expression +/- 2 h from sunrise and sunset, and +/- 20 days from winter and summer solstices. Comparisons with published studies from other conifers shows congruent behavior in clock genes with Japanese cedar (Cryptomeria), and a significant preservation of gene expression patterns for 2278 putative orthologs from Douglas-fir during the summer growing season, and 760 putative orthologs from spruce (Picea) during the transition from fall to winter.

CONCLUSIONS

Our study highlight the extensive diurnal and circannual transcriptome variability demonstrated in conifer needles. At these temporal scales, 29% of expressed transcripts show a significant diurnal cycle, and 58.7% show a significant circannual cycle. Remarkably, thousands of genes reach their annual peak activity during winter dormancy. Our study establishes the fine-scale timing of daily and annual maximum gene expression for diverse needle genes in Douglas-fir, and it highlights the potential for using this information for evaluating hypotheses concerning the daily or seasonal timing of gene activity in temperate-zone conifers, and for identifying cyclic transcriptome components in other conifer species.

The Draft Genome of Globodera ellingtonae

Globodera ellingtonae is a newly described potato cyst nematode (PCN) found in Idaho, Oregon, and Argentina. Here, we present a genome assembly for G. ellingtonae, a relative of the quarantine nematodes G. pallida and G. rostochiensis, produced using data from Illumina and Pacific Biosciences DNA sequencing technologies.

Genomic evidence for plant-parasitic nematodes as the earliest Wolbachia hosts

Wolbachia, one of the most widespread endosymbionts, is a target for biological control of mosquito-borne diseases (malaria and dengue virus), and antibiotic elimination of infectious filarial nematodes. We sequenced and analyzed the genome of a new Wolbachia strain (wPpe) in the plant-parasitic nematode Pratylenchus penetrans. Phylogenomic analyses placed wPpe as the earliest diverging Wolbachia, suggesting two evolutionary invasions into nematodes. The next branches comprised strains in sap-feeding insects, suggesting Wolbachia may have first evolved as a nutritional mutualist. Genome size, protein content, %GC, and repetitive DNA allied wPpe with mutualistic Wolbachia, whereas gene repertoire analyses placed it between parasite (A, B) and mutualist (C, D, F) groups. Conservation of iron metabolism genes across Wolbachia suggests iron homeostasis as a potential factor in its success. This study enhances our understanding of this globally pandemic endosymbiont, highlighting genetic patterns associated with host changes. Combined with future work on this strain, these genomic data could help provide potential new targets for plant-parasitic nematode control.

Paternal Mitochondrial Transmission in Intra-Species Caenorhabditis briggsae Hybrids

To study mitochondrial–nuclear genetic interactions in the nematode Caenorhabditis briggsae, our three laboratories independently created 38 intra-species cytoplasmic–nuclear hybrid (cybrid) lines. Although the cross design combines maternal mitotypes with paternal nuclear genotypes, eight lines (21%) unexpectedly contained paternal mitotypes. All eight share in common ancestry of one of two genetically related strains. This unexpected parallel observation of paternal mitochondrial transmission, undesirable given our intent of creating cybrids, provides a serendipitous experimental model and framework to study the molecular and evolutionary basis of uniparental mitochondrial inheritance.

The mitochondrial genome of Globodera ellingtonae is composed of two circles with segregated gene content and differential copy numbers

Background

The evolution of animal mitochondrial (mt) genomes has resulted in a highly conserved structure: a single compact circular chromosome approximately 14 to 20 kb long. Within the last two decades exceptions to this conserved structure, such as the division of the genome into multiple chromosomes, have been reported in a diverse set of metazoans. We report on the two circle multipartite mt genome of a newly described cyst nematode, Globodera ellingtonae.

Results

The G. ellingtonae mt genome was found to be comprised of two circles, each larger than any other multipartite circular mt chromosome yet reported, and both were larger than the single mt circle of the model nematode Caenorhabditis elegans. The genetic content of the genome was disproportionately divided between the two circles, although they shared a ~6.5 kb non-coding region. The 17.8 kb circle (mtDNA-I) contained ten protein-coding genes and two tRNA genes, whereas the 14.4 kb circle (mtDNA-II) contained two protein-coding genes, 20 tRNA genes and both rRNA genes. Perhaps correlated with this division of genetic content, the copy number of mtDNA-II was more than four-fold that of mtDNA-I in individual nematodes. The difference in copy number increased between second-stage and fourth-stage juveniles.

Conclusions

The segregation of gene types to different mt circles in G. ellingtonae could provide benefit by localizing gene functional types to independent transcriptional units. This is the first report of both two-circle and several-circle mt genomes within a single genus. The differential copy number associated with this multipartite mt organization could provide a model system for deconstructing mechanisms regulating mtDNA copy number both in somatic cells and during germline development.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3047-x) contains supplementary material, which is available to authorized users.

Paths of Heritable Mitochondrial DNA Mutation and Heteroplasmy in Reference and gas-1 Strains of Caenorhabditis elegans

Heteroplasmy—the presence of more than one mitochondrial DNA (mtDNA) sequence type in a cell, tissue, or individual—impacts human mitochondrial disease and numerous aging-related syndromes. Understanding the trans-generational dynamics of mtDNA is critical to understanding the underlying mechanisms of mitochondrial disease and evolution. We investigated mtDNA mutation and heteroplasmy using a set of wild-type (N2 strain) and mitochondrial electron transport chain (ETC) mutant (gas-1) mutant Caenorhabditis elegans mutation-accumulation (MA) lines. The N2 MA lines, derived from a previous experiment, were bottlenecked for 250 generations. The gas-1 MA lines were created for this study, and bottlenecked in the laboratory for up to 50 generations. We applied Illumina-MiSeq DNA sequencing to L1 larvae from five gas-1 MA lines and five N2 MA lines to detect and characterize mtDNA mutation and heteroplasmic inheritance patterns evolving under extreme drift. mtDNA copy number increased in both sets of MA lines: three-fold on average among the gas-1 MA lines and five-fold on average among N2 MA lines. Eight heteroplasmic single base substitution polymorphisms were detected in the gas-1 MA lines; only one was observed in the N2 MA lines. Heteroplasmy frequencies ranged broadly in the gas-1 MA lines, from as low as 2.3% to complete fixation (homoplasmy). An initially low-frequency (<5%) heteroplasmy discovered in the gas-1 progenitor was observed to fix in one gas-1 MA line, achieve higher frequency (37.4%) in another, and be lost in the other three lines. A similar low-frequency heteroplasmy was detected in the N2 progenitor, but was lost in all five N2 MA lines. We identified three insertion-deletion (indel) heteroplasmies in gas-1 MA lines and six indel variants in the N2 MA lines, most occurring at homopolymeric nucleotide runs. The observed bias toward accumulation of single nucleotide polymorphisms in gas-1 MA lines is consistent with the idea that impaired mitochondrial activity renders mtDNA more vulnerable to this type of mutation. The consistent increases in mtDNA copy number implies that extreme genetic drift provides a permissive environment for elevated organelle genome copy number in C. elegans reference and gas-1 strains. This study broadens our understanding of the heteroplasmic mitochondrial mutation process in a multicellular model organism.

The genomic basis of parasitism in the Strongyloides clade of nematodes

Soil-transmitted nematodes, including the Strongyloides genus, cause one of the most prevalent neglected tropical diseases. Here we compare the genomes of four Strongyloides species, including the human pathogen Strongyloides stercoralis, and their close relatives that are facultatively parasitic (Parastrongyloides trichosuri) and free-living (Rhabditophanes sp. KR3021). A significant paralogous expansion of key gene families--families encoding astacin-like and SCP/TAPS proteins--is associated with the evolution of parasitism in this clade. Exploiting the unique Strongyloides life cycle, we compare the transcriptomes of the parasitic and free-living stages and find that these same gene families are upregulated in the parasitic stages, underscoring their role in nematode parasitism.

Genome-wide polymorphism and signatures of selection in the symbiotic sea anemone Aiptasia

Background

Coral reef ecosystems are declining in response to global climate change and anthropogenic impacts. Yet patterns of standing genetic variation within cnidarian species, a major determinant of adaptive potential, are virtually unknown at genome-scale resolution. We explore patterns of genome-wide polymorphism and identify candidate loci under selection in the sea anemone Aiptasia, an important laboratory model system for studying the symbiosis between corals and dinoflagellate algae of the genus Symbiodinium.

Results

Low coverage genome sequencing revealed large genetic distances among globally widespread lineages, novel candidate targets of selection, and considerably higher heterozygosity than previously reported for Aiptasia. More than 670,000 single nucleotide polymorphisms were identified among 10 Aiptasia individuals including two pairs of genetic clones. Evolutionary relationships based on genome-wide polymorphism supported the current paradigm of a genetically distinct population from the US South Atlantic that harbors diverse Symbiodinium clades. However, anemones from the US South Atlantic demonstrated a striking lack of shared derived polymorphism. Heterozygosity was an important feature shaping nucleotide diversity patterns: at any given SNP site, more than a third of individuals genotyped were heterozygotes, and heterozygosity within individual genomes ranged from 0.37–0.58 %. Analysis of nonsynonymous and synonymous sites suggested that highly heterozygous regions are evolving under relaxed purifying selection compared to the rest of the Aiptasia genome. Genes previously identified as having elevated evolutionary rates in Aiptasia compared to other cnidarians were found in our study to be under strong purifying selection within Aiptasia. Candidate targets of selection, including lectins and genes involved in Rho GTPase signalling, were identified based on unusual signatures of nucleotide diversity, Tajima’s D, and heterozygosity compared to genome-wide averages.

Conclusions

This study represents the first genome-wide analysis of Tajima’s D in a cnidarian. Our results shed light on patterns of intraspecific genome-wide polymorphism in a model for studies of coral-algae symbiosis and present genetic targets for future research on evolutionary and cellular processes in early-diverging metazoans.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2488-6) contains supplementary material, which is available to authorized users.

Comparative Genomics of a Plant-Parasitic Nematode Endosymbiont Suggest a Role in Nutritional Symbiosis

Bacterial mutualists can modulate the biochemical capacity of animals. Highly coevolved nutritional mutualists do this by synthesizing nutrients missing from the host’s diet. Genomics tools have advanced the study of these partnerships. Here we examined the endosymbiont Xiphinematobacter (phylum Verrucomicrobia) from the dagger nematode Xiphinema americanum, a migratory ectoparasite of numerous crops that also vectors nepovirus. Previously, this endosymbiont was identified in the gut, ovaries, and eggs, but its role was unknown. We explored the potential role of this symbiont using fluorescence in situ hybridization, genome sequencing, and comparative functional genomics. We report the first genome of an intracellular Verrucomicrobium and the first exclusively intracellular non-Wolbachia nematode symbiont. Results revealed that Xiphinematobacter had a small 0.916-Mb genome with only 817 predicted proteins, resembling genomes of other mutualist endosymbionts. Compared with free-living relatives, conserved proteins were shorter on average, and there was large-scale loss of regulatory pathways. Despite massive gene loss, more genes were retained for biosynthesis of amino acids predicted to be essential to the host. Gene ontology enrichment tests showed enrichment for biosynthesis of arginine, histidine, and aromatic amino acids, as well as thiamine and coenzyme A, diverging from the profiles of relatives Akkermansia muciniphilia (in the human colon), Methylacidiphilum infernorum, and the mutualist Wolbachia from filarial nematodes. Together, these features and the location in the gut suggest that Xiphinematobacter functions as a nutritional mutualist, supplementing essential nutrients that are depleted in the nematode diet. This pattern points to evolutionary convergence with endosymbionts found in sap-feeding insects.

Selfish Mitochondrial DNA Proliferates and Diversifies in Small, but not Large, Experimental Populations of Caenorhabditis briggsae

Evolutionary interactions across levels of biological organization contribute to a variety of fundamental processes including genome evolution, reproductive mode transitions, species diversification, and extinction. Evolutionary theory predicts that so-called “selfish” genetic elements will proliferate when the host effective population size (N_e) is small, but direct tests of this prediction remain few. We analyzed the evolutionary dynamics of deletion-containing mitochondrial DNA (ΔmtDNA) molecules, previously characterized as selfish elements, in six different natural strains of the nematode Caenorhabditis briggsae allowed to undergo experimental evolution in a range of population sizes (N = 1, 10, 100, and 1,000) for a maximum of 50 generations. Mitochondrial DNA (mtDNA) was analyzed for replicate lineages at each five-generation time point. Ten different ΔmtDNA molecule types were observed and characterized across generations in the experimental populations. Consistent with predictions from evolutionary theory, lab lines evolved in small-population sizes (e.g., nematode N = 1) were more susceptible to accumulation of high levels of preexisting ΔmtDNA compared with those evolved in larger populations. New ΔmtDNA elements were observed to increase in frequency and persist across time points, but almost exclusively at small population sizes. In some cases, ΔmtDNA levels decreased across generations when population size was large (nematode N = 1,000). Different natural strains of C. briggsae varied in their susceptibilities to ΔmtDNA accumulation, owing in part to preexisting compensatory mtDNA alleles in some strains that prevent deletion formation. This analysis directly demonstrates that the evolutionary trajectories of ΔmtDNA elements depend upon the population-genetic environments and molecular-genetic features of their hosts.

Endogenous ROS levels in C. elegans under exogenous stress support revision of oxidative stress theory of life-history tradeoffs

BACKGROUND

The oxidative stress theory of life-history tradeoffs states that oxidative stress caused by damaging free radicals directly underpins tradeoffs between reproduction and longevity by altering the allocation of energetic resources between these tasks. We test this theory by characterizing the effects of exogenous oxidative insult and its interaction with thermal stress and diet quality on a suite of life-history traits and correlations in Caenorhabditis elegans nematodes. We also quantify demographic aging rates and endogenous reactive oxygen species (ROS) levels in live animals.

RESULTS

Our findings indicate a tradeoff between investment in reproduction and antioxidant defense (somatic maintenance) consistent with theoretical predictions, but correlations between standard life-history traits yield little evidence that oxidative stress generates strict tradeoffs. Increasing oxidative insult, however, shows a strong tendency to uncouple positive phenotypic correlations and, in particular, to reduce the correlation between reproduction and lifespan. We also found that mild oxidative insult results in lower levels of endogenous ROS accompanied by hormetic changes in lifespan, demographic aging, and reproduction that disappear in combined-stress treatments--consistent with the oxidative stress theory of aging.

CONCLUSIONS

Our findings demonstrate that oxidative stress is a direct contributor to life-history trait variation and that traditional tradeoffs are not necessary to invoke oxidative stress as a mediator of relationships between life-history traits, supporting previous calls for revisions to theory.

A broadly implementable research course in phage discovery and genomics for first-year undergraduate students

Engaging large numbers of undergraduates in authentic scientific discovery is desirable but difficult to achieve. We have developed a general model in which faculty and teaching assistants from diverse academic institutions are trained to teach a research course for first-year undergraduate students focused on bacteriophage discovery and genomics. The course is situated within a broader scientific context aimed at understanding viral diversity, such that faculty and students are collaborators with established researchers in the field. The Howard Hughes Medical Institute (HHMI) Science Education Alliance Phage Hunters Advancing Genomics and Evolutionary Science (SEA-PHAGES) course has been widely implemented and has been taken by over 4,800 students at 73 institutions. We show here that this alliance-sourced model not only substantially advances the field of phage genomics but also stimulates students' interest in science, positively influences academic achievement, and enhances persistence in science, technology, engineering, and mathematics (STEM) disciplines. Broad application of this model by integrating other research areas with large numbers of early-career undergraduate students has the potential to be transformative in science education and research training.

IMPORTANCE

Engagement of undergraduate students in scientific research at early stages in their careers presents an opportunity to excite students about science, technology, engineering, and mathematics (STEM) disciplines and promote continued interests in these areas. Many excellent course-based undergraduate research experiences have been developed, but scaling these to a broader impact with larger numbers of students is challenging. The Howard Hughes Medical Institute (HHMI) Science Education Alliance Phage Hunting Advancing Genomics and Evolutionary Science (SEA-PHAGES) program takes advantage of the huge size and diversity of the bacteriophage population to engage students in discovery of new viruses, genome annotation, and comparative genomics, with strong impacts on bacteriophage research, increased persistence in STEM fields, and student self-identification with learning gains, motivation, attitude, and career aspirations.

Evolution of a higher intracellular oxidizing environment in Caenorhabditis elegans under relaxed selection

We explored the relationship between relaxed selection, oxidative stress, and spontaneous mutation in a set of mutation-accumulation (MA) lines of the nematode Caenorhabditis elegans and in their common ancestor. We measured steady-state levels of free radicals and oxidatively damaged guanosine nucleosides in the somatic tissues of five MA lines for which nuclear genome base substitution and GC-TA transversion frequencies are known. The two markers of oxidative stress are highly correlated and are elevated in the MA lines relative to the ancestor; point estimates of the per-generation rate of mutational decay (ΔM) of these measures of oxidative stress are similar to those reported for fitness-related traits. Conversely, there is no significant relationship between either marker of oxidative stress and the per-generation frequencies of base substitution or GC-TA transversion. Although these results provide no direct evidence for a causative relationship between oxidative damage and base substitution mutations, to the extent that oxidative damage may be weakly mutagenic in the germline, the case for condition-dependent mutation is advanced.

Natural variation in Caenorhabditis briggsae mitochondrial form and function suggests a novel model of organelle dynamics

Mitochondrial functioning and morphology are known to be connected through cycles of organelle fusion and fission that depend upon the mitochondrial membrane potential (ΔΨM); however, we lack an understanding of the features and dynamics of natural mitochondrial populations. Using data from our recent study of univariate mitochondrial phenotypic variation in Caenorhabditis briggsae nematodes, we analyzed patterns of phenotypic correlation for 24 mitochondrial traits. Our findings support a role for ΔΨM in shaping mitochondrial dynamics, but no role for mitochondrial ROS. Further, our study suggests a novel model of mitochondrial population dynamics dependent upon cellular environmental context and with implications for mitochondrial genome integrity.

In vivo quantification reveals extensive natural variation in mitochondrial form and function in Caenorhabditis briggsae

We have analyzed natural variation in mitochondrial form and function among a set of Caenorhabditis briggsae isolates known to harbor mitochondrial DNA structural variation in the form of a heteroplasmic nad5 gene deletion (nad5Δ) that correlates negatively with organismal fitness. We performed in vivo quantification of 24 mitochondrial phenotypes including reactive oxygen species level, membrane potential, and aspects of organelle morphology, and observed significant among-isolate variation in 18 traits. Although several mitochondrial phenotypes were non-linearly associated with nad5Δ levels, most of the among-isolate phenotypic variation could be accounted for by phylogeographic clade membership. In particular, isolate-specific mitochondrial membrane potential was an excellent predictor of clade membership. We interpret this result in light of recent evidence for local adaptation to temperature in C. briggsae. Analysis of mitochondrial-nuclear hybrid strains provided support for both mtDNA and nuclear genetic variation as drivers of natural mitochondrial phenotype variation. This study demonstrates that multicellular eukaryotic species are capable of extensive natural variation in organellar phenotypes and highlights the potential of integrating evolutionary and cell biology perspectives.

The worm in the world and the world in the worm

Caenorhabditis elegans is a preeminent model organism, but the natural ecology of this nematode has been elusive. A four-year survey of French orchards published in BMC Biology reveals thriving populations of C. elegans (and Caenorhabditis briggsae) in rotting fruit and plant stems. Rather than being simply a 'soil nematode', C. elegans appears to be a 'plant-rot nematode'. These studies signal a growing interest in the integrated genomics and ecology of these tractable animals.

Variation in base-substitution mutation in experimental and natural lineages of Caenorhabditis nematodes

Variation among lineages in the mutation process has the potential to impact diverse biological processes ranging from susceptibilities to genetic disease to the mode and tempo of molecular evolution. The combination of high-throughput DNA sequencing (HTS) with mutation-accumulation (MA) experiments has provided a powerful approach to genome-wide mutation analysis, though insights into mutational variation have been limited by the vast evolutionary distances among the few species analyzed. We performed a HTS analysis of MA lines derived from four Caenorhabditis nematode natural genotypes: C. elegans N2 and PB306 and C. briggsae HK104 and PB800. Total mutation rates did not differ among the four sets of MA lines. A mutational bias toward G:C→A:T transitions and G:C→T:A transversions was observed in all four sets of MA lines. Chromosome-specific rates were mostly stable, though there was some evidence for a slightly elevated X chromosome mutation rate in PB306. Rates were homogeneous among functional coding sequence types and across autosomal cores, arms, and tips. Mutation spectra were similar among the four MA line sets but differed significantly when compared with patterns of natural base-substitution polymorphism for 13/14 comparisons performed. Our findings show that base-substitution mutation processes in these closely related animal lineages are mostly stable but differ from natural polymorphism patterns in these two species.

Fitness recovery and compensatory evolution in natural mutant lines of C. elegans

Deleterious mutation accumulation plays a central role in evolutionary genetics, conservation biology, human health, and evolutionary medicine (e.g., methods of viral attenuation for live vaccines). It is therefore important to understand whether and how quickly populations with accumulated deleterious mutational loads can recover fitness through adaptive evolution. We used laboratory experimental evolution with four long-term mutation-accumulation (MA) lines of Caenorhabditis elegans nematodes to study the dynamics of such fitness evolution. We previously showed that when homozygous mutant populations are evolved in large population sizes, they can rapidly achieve wild-type fitness through the accumulation of new beneficial or compensatory epistatic mutations. Here, we expand this approach to demonstrate that when replicate lineages are initiated from the same mutant genotype, phenotypic evolution is only sometimes repeatable. MA genotypes that recovered ancestral fitness in the previous experiment did not always do so here. Further, the pattern of adaptive evolution in independently evolved replicates was contingent upon the MA genotype and varied among fitness-related traits. Our findings suggest that new beneficial mutations can drive rapid fitness evolution, but that the adaptive process is rendered somewhat unpredictable by its susceptibility to chance events and sensitivity to the evolutionary history of the starting population.

Expanding the diversity of mycobacteriophages: insights into genome architecture and evolution

Mycobacteriophages are viruses that infect mycobacterial hosts such as Mycobacterium smegmatis and Mycobacterium tuberculosis. All mycobacteriophages characterized to date are dsDNA tailed phages, and have either siphoviral or myoviral morphotypes. However, their genetic diversity is considerable, and although sixty-two genomes have been sequenced and comparatively analyzed, these likely represent only a small portion of the diversity of the mycobacteriophage population at large. Here we report the isolation, sequencing and comparative genomic analysis of 18 new mycobacteriophages isolated from geographically distinct locations within the United States. Although no clear correlation between location and genome type can be discerned, these genomes expand our knowledge of mycobacteriophage diversity and enhance our understanding of the roles of mobile elements in viral evolution. Expansion of the number of mycobacteriophages grouped within Cluster A provides insights into the basis of immune specificity in these temperate phages, and we also describe a novel example of apparent immunity theft. The isolation and genomic analysis of bacteriophages by freshman college students provides an example of an authentic research experience for novice scientists.

Natural variation in life history and aging phenotypes is associated with mitochondrial DNA deletion frequency in Caenorhabditis briggsae

BACKGROUND

Mutations that impair mitochondrial functioning are associated with a variety of metabolic and age-related disorders. A barrier to rigorous tests of the role of mitochondrial dysfunction in aging processes has been the lack of model systems with relevant, naturally occurring mitochondrial genetic variation. Toward the goal of developing such a model system, we studied natural variation in life history, metabolic, and aging phenotypes as it relates to levels of a naturally-occurring heteroplasmic mitochondrial ND5 deletion recently discovered to segregate among wild populations of the soil nematode, Caenorhabditis briggsae. The normal product of ND5 is a central component of the mitochondrial electron transport chain and integral to cellular energy metabolism.

RESULTS

We quantified significant variation among C. briggsae isolates for all phenotypes measured, only some of which was statistically associated with isolate-specific ND5 deletion frequency. We found that fecundity-related traits and pharyngeal pumping rate were strongly inversely related to ND5 deletion level and that C. briggsae isolates with high ND5 deletion levels experienced a tradeoff between early fecundity and lifespan. Conversely, oxidative stress resistance was only weakly associated with ND5 deletion level while ATP content was unrelated to deletion level. Finally, mean levels of reactive oxygen species measured in vivo showed a significant non-linear relationship with ND5 deletion level, a pattern that may be driven by among-isolate variation in antioxidant or other compensatory mechanisms.

CONCLUSIONS

Our findings suggest that the ND5 deletion may adversely affect fitness and mitochondrial functioning while promoting aging in natural populations, and help to further establish this species as a useful model for explicit tests of hypotheses in aging biology and mitochondrial genetics.

Selective sweeps and parallel mutation in the adaptive recovery from deleterious mutation in Caenorhabditis elegans

Deleterious mutation poses a serious threat to human health and the persistence of small populations. Although adaptive recovery from deleterious mutation has been well-characterized in prokaryotes, the evolutionary mechanisms by which multicellular eukaryotes recover from deleterious mutation remain unknown. We applied high-throughput DNA sequencing to characterize genomic divergence patterns associated with the adaptive recovery from deleterious mutation using a Caenorhabditis elegans recovery-line system. The C. elegans recovery lines were initiated from a low-fitness mutation-accumulation (MA) line progenitor and allowed to independently evolve in large populations (N ∼ 1000) for 60 generations. All lines rapidly regained levels of fitness similar to the wild-type (N2) MA line progenitor. Although there was a near-zero probability of a single mutation fixing due to genetic drift during the recovery experiment, we observed 28 fixed mutations. Cross-generational analysis showed that all mutations went from undetectable population-level frequencies to a fixed state in 10-20 generations. Many recovery-line mutations fixed at identical timepoints, suggesting that the mutations, if not beneficial, hitchhiked to fixation during selective sweep events observed in the recovery lines. No MA line mutation reversions were detected. Parallel mutation fixation was observed for two sites in two independent recovery lines. Analysis using a C. elegans interactome map revealed many predicted interactions between genes with recovery line-specific mutations and genes with previously accumulated MA line mutations. Our study suggests that recovery-line mutations identified in both coding and noncoding genomic regions might have beneficial effects associated with compensatory epistatic interactions.

Spontaneous mutations decrease sensitivity of gene expression to random environmental variation in Caenorhabditis elegans

BACKGROUND

Biological phenotypes are described as "canalized" if they are robust to minor variation of environment and/or genetic background. The existence of a robust phenotype logically implies that some underlying mechanism must be variable, in the sense of "able to vary", in order to compensate for variation in the environment and/or genetic effects. Several lines of evidence lead to the conclusion that deleterious mutations predictably render morphological, developmental, and life-history traits more sensitive to small random environmental perturbations--that is, mutations de-canalize the phenotype.

METHODOLOGY/PRINCIPAL FINDINGS

Using conventional dye-swap microarray methodology, we compared transcript abundance in a sample of >7,000 genes between four mutation accumulation (MA) lines of the nematode Caenorhabditis elegans and the common (unmutated) ancestor. There was significantly less environmental variance in the MA lines than in the ancestor, both among replicates of the same gene and among genes.

CONCLUSIONS/SIGNIFICANCE

Deleterious mutations consistently decrease the within-line component of variance in transcript abundance, which is straightforwardly interpreted as reducing the sensitivity of gene expression to small random variation in the environment. This finding is consistent with the idea that underlying variability in gene expression might be mechanistically responsible for phenotypic robustness.

High rate of large deletions in Caenorhabditis briggsae mitochondrial genome mutation processes

Mitochondrial DNA (mtDNA) mutations underlie a variety of human genetic disorders and are associated with the aging process. mtDNA polymorphisms are widely used in a variety of evolutionary applications. Although mtDNA mutation spectra are known to differ between distantly related model organisms, the extent to which mtDNA mutation processes vary between more closely related species and within species remains enigmatic. We analyzed mtDNA divergence in two sets of 250-generation Caenorhabditis briggsae mutation-accumulation (MA) lines, each derived from a different natural isolate progenitor: strain HK104 from Okayama, Japan, and strain PB800 from Ohio, United States. Both sets of C. briggsae MA lines accumulated numerous large heteroplasmic mtDNA deletions, whereas only one similar event was observed in a previous analysis of Caenorhabditis elegans MA line mtDNA. Homopolymer length change mutations were frequent in both sets of C. briggsae MA lines and occurred in both intergenic and protein-coding gene regions. The spectrum of C. briggsae mtDNA base substitution mutations differed from the spectrum previously observed in C. elegans. In C. briggsae, the HK104 MA lines experienced many different base substitution types, whereas the PB800 lines displayed only C:G --> T:A transitions, although the difference was not significant. Over half of the mtDNA base substitutions detected in the C. briggsae MA lines were in a heteroplasmic state, whereas all those previously characterized in C. elegans MA line mtDNA were fixed changes, indicating a narrower mtDNA bottleneck in C. elegans as compared with C. briggsae. Our results show that C. briggsae mtDNA is highly susceptible to large deletions and that the mitochondrial mutation process varies between Caenorhabditis nematode species.

Evolution of Caenorhabditis mitochondrial genome pseudogenes and Caenorhabditis briggsae natural isolates

Although most metazoan mitochondrial genomes are highly streamlined and encode little noncoding DNA outside of the "AT" region, the accumulation of mitochondrial pseudogenes and other types of noncoding DNA has been observed in a growing number of animal groups. The nematode species Caenorhabditis briggsae harbors two mitochondrial DNA (mtDNA) pseudogenes, named Psinad5-1 and Psinad5-2, presumably derived from the nad5 protein-coding gene. Here, we provide an in-depth analysis of mtDNA pseudogene evolution in C. briggsae natural isolates and related Caenorhabditis species. Mapping the observed presence and absence of the pseudogenes onto phylogenies suggests that Psinad5-1 originated in the ancestor to C. briggsae and its recently discovered outcrossing relative species Caenorhabditis sp. 5 and Caenorhabditis sp. 9. However, Psinad5-1 was not detected in Caenorhabditis sp. 9 natural isolates, suggesting a lineage-specific loss of this pseudogene in this species. Our results corroborated the previous finding that Psinad5-2 originated within C. briggsae. The observed pattern of mitochondrial pseudogene gain and loss in Caenorhabditis was inconsistent with predictions of the tandem duplication-random loss model of mitochondrial genome evolution and suggests that intralineage recombination-like mechanisms might play a major role in Caenorhabditis mtDNA evolution. Natural variation was analyzed at the pseudogenes and flanking mtDNA sequences in 141 geographically diverse C. briggsae natural isolates. Although phylogenetic analysis placed the majority of isolates into the three previously established major intraspecific clades of C. briggsae, two new and unexpected haplotypes fell outside of these conventional groupings. Psinad5-2 copy number variation was observed among C. briggsae isolates collected from the same geographic site. Patterns of nucleotide diversity were analyzed in Psinad5-1 and Psinad5-2, and confidence intervals were found to overlap values from synonymous sites in protein-coding genes, consistent with neutral expectations. Our findings provide new insights into the mode and tempo of mitochondrial genome and pseudogene evolution both within and between Caenorhabditis nematode species.

High mitogenomic evolutionary rates and time dependency

Using entire modern and ancient mitochondrial genomes of Adélie penguins (Pygoscelis adeliae) that are up to 44000 years old, we show that the rates of evolution of the mitochondrial genome are two to six times greater than those estimated from phylogenetic comparisons. Although the rate of evolution at constrained sites, including nonsynonymous positions and RNAs, varies more than twofold with time (between shallow and deep nodes), the rate of evolution at synonymous sites remains the same. The time-independent neutral evolutionary rates reported here would be useful for the study of recent evolutionary events.

Molecular evolution in Panagrolaimus nematodes: origins of parthenogenesis, hermaphroditism and the Antarctic species P. davidi

BACKGROUND

As exemplified by the famously successful model organism Caenorhabditis elegans, nematodes offer outstanding animal systems for investigating diverse biological phenomena due to their small genome sizes, short generation times and ease of laboratory maintenance. Nematodes in the genus Panagrolaimus have served in comparative development and anhydrobiosis studies, and the Antarctic species P. davidi offers a powerful paradigm for understanding the biological mechanisms of extreme cold tolerance. Panagrolaimus nematodes are also unique in that examples of gonochoristic, hermaphroditic and parthenogenetic reproductive modes have been reported for members of this genus. The evolutionary origins of these varying reproductive modes and the Antarctic species P. davidi, however, remain enigmatic.

RESULTS

We collected nuclear ribosomal RNA gene and mitochondrial protein-coding gene sequences from diverse Panagrolaimus species and strains, including newly discovered isolates from Oregon, to investigate phylogenetic relationships in this nematode genus. Nuclear phylogenies showed that the species and strains historically identified as members of Panagrolaimus constitute a paraphyletic group, suggesting that taxonomic revision is required for Panagrolaimus and related nematode lineages. Strain-specific reproductive modes were mapped onto the molecular phylogeny to show a single origin of parthenogenesis from a presumably gonochoristic ancestor. The hermaphroditic strains were all placed outside a major monophyletic clade that contained the majority of other Panagrolaimus nematodes. Phylogenetic analyses of mitochondrial sequences showed that substantial molecular and geographic diversity exists within the clade of parthenogenetic strains. The Antarctic species P. davidi was found to be very closely related to two Panagrolaimus strains from southern California. Phylogenetic and molecular clock analyses suggested that P. davidi and the California strain PS1579 shared a common ancestor in the very recent evolutionary past.

CONCLUSION

Our study provides a phylogenetic framework for understanding the evolutionary origins and diversification patterns of varying reproductive modes within Panagrolaimus and important insights into the origin of the Antarctic species P. davidi. Panagrolaimus offers a powerful nematode model for understanding diverse evolutionary phenomena including the evolution of asexuality and the adaptive evolution of extreme cold tolerance.

Natural selection governs local, but not global, evolutionary gene coexpression networks in Caenorhabditis elegans

Background

Large-scale evaluation of gene expression variation among Caenorhabditis elegans lines that have diverged from a common ancestor allows for the analysis of a novel class of biological networks – evolutionary gene coexpression networks. Comparative analysis of these evolutionary networks has the potential to uncover the effects of natural selection in shaping coexpression network topologies since C. elegans mutation accumulation (MA) lines evolve essentially free from the effects of natural selection, whereas natural isolate (NI) populations are subject to selective constraints.

Results

We compared evolutionary gene coexpression networks for C. elegans MA lines versus NI populations to evaluate the role that natural selection plays in shaping the evolution of network topologies. MA and NI evolutionary gene coexpression networks were found to have very similar global topological properties as measured by a number of network topological parameters. Observed MA and NI networks show node degree distributions and average values for node degree, clustering coefficient, path length, eccentricity and betweeness that are statistically indistinguishable from one another yet highly distinct from randomly simulated networks. On the other hand, at the local level the MA and NI coexpression networks are highly divergent; pairs of genes coexpressed in the MA versus NI lines are almost entirely different as are the connectivity and clustering properties of individual genes.

Conclusion

It appears that selective forces shape how local patterns of coexpression change over time but do not control the global topology of C. elegans evolutionary gene coexpression networks. These results have implications for the evolutionary significance of global network topologies, which are known to be conserved across disparate complex systems.

TileQC: a system for tile-based quality control of Solexa data

Background

Next-generation DNA sequencing technologies such as Illumina's Solexa platform and Roche's 454 approach provide new avenues for investigating genome-scale questions. However, they also present novel analytical challenges that must be met for their effective application to biological questions.

Results

Here we report the availability of tileQC, a tile-based quality control system for Solexa data written in the R language. TileQC provides a means of recognizing bias and error in Solexa output by graphically representing data generated by flow cell tiles. The data represented in the images is then made available in the R environment for further analysis and automation of error detection.

Conclusion

TileQC offers a highly adaptable and powerful tool for the quality control of Solexa-based DNA sequence data.

Muller's Ratchet and compensatory mutation in Caenorhabditis briggsae mitochondrial genome evolution

Background

The theory of Muller' Ratchet predicts that small asexual populations are doomed to accumulate ever-increasing deleterious mutation loads as a consequence of the magnified power of genetic drift and mutation that accompanies small population size. Evidence for Muller's Ratchet and knowledge on its underlying molecular mechanisms, however, are lacking for natural populations.

Results

We characterized mitochondrial genome evolutionary processes in Caenorhabditis briggsae natural isolates to show that numerous lineages experience a high incidence of nonsynonymous substitutions in protein-coding genes and accumulate unusual deleterious noncoding DNA stretches with associated heteroplasmic function-disrupting genome deletions. Isolate-specific deletion proportions correlated negatively with nematode fecundity, suggesting that these deletions might negatively affect C. briggsae fitness. However, putative compensatory mutations were also observed that are predicted to reduce heteroplasmy levels of deleterious deletions. Paradoxically, compensatory mutations were observed in one major intraspecific C. briggsae clade where population sizes are estimated to be very small (and selection is predicted to be relatively weak), but not in a second major clade where population size estimates are much larger and selection is expected to be more efficient.

Conclusion

This study provides evidence that the mitochondrial genomes of animals evolving in nature are susceptible to Muller's Ratchet, suggests that context-dependent compensatory mutations can accumulate in small populations, and predicts that Muller's Ratchet can affect fundamental evolutionary forces such as the rate of mutation.

High intraspecific recombination rate in a native population of Candidatus pelagibacter ubique (SAR11)

Recombination is an important process in microbial evolution. Rates of recombination with extracellular DNA matter because models of microbial population structure are profoundly influenced by the degree to which recombination is occurring within the population. Low rates of recombination may be sufficient to ensure the lateral propagation of genes that have a high selective advantage without disrupting the clonal pattern of inheritance for other genes. High rates of recombination potentially can obscure clonal patterns, leading to linkage equilibrium, and give microbial populations a population genetic structure more akin to sexually interbreeding eukaryotic populations. We examined eight loci from nine strains of candidatus Pelagibacter ubique (SAR11), isolated from a single 2L niskin sample of natural seawater, for evidence of genetic recombination between strains. The Shimodaira-Hasegawa test revealed significant phylogenetic incongruence in seven of the genes, indicating that frequent recombination obscures phylogenetic signals from the linear inheritance of genes in this population. Statistical evidence for intragenic recombination was found for six loci. An informative sites matrix showed extensive evidence for a widespread breakdown of linkage disequilibrium. Although the mechanisms of genetic transfer in native SAR11 populations are unknown, we measured recombination rates, rho, that are much higher than point mutation rates, theta, as a source of genetic diversity in this clade. The eukaryotic model of species sharing a common pool of alleles is more apt for this SAR11 population than a strictly clonal model of inheritance in which allelic diversity is controlled by periodic selection.

Mutation rate variation in multicellular eukaryotes: causes and consequences

A basic knowledge about mutation rates is central to our understanding of a myriad of evolutionary phenomena, including the maintenance of sex and rates of molecular evolution. Although there is substantial evidence that mutation rates vary among taxa, relatively little is known about the factors that underlie this variation at an empirical level, particularly in multicellular eukaryotes. Here we integrate several disparate lines of theoretical and empirical inquiry into a unified framework to guide future studies that are aimed at understanding why and how mutation rates evolve in multicellular species.

The relative roles of three DNA repair pathways in preventing Caenorhabditis elegans mutation accumulation

Mutation is a central biological process whose rates and spectra are influenced by a variety of complex and interacting forces. Although DNA repair pathways are generally known to play key roles in maintaining genetic stability, much remains to be understood about the relative roles of different pathways in preventing the accumulation of mutations and the extent of heterogeneity in pathway-specific repair efficiencies across different genomic regions. In this study we examine mutation processes in base excision repair-deficient (nth-1) and nucleotide excision repair-deficient (xpa-1) Caenorhabditis elegans mutation-accumulation (MA) lines across 24 regions of the genome and compare our observations to previous data from mismatch repair-deficient (msh-2 and msh-6) and wild-type (N2) MA lines. Drastic variation in both average and locus-specific mutation rates, ranging two orders of magnitude for the latter, was detected among the four sets of repair-deficient MA lines. Our work provides critical insights into the relative roles of three DNA repair pathways in preventing C. elegans mutation accumulation and provides evidence for the presence of pathway-specific DNA repair territories in the C. elegans genome.