Caenorhabditis evolution in the wild

Patterns of Genomic Diversity in a Fig-Associated Close Relative of Caenorhabditis elegans

The evolution of reproductive mode is expected to have profound impacts on the genetic composition of populations. At the same time, ecological interactions can generate close associations among species, which can in turn generate a high degree of overlap in their spatial distributions. Caenorhabditis elegans is a hermaphroditic nematode that has enabled extensive advances in developmental genetics. Caenorhabditis inopinata, the sister species of C. elegans, is a gonochoristic nematode that thrives in figs and obligately disperses on fig wasps. Here, we describe patterns of genomic diversity in C. inopinata. We performed RAD-seq on individual worms isolated from the field across three Okinawan island populations. C. inopinata is about five times more diverse than C. elegans. Additionally, C. inopinata harbors greater differences in diversity among functional genomic regions (such as between genic and intergenic sequences) than C. elegans. Conversely, C. elegans harbors greater differences in diversity between high-recombining chromosome arms and low-recombining chromosome centers than C. inopinata. FST is low among island population pairs, and clear population structure could not be easily detected among islands, suggesting frequent migration of wasps between islands. These patterns of population differentiation appear comparable with those previously reported in its fig wasp vector. These results confirm many theoretical population genetic predictions regarding the evolution of reproductive mode and suggest C. inopinata population dynamics may be driven by wasp dispersal. This work sets the stage for future evolutionary genomic studies aimed at understanding the evolution of sex as well as the evolution of ecological interactions.

Colony level fitness analysis identifies a trade-off between population growth rate and dauer yield in Caenorhabditis elegans

BACKGROUND

In the evolution from unicellular to multicellular life forms, natural selection favored reduced cell proliferation and even programmed cell death if this increased organismal fitness. Could reduced individual fertility or even programmed organismal death similarly increase the fitness of colonies of closely-related metazoan organisms? This possibility is at least consistent with evolutionary theory, and has been supported by computer modelling. Caenorhabditis elegans has a boom and bust life history, where populations of nematodes that are sometimes near clonal subsist on and consume food patches, and then generate dauer larva dispersal propagules. A recent study of an in silico model of C. elegans predicted that one determinant of colony fitness (measured as dauer yield) is minimization of futile food consumption (i.e. that which does not contribute to dauer yield). One way to achieve this is to optimize colony population structure by adjustment of individual fertility.

RESULTS

Here we describe development of a C. elegans colony fitness assay, and its use to investigate the effect of altering population structure on colony fitness after population bust. Fitness metrics measured were speed of dauer production, and dauer yield, an indirect measure of efficiency of resource utilization (i.e. conversion of food into dauers). We find that with increasing founder number, speed of dauer production increases (due to earlier bust) but dauer yield rises and falls. In addition, some dauer recovery was detected soon after the post-colony bust peak of dauer yield, suggesting possible bet hedging among dauers.

CONCLUSIONS

These results suggest the presence of a fitness trade-off at colony level between speed and efficiency of resource utilization in C. elegans. They also provide indirect evidence that population structure is a determinant of colony level fitness, potentially by affecting level of futile food consumption.

Evolutionary codependency: insights into the mitonuclear interaction landscape from experimental and wild Caenorhabditis nematodes

Aided by new technologies, the upsurgence of research into mitochondrial genome biology during the past 15 years suggests that we have misunderstood, and perhaps dramatically underestimated, the ongoing biological and evolutionary significance of our long-time symbiotic partner. While we have begun to scratch the surface of several topics, many questions regarding the nature of mutation and selection in the mitochondrial genome, and the nature of its relationship to the nuclear genome, remain unanswered. Although best known for their contributions to studies of developmental and aging biology, Caenorhabditis nematodes are increasingly recognized as excellent model systems to advance understanding in these areas. We review recent discoveries with relevance to mitonuclear coevolution and conflict and offer several fertile areas for future work.

Genomic diversity landscapes in outcrossing and selfing Caenorhabditis nematodes

Caenorhabditis nematodes form an excellent model for studying how the mode of reproduction affects genetic diversity, as some species reproduce via outcrossing whereas others can self-fertilize. Currently, chromosome-level patterns of diversity and recombination are only available for self-reproducing Caenorhabditis, making the generality of genomic patterns across the genus unclear given the profound potential influence of reproductive mode. Here we present a whole-genome diversity landscape, coupled with a new genetic map, for the outcrossing nematode C. remanei. We demonstrate that the genomic distribution of recombination in C. remanei, like the model nematode C. elegans, shows high recombination rates on chromosome arms and low rates toward the central regions. Patterns of genetic variation across the genome are also similar between these species, but differ dramatically in scale, being tenfold greater for C. remanei. Historical reconstructions of variation in effective population size over the past million generations echo this difference in polymorphism. Evolutionary simulations demonstrate how selection, recombination, mutation, and selfing shape variation along the genome, and that multiple drivers can produce patterns similar to those observed in natural populations. The results illustrate how genome organization and selection play a crucial role in shaping the genomic pattern of diversity whereas demographic processes scale the level of diversity across the genome as a whole.

Developing an empirical model for spillover and emergence: Orsay virus host range in Caenorhabditis

A lack of tractable experimental systems in which to test hypotheses about the ecological and evolutionary drivers of disease spillover and emergence has limited our understanding of these processes. Here we introduce a promising system: Caenorhabditis hosts and Orsay virus, a positive-sense single-stranded RNA virus that naturally infects C. elegans. We assayed species across the Caenorhabditis tree and found Orsay virus susceptibility in 21 of 84 wild strains belonging to 14 of 44 species. Confirming patterns documented in other systems, we detected effects of host phylogeny on susceptibility. We then tested whether susceptible strains were capable of transmitting Orsay virus by transplanting exposed hosts and determining whether they transmitted infection to conspecifics during serial passage. We found no evidence of transmission in 10 strains (virus undetectable after passaging in all replicates), evidence of low-level transmission in 5 strains (virus lost between passage 1 and 5 in at least one replicate) and evidence of sustained transmission in 6 strains (including all three experimental C. elegans strains) in at least one replicate. Transmission was strongly associated with viral amplification in exposed populations. Variation in Orsay virus susceptibility and transmission among Caenorhabditis strains suggests that the system could be powerful for studying spillover and emergence.

Adaptation to environmental temperature in divergent clades of the nematode Pristionchus pacificus

Because of ongoing climate change, populations of organisms are being subjected to stressful temperatures more often. This is especially problematic for ectothermic organisms, which are likely to be more sensitive to changes in temperature. Therefore, we need to know if ectotherms have adapted to environmental temperature and, if so, what are the evolutionary mechanisms behind such adaptation. Here, we use the nematode Pristionchus pacificus as a case study to investigate thermal adaptation on the Indian Ocean island of La Réunion, which experiences a range of temperatures from coast to summit. We study the evolution of high-temperature tolerance by constructing a phylogenetic tree of strains collected from many different thermal niches. We show that populations of P. pacificus at low altitudes have higher fertility at warmer temperatures. Most likely, this phenotype has arisen recently and at least twice independently, consistent with parallel evolution. We also studied low-temperature tolerance and showed that populations from high altitudes have increased their fertility at cooler temperatures. Together, these data indicate that P. pacificus strains on La Réunion are subject to divergent selection, adapting to hot and cold niches at the coast and summit of the volcano. Precisely defining these thermal niches provides essential information for models that predict the impact of future climate change on these populations.

Opposing directions of stage-specific body shape change in a close relative of C. elegans

Background

Body size is a fundamental organismal trait. However, as body size and ecological contexts change across developmental time, evolutionary divergence may cause unexpected patterns of body size diversity among developmental stages. This may be particularly evident in polyphenic developmental stages specialized for dispersal. The dauer larva is such a stage in nematodes, and Caenorhabditis species disperse by traveling on invertebrate carriers. Here, we describe the morphology of a stress-resistant, dauer-like larval stage of the nematode Caenorhabditis inopinata, whose adults can grow to be nearly twice as long as its close relative, the model organism C. elegans.

Results

We find that a dauer-like, stress-resistant larval stage in two isolates of C. inopinata is on average 13% shorter and 30% wider than the dauer larvae of C. elegans, despite its much longer adult stage. Additionally, many C. inopinata dauer-like larvae were ensheathed, a possible novelty in this lineage reminiscent of the infective juveniles of parasitic nematodes. Variation in dauer-like larva formation frequency among twenty-four wild isolates of C. inopinata was also observed, although frequencies were low across all isolates (< 2%), with many isolates unable to produce dauer-like larvae under conventional laboratory conditions.

Conclusion

Most Caenorhabditis species thrive on rotting plants and disperse on snails, slugs, or isopods (among others) whereas C. inopinata is ecologically divergent and thrives in fresh Ficus septica figs and disperses on their pollinating wasps. While there is some unknown factor of the fig environment that promotes elongated body size in C. inopinata adults, the small size or unique life history of its fig wasp carrier may be driving the divergent morphology of its stress-resistant larval stages. Further characterization of the behavior, development, and morphology of this stage will refine connections to homologous developmental stages in other species and determine whether ecological divergence across multiple developmental stages can promote unexpected and opposing changes in body size dimensions within a single species.

Caenorhabditis nematodes colonize ephemeral resource patches in neotropical forests

Factors shaping the distribution and abundance of species include life-history traits, population structure, and stochastic colonization-extinction dynamics. Field studies of model species groups help reveal the roles of these factors. Species of Caenorhabditis nematodes are highly divergent at the sequence level but exhibit highly conserved morphology, and many of these species live in sympatry on microbe-rich patches of rotten material. Here, we use field experiments and large-scale opportunistic collections to investigate species composition, abundance, and colonization efficiency of Caenorhabditis species in two of the world's best-studied lowland tropical field sites: Barro Colorado Island in Panamá and La Selva in Sarapiquí, Costa Rica. We observed seven species of Caenorhabditis, four of them known only from these collections. We formally describe two species and place them within the Caenorhabditis phylogeny. While these localities contain species from many parts of the phylogeny, both localities were dominated by globally distributed androdiecious species. We found that Caenorhabditis individuals were able to colonize baits accessible only through phoresy and preferentially colonized baits that were in direct contact with the ground. We estimate the number of colonization events per patch to be low.

Local adaptation and spatiotemporal patterns of genetic diversity revealed by repeated sampling of Caenorhabditis elegans across the Hawaiian Islands

The nematode Caenorhabditis elegans is among the most widely studied organisms, but relatively little is known about its natural ecology. Genetic diversity is low across much of the globe but high in the Hawaiian Islands and across the Pacific Rim. To characterize the niche and genetic diversity of C. elegans on the Hawaiian Islands and to explore how genetic diversity might be influenced by local adaptation, we repeatedly sampled nematodes over a three-year period, measured various environmental parameters at each sampling site, and whole-genome sequenced the C. elegans isolates that we identified. We found that the typical Hawaiian C. elegans niche comprises moderately moist native forests at high elevations (500-1,500 m) where ambient air temperatures are cool (15-20°C). Compared to other Caenorhabditis species found on the Hawaiian Islands (e.g., Caenorhabditis briggsae and Caenorhabditis tropicalis), we found that C. elegans were enriched in native habitats. We measured levels of genetic diversity and differentiation among Hawaiian C. elegans and found evidence of seven genetically distinct groups distributed across the islands. Then, we scanned these genomes for signatures of local adaptation and identified 18 distinct regions that overlap with hyper-divergent regions, which may be maintained by balancing selection and are enriched for genes related to environmental sensing, xenobiotic detoxification, and pathogen resistance. These results provide strong evidence of local adaptation among Hawaiian C. elegans and contribute to our understanding of the forces that shape genetic diversity on the most remote volcanic archipelago in the world.

Mating behavior and reproductive morphology predict macroevolution of sex allocation in hermaphroditic flatworms

Background

Sex allocation is the distribution of resources to male or female reproduction. In hermaphrodites, this concerns an individual’s resource allocation to, for example, the production of male or female gametes. Macroevolutionary studies across hermaphroditic plants have revealed that the self-pollination rate and the pollination mode are strong predictors of sex allocation. Consequently, we expect similar factors such as the selfing rate and aspects of the reproductive biology, like the mating behaviour and the intensity of postcopulatory sexual selection, to predict sex allocation in hermaphroditic animals. However, comparative work on hermaphroditic animals is limited. Here, we study sex allocation in 120 species of the hermaphroditic free-living flatworm genus Macrostomum. We ask how hypodermic insemination, a convergently evolved mating behaviour where sperm are traumatically injected through the partner’s epidermis, affects the evolution of sex allocation. We also test the commonly-made assumption that investment into male and female reproduction should trade-off. Finally, we ask if morphological indicators of the intensity of postcopulatory sexual selection (female genital complexity, male copulatory organ length, and sperm length) can predict sex allocation.

Results

We find that the repeated evolution of hypodermic insemination predicts a more female-biased sex allocation (i.e., a relative shift towards female allocation). Moreover, transcriptome-based estimates of heterozygosity reveal reduced heterozygosity in hypodermically mating species, indicating that this mating behavior is linked to increased selfing or biparental inbreeding. Therefore, hypodermic insemination could represent a selfing syndrome. Furthermore, across the genus, allocation to male and female gametes is negatively related, and larger species have a more female-biased sex allocation. Finally, increased female genital complexity, longer sperm, and a longer male copulatory organ predict a more male-biased sex allocation.

Conclusions

Selfing syndromes have repeatedly originated in plants. Remarkably, this macroevolutionary pattern is replicated in Macrostomum flatworms and linked to repeated shifts in reproductive behavior. We also find a trade-off between male and female reproduction, a fundamental assumption of most theories of sex allocation. Beyond that, no theory predicts a more female-biased allocation in larger species, suggesting avenues for future work. Finally, morphological indicators of more intense postcopulatory sexual selection appear to predict more intense sperm competition.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01234-1.

Natural variation in reproductive timing and X-chromosome non-disjunction in Caenorhabditis elegans

Caenorhabditis elegans hermaphrodites first produce a limited number of sperm cells, before their germline switches to oogenesis. Production of progeny then ensues until sperm is depleted. Male production in the self-progeny of hermaphrodites occurs following X-chromosome non-disjunction during gametogenesis, and in the reference strain increases with age of the hermaphrodite parent. To enhance our understanding of the reproductive timecourse in Caenorhabditis elegans, we measured and compared progeny production and male proportion during the early and late reproductive periods of hermaphrodites for 96 wild Caenorhabditis elegans strains. We found that the two traits exhibited natural phenotypic variation with few outliers and a similar reproductive timing pattern as previous reports. Progeny number and male proportion were not correlated in the wild isolates, implying that isolates with a large brood size did not produce males at a higher rate. We also identified loci and candidate genetic variants significantly associated with male-production rate in the late and total reproductive periods. Our results provide an insight into life history traits in wild Caenorhabditis elegans strains.

Gene-level quantitative trait mapping in Caenorhabditis elegans

The Caenorhabditis elegans multiparental experimental evolution (CeMEE) panel is a collection of genome-sequenced, cryopreserved recombinant inbred lines useful for mapping the evolution and genetic basis of quantitative traits. We have expanded the resource with new lines and new populations, and here report the genotype and haplotype composition of CeMEE version 2, including a large set of putative de novo mutations, and updated additive and epistatic mapping simulations. Additive quantitative trait loci explaining 4% of trait variance are detected with >80% power, and the median detection interval approaches single-gene resolution on the highly recombinant chromosome arms. Although CeMEE populations are derived from a long-term evolution experiment, genetic structure is dominated by variation present in the ancestral population.

Host genetic drift and adaptation in the evolution and maintenance of parasite resistance

Host-parasite interactions may often be subject to opposing evolutionary forces, which likely influence the evolutionary trajectories of both partners. Natural selection and genetic drift are two major evolutionary forces that act in host and parasite populations. Further, population size is a significant determinant of the relative strengths of these forces. In small populations, drift may undermine the persistence of beneficial alleles, potentially impeding host adaptation to parasites. Here, we investigate two questions: (a) can selection pressure for increased resistance in small, susceptible host populations overcome the effects of drift and (b) can resistance be maintained in small host populations? To answer these questions, we experimentally evolved the host Caenorhabditis elegans against its bacterial parasite, Serratia marcescens, for 13 host generations. We found that strong selection favouring increased host resistance was insufficient to counteract drift in small populations, resulting in persistently high host mortality. Additionally, in small populations of resistant hosts, we found that selection for the maintenance of resistance is not always sufficient to curb the loss of resistance. We compared these results with selection in large host populations. We found that initially resistant, large host populations were able to maintain high levels of resistance. Likewise, initially susceptible, large host populations were able to gain resistance to the parasite. These results show that strong selection pressure for survival is not always sufficient to counteract drift. In consideration of C. elegans natural population dynamics, we suggest that drift may often impede selection in nature.

Evolution: Drivers in a Traffic Jam

Genetic loci coding for a toxin and its antidote behave like selfish elements. Two new studies find an accumulation of such elements in one species.

Selfing is the safest sex for Caenorhabditis tropicalis

Mating systems have profound effects on genetic diversity and compatibility. The convergent evolution of self-fertilization in three Caenorhabditis species provides a powerful lens to examine causes and consequences of mating system transitions. Among the selfers, Caenorhabditis tropicalis is the least genetically diverse and most afflicted by outbreeding depression. We generated a chromosomal-scale genome for C. tropicalis and surveyed global diversity. Population structure is very strong, and islands of extreme divergence punctuate a genomic background that is highly homogeneous around the globe. Outbreeding depression in the laboratory is caused largely by multiple Medea-like elements, genetically consistent with maternal toxin/zygotic antidote systems. Loci with Medea activity harbor novel and duplicated genes, and their activity is modified by mito-nuclear background. Segregating Medea elements dramatically reduce fitness, and simulations show that selfing limits their spread. Frequent selfing in C. tropicalis may therefore be a strategy to avoid Medea-mediated outbreeding depression.

Degradation of the Repetitive Genomic Landscape in a Close Relative of Caenorhabditis elegans

The abundance, diversity, and genomic distribution of repetitive elements is highly variable among species. These patterns are thought to be driven in part by reproductive mode and the interaction of selection and recombination, and recombination rates typically vary by chromosomal position. In the nematode Caenorhabditis elegans, repetitive elements are enriched at chromosome arms and depleted on centers, and this mirrors the chromosomal distributions of other genomic features such as recombination rate. How conserved is this genomic landscape of repeats, and what evolutionary forces maintain it? To address this, we compared the genomic organization of repetitive elements across five Caenorhabditis species with chromosome-level assemblies. As previously reported, repeat content is enriched on chromosome arms in most Caenorhabditis species, and no obvious patterns of repeat content associated with reproductive mode were observed. However, the fig-associated C. inopinata has experienced repetitive element expansion and reveals no association of global repeat density with chromosome position. Patterns of repeat superfamily specific distributions reveal this global pattern is driven largely by a few repeat superfamilies that in C. inopinata have expanded in number and have weak associations with chromosome position. Additionally, 15% of predicted protein-coding genes in C. inopinata align to transposon-related proteins. When these are excluded, C. inopinata has no enrichment of genes in chromosome centers, in contrast to its close relatives who all have such clusters. Forward evolutionary simulations reveal that chromosomal heterogeneity in recombination rate alone can generate structured repetitive genomic landscapes when insertions are weakly deleterious, whereas chromosomal heterogeneity in the fitness effects of transposon insertion can promote such landscapes across a variety of evolutionary scenarios. Thus, patterns of gene density along chromosomes likely contribute to global repetitive landscapes in this group, although other historical or genomic factors are needed to explain the idiosyncrasy of genomic organization of various transposable element taxa within C. inopinata. Taken together, these results highlight the power of comparative genomics and evolutionary simulations in testing hypotheses regarding the causes of genome organization.

Shorter life and reduced fecundity can increase colony fitness in virtual Caenorhabditis elegans

In the nematode Caenorhabditis elegans, loss of function of many genes leads to increases in lifespan, sometimes of a very large magnitude. Could this reflect the occurrence of programmed death that, like apoptosis of cells, promotes fitness? The notion that programmed death evolves as a mechanism to remove worn out, old individuals in order to increase food availability for kin is not supported by classic evolutionary theory for most species. However, it may apply in organisms with colonies of closely related individuals such as C. elegans in which largely clonal populations subsist on spatially limited food patches. Here, we ask whether food competition between nonreproductive adults and their clonal progeny could favor programmed death by using an in silico model of C. elegans. Colony fitness was estimated as yield of dauer larva propagules from a limited food patch. Simulations showed that not only shorter lifespan but also shorter reproductive span and reduced adult feeding rate can increase colony fitness, potentially by reducing futile food consumption. Early adult death was particularly beneficial when adult food consumption rate was high. These results imply that programmed, adaptive death could promote colony fitness in C. elegans through a consumer sacrifice mechanism. Thus, C. elegans lifespan may be limited not by aging in the usual sense but rather by apoptosis-like programmed death.

Diversification of the Caenorhabditis heat shock response by Helitron transposable elements

Heat Shock Factor 1 (HSF-1) is a key regulator of the heat shock response (HSR). Upon heat shock, HSF-1 binds well-conserved motifs, called Heat Shock Elements (HSEs), and drives expression of genes important for cellular protection during this stress. Remarkably, we found that substantial numbers of HSEs in multiple Caenorhabditis species reside within Helitrons, a type of DNA transposon. Consistent with Helitron-embedded HSEs being functional, upon heat shock they display increased HSF-1 and RNA polymerase II occupancy and up-regulation of nearby genes in C. elegans. Interestingly, we found that different genes appear to be incorporated into the HSR by species-specific Helitron insertions in C. elegans and C. briggsae and by strain-specific insertions among different wild isolates of C. elegans. Our studies uncover previously unidentified targets of HSF-1 and show that Helitron insertions are responsible for rewiring and diversifying the Caenorhabditis HSR.

Reproductive transitions in plants and animals: selfing syndrome, sexual selection and speciation

The evolution of predominant self-fertilisation frequently coincides with the evolution of a collection of phenotypes that comprise the 'selfing syndrome', in both plants and animals. Genomic features also display a selfing syndrome. Selfing syndrome traits often involve changes to male and female reproductive characters that were subject to sexual selection and sexual conflict in the obligatorily outcrossing ancestor, including the gametic phase for both plants and animals. Rapid evolution of reproductive traits, due to both relaxed selection and directional selection under the new status of predominant selfing, lays the genetic groundwork for reproductive isolation. Consequently, shifts in sexual selection pressures coupled to transitions to selfing provide a powerful paradigm for investigating the speciation process. Plant and animal studies, however, emphasise distinct selective forces influencing reproductive-mode transitions: genetic transmission advantage to selfing or reproductive assurance outweighing the costs of inbreeding depression vs the costs of males and meiosis. Here, I synthesise links between sexual selection, evolution of selfing and speciation, with particular focus on identifying commonalities and differences between plant and animal systems and pointing to areas warranting further synergy.

Males, Outcrossing, and Sexual Selection in Caenorhabditis Nematodes

Males of Caenorhabditis elegans provide a crucial practical tool in the laboratory, but, as the rarer and more finicky sex, have not enjoyed the same depth of research attention as hermaphrodites. Males, however, have attracted the attention of evolutionary biologists who are exploiting the C. elegans system to test longstanding hypotheses about sexual selection, sexual conflict, transitions in reproductive mode, and genome evolution, as well as to make new discoveries about Caenorhabditis organismal biology. Here, we review the evolutionary concepts and data informed by study of males of C. elegans and other Caenorhabditis We give special attention to the important role of sperm cells as a mediator of inter-male competition and male-female conflict that has led to drastic trait divergence across species, despite exceptional phenotypic conservation in many other morphological features. We discuss the evolutionary forces important in the origins of reproductive mode transitions from males being common (gonochorism: females and males) to rare (androdioecy: hermaphrodites and males) and the factors that modulate male frequency in extant androdioecious populations, including the potential influence of selective interference, host-pathogen coevolution, and mutation accumulation. Further, we summarize the consequences of males being common vs rare for adaptation and for trait divergence, trait degradation, and trait dimorphism between the sexes, as well as for molecular evolution of the genome, at both micro-evolutionary and macro-evolutionary timescales. We conclude that C. elegans male biology remains underexploited and that future studies leveraging its extensive experimental resources are poised to discover novel biology and to inform profound questions about animal function and evolution.

Molecular evolution across developmental time reveals rapid divergence in early embryogenesis

Ontogenetic development hinges on the changes in gene expression in time and space within an organism, suggesting that the demands of ontogenetic growth can impose or reveal predictable pattern in the molecular evolution of genes expressed dynamically across development. Here, we characterize coexpression modules of the Caenorhabditis elegans transcriptome, using a time series of 30 points from early embryo to adult. By capturing the functional form of expression profiles with quantitative metrics, we find fastest evolution in the distinctive set of genes with transcript abundance that declines through development from a peak in young embryos. These genes are highly enriched for oogenic function and transient early zygotic expression, are nonrandomly distributed in the genome, and correspond to a life stage especially prone to inviability in interspecies hybrids. These observations conflict with the "early conservation model" for the evolution of development, although expression-weighted sequence divergence analysis provides some support for the "hourglass model." Genes in coexpression modules that peak toward adulthood also evolve fast, being hyper-enriched for roles in spermatogenesis, implicating a history of sexual selection and relaxation of selection on sperm as key factors driving rapid change to ontogenetically distinguishable coexpression modules of genes. We propose that these predictable trends of molecular evolution for dynamically expressed genes across ontogeny predispose particular life stages, early embryogenesis in particular, to hybrid dysfunction in the speciation process.

Genetically Distinct Behavioral Modules Underlie Natural Variation in Thermal Performance Curves

Thermal reaction norms pervade organismal traits as stereotyped responses to temperature, a fundamental environmental input into sensory and physiological systems. Locomotory behavior represents an especially plastic read-out of animal response, with its dynamic dependence on environmental stimuli presenting a challenge for analysis and for understanding the genomic architecture of heritable variation. Here we characterize behavioral reaction norms as thermal performance curves for the nematode Caenorhabditis briggsae, using a collection of 23 wild isolate genotypes and 153 recombinant inbred lines to quantify the extent of genetic and plastic variation in locomotory behavior to temperature changes. By reducing the dimensionality of the multivariate phenotypic response with a function-valued trait framework, we identified genetically distinct behavioral modules that contribute to the heritable variation in the emergent overall behavioral thermal performance curve. Quantitative trait locus mapping isolated regions on Chromosome II associated with locomotory activity at benign temperatures and Chromosome V loci related to distinct aspects of sensitivity to high temperatures, with each quantitative trait locus explaining up to 28% of trait variation. These findings highlight how behavioral responses to environmental inputs as thermal reaction norms can evolve through independent changes to genetically distinct modular components of such complex phenotypes.

Evolution of sex ratio through gene loss

Several species of Caenorhabditis nematodes, including Caenorhabditis elegans, have recently evolved self-fertile hermaphrodites from female/male ancestors. These hermaphrodites can either self-fertilize or mate with males, and the extent of outcrossing determines subsequent male frequency. Using experimental evolution, the authors show that a gene family with a historical role in sperm competition plays a large role in regulating male frequency after self-fertility evolves. By reducing, but not completely eliminating outcrossing, loss of the mss genes contributes to adaptive tuning of the sex ratio in a newly self-fertile species.

The maintenance of males at intermediate frequencies is an important evolutionary problem. Several species of Caenorhabditis nematodes have evolved a mating system in which selfing hermaphrodites and males coexist. While selfing produces XX hermaphrodites, cross-fertilization produces 50% XO male progeny. Thus, male mating success dictates the sex ratio. Here, we focus on the contribution of the male secreted short (mss) gene family to male mating success, sex ratio, and population growth. The mss family is essential for sperm competitiveness in gonochoristic species, but has been lost in parallel in androdioecious species. Using a transgene to restore mss function to the androdioecious Caenorhabditis briggsae, we examined how mating system and population subdivision influence the fitness of the mss+ genotype. Consistent with theoretical expectations, when mss+ and mss-null (i.e., wild type) genotypes compete, mss+ is positively selected in both mixed-mating and strictly outcrossing situations, though more strongly in the latter. Thus, while sexual mode alone affects the fitness of mss+, it is insufficient to explain its parallel loss. However, in genetically homogenous androdioecious populations, mss+ both increases male frequency and depresses population growth. We propose that the lack of inbreeding depression and the strong subdivision that characterize natural Caenorhabditis populations impose selection on sex ratio that makes loss of mss adaptive after self-fertility evolves.

A large close relative of C. elegans is slow-developing but not long-lived

BACKGROUND

Variation in body size is thought to be a major driver of a wide variety of ecological and evolutionary patterns, including changes in development, reproduction, and longevity. Additionally, drastic changes in natural context often have profound effects on multiple fitness-related traits. Caenorhabditis inopinata is a recently-discovered fig-associated nematode that is unusually large relative to other members of the genus, including the closely related model system C. elegans. Here we test whether the dramatic increase in body size and shift in ecological context has led to correlated changes in key life history and developmental parameters within this species.

RESULTS

Using four developmental milestones, C. inopinata was found to have a slower rate of development than C. elegans across a range of temperatures. Despite this, C. inopinata did not reveal any differences in adult lifespan from C. elegans after accounting for differences in developmental timing and reproductive mode. C. inopinata fecundity was generally lower than that of C. elegans, but fitness improved under continuous-mating, consistent with sperm-limitation under gonochoristic (male/female) reproduction. C. inopinata also revealed greater fecundity and viability at higher temperatures.

CONCLUSION

Consistent with observations in other ectotherms, slower growth in C. inopinata indicates a potential trade-off between body size and developmental timing, whereas its unchanged lifespan suggests that longevity is largely uncoupled from its increase in body size. Additionally, temperature-dependent patterns of fitness in C. inopinata are consistent with its geographic origins in subtropical Okinawa. Overall, these results underscore the extent to which changes in ecological context and body size can shape life history traits.

Does senescence promote fitness in Caenorhabditis elegans by causing death?

A widely appreciated conclusion from evolutionary theory is that senescence (aging) is of no adaptive value to the individual that it afflicts. Yet studies of Caenorhabditis elegans and Saccharomyces cerevisiae are increasingly revealing the presence of processes which actively cause senescence and death, leading some biogerontologists to wonder about the established theory. Here we argue that programmed death that increases fitness could occur in C. elegans and S. cerevisiae, and that this is consistent with the classic evolutionary theory of aging. This is because of the special conditions under which these organisms have evolved, particularly the existence of clonal populations with limited dispersal and, in the case of C. elegans, the brevity of the reproductive period caused by protandrous hermaphroditism. Under these conditions, death-promoting mechanisms could promote worm fitness by enhancing inclusive fitness, or worm colony fitness through group selection. Such altruistic, adaptive death is not expected to evolve in organisms with outbred, dispersed populations (e.g. most vertebrate species). The plausibility of adaptive death in C. elegans is supported by computer modelling studies, and new knowledge about the ecology of this species. To support these arguments we also review the biology of adaptive death, and distinguish three forms: consumer sacrifice, biomass sacrifice and defensive sacrifice.

Demographic consequences of reproductive interference in multi-species communities

BACKGROUND

Reproductive interference can mediate interference competition between species through sexual interactions that reduce the fitness of one species by another. Theory shows that the positive frequency-dependent effects of such costly errors in mate recognition can dictate species coexistence or exclusion even with countervailing resource competition differences between species. While usually framed in terms of pre-mating or post-zygotic costs, reproductive interference manifests between individual Caenorhabditis nematodes from negative interspecies gametic interactions: sperm cells from interspecies matings can migrate ectopically to induce female sterility and premature death. The potential for reproductive interference to exert population level effects on Caenorhabditis trait evolution and community structure, however, remains unknown.

RESULTS

Here we test whether a species that is superior in individual-level reproductive interference (C. nigoni) can exact negative demographic effects on competitor species that are superior in resource competition (C. briggsae and C. elegans). We observe coexistence over six generations and find evidence of demographic reproductive interference even under conditions unfavorable to its influence. C. briggsae and C. elegans show distinct patterns of reproductive interference in competitive interactions with C. nigoni.

CONCLUSIONS

These results affirm that individual level negative effects of reproductive interference mediated by gamete interactions can ramify to population demography, with the potential to influence patterns of species coexistence separately from the effects of direct resource competition.

Strategic adjustment of copulatory plug size in a nematode

Copulatory plugs (CP) are substances produced during copulation that block the genital openings of the female. In several species of Nematoda, males produce CP that are thought to impede female remating and thus sperm competition. The relatively large size of the CP in several nematodes, and its evolutionary loss in self-fertilizing populations of Caenorhabditis elegans, suggests that CP are costly to produce. If CP production is costly, the application of basic concepts of strategic ejaculation theory suggests a modulated allocation of CP in response to sperm competition risk. This hypothesis led us to predict that males perceiving a higher risk of sperm competition will produce larger CP. We tested these ideas with the entomopathogenic, gonochoristic nematode Rhabditis regina. Our first experiment provides evidence suggesting that production of CP is costly, because the size of CP is negatively affected by stressful conditions (high population density, small male adult size, and suboptimal food type). The results of our second experiment support the prediction that males adjust the size of CP to sperm competition risk: the average size of CP increased as the number of males competing for one female increased. Overall, our study supports the idea that in R. regina the production of CP is costly for males and that the size of the CP produced is influenced by sperm competition risk.

Field studies reveal a close relative of C. elegans thrives in the fresh figs of Ficus septica and disperses on its Ceratosolen pollinating wasps

BACKGROUND

Biotic interactions are ubiquitous and require information from ecology, evolutionary biology, and functional genetics in order to be understood. However, study systems that are amenable to investigations across such disparate fields are rare. Figs and fig wasps are a classic system for ecology and evolutionary biology with poor functional genetics; Caenorhabditis elegans is a classic system for functional genetics with poor ecology. In order to help bridge these disciplines, here we describe the natural history of a close relative of C. elegans, Caenorhabditis inopinata, that is associated with the fig Ficus septica and its pollinating Ceratosolen wasps.

RESULTS

To understand the natural context of fig-associated Caenorhabditis, fresh F. septica figs from four Okinawan islands were sampled, dissected, and observed under microscopy. C. inopinata was found in all islands where F. septica figs were found. C.i nopinata was routinely found in the fig interior and almost never observed on the outside surface. C. inopinata was only found in pollinated figs, and C. inopinata was more likely to be observed in figs with more foundress pollinating wasps. Actively reproducing C. inopinata dominated early phase figs, whereas late phase figs with emerging wasp progeny harbored C. inopinata dauer larvae. Additionally, C. inopinata was observed dismounting from Ceratosolen pollinating wasps that were placed on agar plates. C. inopinata was not found on non-pollinating, parasitic Philotrypesis wasps. Finally, C. inopinata was only observed in F. septica figs among five Okinawan Ficus species sampled.

CONCLUSION

These are the first detailed field observations of C. inopinata, and they suggest a natural history where this species proliferates in early phase F. septica figs and disperses from late phase figs on Ceratosolen pollinating fig wasps. While consistent with other examples of nematode diversification in the fig microcosm, the fig and wasp host specificity of C. inopinata is highly divergent from the life histories of its close relatives and frames hypotheses for future investigations. This natural co-occurrence of the fig/fig wasp and C. inopinata study systems sets the stage for an integrated research program that can help to explain the evolution of interspecific interactions.

Genetic basis to hybrid inviability is more complex than hybrid male sterility in Caenorhabditis nematodes

Hybrid male sterility often evolves before female sterility or inviability of hybrids, implying that the accumulation of divergence between separated lineages should lead hybrid male sterility to have a more polygenic basis. However, experimental evidence is mixed. Here, we use the nematodes Caenorhabditis remanei and C. latens to characterize the underlying genetic basis of asymmetric hybrid male sterility and hybrid inviability. We demonstrate that hybrid male sterility is consistent with a simple genetic basis, involving a single X-autosome incompatibility. We also show that hybrid inviability involves more genomic compartments, involving diverse nuclear-nuclear incompatibilities, a mito-nuclear incompatibility, and maternal effects. These findings demonstrate that male sensitivity to genetic perturbation may be genetically simple compared to hybrid inviability in Caenorhabditis and motivates tests of generality for the genetic architecture of hybrid incompatibility across the breadth of phylogeny.

Turnover in local parasite populations temporarily favors host outcrossing over self-fertilization during experimental evolution

The ubiquity of outcrossing in plants and animals is difficult to explain given its costs relative to self-fertilization. Despite these costs, exposure to changing environmental conditions can temporarily favor outcrossing over selfing. Therefore, recurring episodes of environmental change are predicted to favor the maintenance of outcrossing. Studies of host-parasite coevolution have provided strong support for this hypothesis. However, it is unclear whether multiple exposures to novel parasite genotypes in the absence of coevolution are sufficient to favor outcrossing. Using the nematode Caenorhabditis elegans and the bacterial parasite Serratia marcescens, we studied host responses to parasite turnover. We passaged several replicates of a host population that was well-adapted to the S. marcescens strain Sm2170 with either Sm2170 or one of three novel S. marcescens strains, each derived from Sm2170, for 18 generations. We found that hosts exposed to novel parasites maintained higher outcrossing rates than hosts exposed to Sm2170. Nonetheless, host outcrossing rates declined over time against all but the most virulent novel parasite strain. Hosts exposed to the most virulent novel strain exhibited increased outcrossing rates for approximately 12 generations, but did not maintain elevated levels of outcrossing throughout the experiment. Thus, parasite turnover can transiently increase host outcrossing. These results suggest that recurring episodes of parasite turnover have the potential to favor the maintenance of host outcrossing. However, such maintenance may require frequent exposure to novel virulent parasites, rapid rates of parasite turnover, and substantial host gene flow.

The Local Coexistence Pattern of Selfing Genotypes in Caenorhabditis elegans Natural Metapopulations

To study the interplay of rare outcrossing and metapopulation structure, we focus on the nematode Caenorhabditis elegans Its remarkably low outcrossing rate is at the extreme end of the spectrum for facultative selfing organisms. At the demographic level, C. elegans natural populations undergo boom and bust dynamics on ephemeral resources, with the dauer diapause larva acting as the dispersal form. Here we investigate the small-scale genetic structure of C. elegans populations in two localities over several years, using 2b restriction-associated DNA sequencing of nearly 1000 individuals. We find a remarkably small number of genome-wide haplotypes, almost exclusively in the homozygous state, confirming the low effective outcrossing rate. Most strikingly, the major haplotypes in a locality remain intact and do not effectively recombine over several years. From the spatial pattern of diversity, we estimate that each subpopulation or deme is seeded by a mean of 3-10 immigrating individuals. Populations are thus formed by clones that compete at two levels, within a subpopulation and at the metapopulation level. We test for the presence of local phenotypic variation in pathogen resistance and dauer larva nictation, which could possibly explain the maintenance of different genotypes by heterogeneous selection in different local environments or lifecycles. This study is the first to address the local spatiotemporal genetic structure of C. elegans on feeding substrates. We conclude that these animals coexist as competing homozygous clones at the smallest population scale as well as in the metapopulation.

Rapid genome shrinkage in a self-fertile nematode reveals sperm competition proteins

To reveal impacts of sexual mode on genome content, we compared chromosome-scale assemblies of the outcrossing nematode Caenorhabditis nigoni to its self-fertile sibling species, C. briggsaeC. nigoni's genome resembles that of outcrossing relatives but encodes 31% more protein-coding genes than C. briggsaeC. nigoni genes lacking C. briggsae orthologs were disproportionately small and male-biased in expression. These include the male secreted short (mss) gene family, which encodes sperm surface glycoproteins conserved only in outcrossing species. Sperm from mss-null males of outcrossing C. remanei failed to compete with wild-type sperm, despite normal fertility in noncompetitive mating. Restoring mss to C. briggsae males was sufficient to enhance sperm competitiveness. Thus, sex has a pervasive influence on genome content that can be used to identify sperm competition factors.

Ephemeral-habitat colonization and neotropical species richness of Caenorhabditis nematodes

Background

The drivers of species co-existence in local communities are especially enigmatic for assemblages of morphologically cryptic species. Here we characterize the colonization dynamics and abundance of nine species of Caenorhabditis nematodes in neotropical French Guiana, the most speciose known assemblage of this genus, with resource use overlap and notoriously similar external morphology despite deep genomic divergence.

Methods

To characterize the dynamics and specificity of colonization and exploitation of ephemeral resource patches, we conducted manipulative field experiments and the largest sampling effort to date for Caenorhabditis outside of Europe. This effort provides the first in-depth quantitative analysis of substrate specificity for Caenorhabditis in natural, unperturbed habitats.

Results

We amassed a total of 626 strain isolates from nine species of Caenorhabditis among 2865 substrate samples. With the two new species described here (C. astrocarya and C. dolens), we estimate that our sampling procedures will discover few additional species of these microbivorous animals in this tropical rainforest system. We demonstrate experimentally that the two most prevalent species (C. nouraguensis and C. tropicalis) rapidly colonize fresh resource patches, whereas at least one rarer species shows specialist micro-habitat fidelity.

Conclusion

Despite the potential to colonize rapidly, these ephemeral patchy resources of rotting fruits and flowers are likely to often remain uncolonized by Caenorhabditis prior to their complete decay, implying dispersal-limited resource exploitation. We hypothesize that a combination of rapid colonization, high ephemerality of resource patches, and species heterogeneity in degree of specialization on micro-habitats and life histories enables a dynamic co-existence of so many morphologically cryptic species of Caenorhabditis.

Electronic supplementary material

The online version of this article (10.1186/s12898-017-0150-z) contains supplementary material, which is available to authorized users.

X exceptionalism in Caenorhabditis speciation

Speciation genetics research in diverse organisms shows the X-chromosome to be exceptional in how it contributes to "rules" of speciation. Until recently, however, the nematode phylum has been nearly silent on this issue, despite the model organism Caenorhabditis elegans having touched most other topics in biology. Studies of speciation with Caenorhabditis accelerated with the recent discovery of species pairs showing partial interfertility. The resulting genetic analyses of reproductive isolation in nematodes demonstrate key roles for the X-chromosome in hybrid male sterility and inviability, opening up new understanding of the genetic causes of Haldane's rule, Darwin's corollary to Haldane's rule, and enabling tests of the large-X effect hypothesis. Studies to date implicate improper chromatin regulation of the X-chromosome by small RNA pathways as integral to hybrid male dysfunction. Sexual transitions in reproductive mode to self-fertilizing hermaphroditism inject distinctive molecular evolutionary features into the speciation process for some species. Caenorhabditis also provides unique opportunities for analysis in a system with XO sex determination that lacks a Y-chromosome, sex chromosome-dependent sperm competition differences and mechanisms of gametic isolation, exceptional accessibility to the development process and rapid experimental evolution. As genetic analysis of reproductive isolation matures with investigation of multiple pairs of Caenorhabditis species and new species discovery, nematodes will provide a powerful complement to more established study organisms for deciphering the genetic basis of and rules to speciation.

Polygenicity and Epistasis Underlie Fitness-Proximal Traits in the Caenorhabditis elegans Multiparental Experimental Evolution (CeMEE) Panel

Understanding the genetic basis of complex traits remains a major challenge in biology. Polygenicity, phenotypic plasticity, and epistasis contribute to phenotypic variance in ways that are rarely clear. This uncertainty can be problematic for estimating heritability, for predicting individual phenotypes from genomic data, and for parameterizing models of phenotypic evolution. Here, we report an advanced recombinant inbred line (RIL) quantitative trait locus mapping panel for the hermaphroditic nematode Caenorhabditis elegans, the C. elegans multiparental experimental evolution (CeMEE) panel. The CeMEE panel, comprising 507 RILs at present, was created by hybridization of 16 wild isolates, experimental evolution for 140-190 generations, and inbreeding by selfing for 13-16 generations. The panel contains 22% of single-nucleotide polymorphisms known to segregate in natural populations, and complements existing C. elegans mapping resources by providing fine resolution and high nucleotide diversity across > 95% of the genome. We apply it to study the genetic basis of two fitness components, fertility and hermaphrodite body size at time of reproduction, with high broad-sense heritability in the CeMEE. While simulations show that we should detect common alleles with additive effects as small as 5%, at gene-level resolution, the genetic architectures of these traits do not feature such alleles. We instead find that a significant fraction of trait variance, approaching 40% for fertility, can be explained by sign epistasis with main effects below the detection limit. In congruence, phenotype prediction from genomic similarity, while generally poor ([Formula: see text]), requires modeling epistasis for optimal accuracy, with most variance attributed to the rapidly evolving chromosome arms.

Experimental Evolution with Caenorhabditis Nematodes

The hermaphroditic nematode Caenorhabditis elegans has been one of the primary model systems in biology since the 1970s, but only within the last two decades has this nematode also become a useful model for experimental evolution. Here, we outline the goals and major foci of experimental evolution with C. elegans and related species, such as C. briggsae and C. remanei, by discussing the principles of experimental design, and highlighting the strengths and limitations of Caenorhabditis as model systems. We then review three exemplars of Caenorhabditis experimental evolution studies, underlining representative evolution experiments that have addressed the: (1) maintenance of genetic variation; (2) role of natural selection during transitions from outcrossing to selfing, as well as the maintenance of mixed breeding modes during evolution; and (3) evolution of phenotypic plasticity and its role in adaptation to variable environments, including host-pathogen coevolution. We conclude by suggesting some future directions for which experimental evolution with Caenorhabditis would be particularly informative.

The Natural Biotic Environment of Caenorhabditis elegans

Organisms evolve in response to their natural environment. Consideration of natural ecological parameters are thus of key importance for our understanding of an organism's biology. Curiously, the natural ecology of the model species Caenorhabditis elegans has long been neglected, even though this nematode has become one of the most intensively studied models in biological research. This lack of interest changed ∼10 yr ago. Since then, an increasing number of studies have focused on the nematode's natural ecology. Yet many unknowns still remain. Here, we provide an overview of the currently available information on the natural environment of C. elegans We focus on the biotic environment, which is usually less predictable and thus can create high selective constraints that are likely to have had a strong impact on C. elegans evolution. This nematode is particularly abundant in microbe-rich environments, especially rotting plant matter such as decomposing fruits and stems. In this environment, it is part of a complex interaction network, which is particularly shaped by a species-rich microbial community. These microbes can be food, part of a beneficial gut microbiome, parasites and pathogens, and possibly competitors. C. elegans is additionally confronted with predators; it interacts with vector organisms that facilitate dispersal to new habitats, and also with competitors for similar food environments, including competitors from congeneric and also the same species. Full appreciation of this nematode's biology warrants further exploration of its natural environment and subsequent integration of this information into the well-established laboratory-based research approaches.

Farming and public goods production in Caenorhabditis elegans populations

The ecological and evolutionary dynamics of populations are shaped by the strategies they use to produce and use resources. However, our understanding of the interplay between the genetic, behavioral, and environmental factors driving these strategies is limited. Here, we report on a Caenorhabditis elegans-Escherichia coli (worm-bacteria) experimental system in which the worm-foraging behavior leads to a redistribution of the bacterial food source, resulting in a growth advantage for both organisms, similar to that achieved via farming. We show experimentally and theoretically that the increased resource growth represents a public good that can benefit all other consumers, regardless of whether or not they are producers. Mutant worms that cannot farm bacteria benefit from farming by other worms in direct proportion to the fraction of farmers in the worm population. The farming behavior can therefore be exploited if it is associated with either energetic or survival costs. However, when the individuals compete for resources with their own type, these costs can result in an increased population density. Altogether, our findings reveal a previously unrecognized mechanism of public good production resulting from the foraging behavior of C. elegans, which has important population-level consequences. This powerful system may provide broad insight into exploration-exploitation tradeoffs, the resultant ecoevolutionary dynamics, and the underlying genetic and neurobehavioral driving forces of multispecies interactions.

Cultivation of Caenorhabditis elegans in Three Dimensions in the Laboratory

The use of genetic model organisms such as Caenorhabditis elegans has led to seminal discoveries in biology over the last five decades. Most of what we know about C. elegans is limited to laboratory cultivation of the nematodes that may not necessarily reflect the environments they normally inhabit in nature. Cultivation of C. elegans in a 3D habitat that is more similar to the 3D matrix that worms encounter in rotten fruits and vegetative compost in nature could reveal novel phenotypes and behaviors not observed in 2D. In addition, experiments in 3D can address how phenotypes we observe in 2D are relevant for the worm in nature. Here, a new method in which C. elegans grows and reproduces normally in three dimensions is presented. Cultivation of C. elegans in Nematode Growth Tube-3D (NGT-3D) can allow us to measure the reproductive fitness of C. elegans strains or different conditions in a 3D environment. We also present a novel method, termed Nematode Growth Bottle-3D (NGB-3D), to cultivate C. elegans in 3D for microscopic analysis. These methods allow scientists to study C. elegans biology in conditions that are more reflective of the environments they encounter in nature. These can help us to understand the overlying evolutionary relevance of the physiology and behavior of C. elegans we observe in the laboratory.

A Large Collection of Novel Nematode-Infecting Microsporidia and Their Diverse Interactions with Caenorhabditis elegans and Other Related Nematodes

Microsporidia are fungi-related intracellular pathogens that may infect virtually all animals, but are poorly understood. The nematode Caenorhabditis elegans has recently become a model host for studying microsporidia through the identification of its natural microsporidian pathogen Nematocida parisii. However, it was unclear how widespread and diverse microsporidia infections are in C. elegans or other related nematodes in the wild. Here we describe the isolation and culture of 47 nematodes with microsporidian infections. N. parisii is found to be the most common microsporidia infecting C. elegans in the wild. In addition, we further describe and name six new species in the Nematocida genus. Our sampling and phylogenetic analysis further identify two subclades that are genetically distinct from Nematocida, and we name them Enteropsectra and Pancytospora. Interestingly, unlike Nematocida, these two genera belong to the main clade of microsporidia that includes human pathogens. All of these microsporidia are horizontally transmitted and most specifically infect intestinal cells, except Pancytospora epiphaga that replicates mostly in the epidermis of its Caenorhabditis host. At the subcellular level in the infected host cell, spores of the novel genus Enteropsectra show a characteristic apical distribution and exit via budding off of the plasma membrane, instead of exiting via exocytosis as spores of Nematocida. Host specificity is broad for some microsporidia, narrow for others: indeed, some microsporidia can infect Oscheius tipulae but not its sister species Oscheius sp. 3, and conversely some microsporidia found infecting Oscheius sp. 3 do not infect O. tipulae. We also show that N. ausubeli fails to strongly induce in C. elegans the transcription of genes that are induced by other Nematocida species, suggesting it has evolved mechanisms to prevent induction of this host response. Altogether, these newly isolated species illustrate the diversity and ubiquity of microsporidian infections in nematodes, and provide a rich resource to investigate host-parasite coevolution in tractable nematode hosts.

Convergent evolution of sperm gigantism and the developmental origins of sperm size variability in Caenorhabditis nematodes

Sperm cells provide essential, if usually diminutive, ingredients to successful sexual reproduction. Despite this conserved function, sperm competition and coevolution with female traits can drive spectacular morphological change in these cells. Here, we characterize four repeated instances of convergent evolution of sperm gigantism in Caenorhabditis nematodes using phylogenetic comparative methods on 26 species. Species at the extreme end of the 50-fold range of sperm-cell volumes across the genus have sperm capable of comprising up to 5% of egg-cell volume, representing severe attenuation of the magnitude of anisogamy. Furthermore, we uncover significant differences in mean and variance of sperm size among genotypes, between sexes, and within and between individuals of identical genotypes. We demonstrate that the developmental basis of sperm size variation, both within and between species, becomes established during an early stage of sperm development at the formation of primary spermatocytes, while subsequent meiotic divisions contribute little further sperm size variability. These findings provide first insights into the developmental determinants of inter- and intraspecific sperm size differences in Caenorhabditis. We hypothesize that life history and ecological differences among species favored the evolution of alternative sperm competition strategies toward either many smaller sperm or fewer larger sperm.