Haplotype diversity of the nematode Pristionchus pacificus on Réunion in the Indian Ocean suggests multiple independent invasions

Multidimensional competition of nematodes affects plastic traits in a beetle ecosystem

Resource competition has driven the evolution of novel polyphenisms in numerous organisms, enhancing fitness in constantly changing environmental conditions. In natural communities, the myriad interactions among diverse species are difficult to disentangle, but the multidimensional microscopic environment of a decaying insect teeming with bacteria and fighting nematodes provides pliable systems to investigate. Necromenic nematodes of the family Diplogastridae live on beetles worldwide, innocuously waiting for their hosts’ deaths to feast on the blooming bacteria. Often, more than one worm species either affiliates with the insect or joins the microbial meal; thus, competition over limited food ensues, and phenotypic plasticity provides perks for species capable of employing polyphenisms. The recently established system of cockchafer Gymnogaster bupthalma and its occasional co-infestation of Pristionchus mayeri and Acrostichus spp. has revealed that these worms will simultaneously utilize two polyphenisms to thrive in a competitive environment. While both genera maintain plastic capacities in mouth form (strictly bacterial-feeding and omnivorous predation) and developmental pathway (direct and arrested development, dauer), P. mayeri employs both when faced with competition from Acrostichus. Here, we took advantage of the malleable system and added a third competitor, model nematode Pristionchus pacificus. Intriguingly, with a third competitor, P. mayeri is quicker to exit dauer and devour available food, while Acrostichus hides in dauer, waiting for the two Pristionchus species to leave the immediate environment before resuming development. Thus, experimental manipulation of short-lived ecosystems can be used to study the roles of polyphenisms in organismal interactions and their potential significance for evolution.

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.

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.

Ecological and morphological differentiation among COI haplotype groups in the plant parasitic nematode species Mesocriconema xenoplax

DNA barcoding with the mitochondrial COI gene reveals distinct haplotype subgroups within the monophyletic and parthenogenetic nematode species, Mesocriconema xenoplax. Biological attributes of these haplotype groups (HG) have not been explored. An analysis of M. xenoplax from 40 North American sites representing both native plant communities and agroecosystems was conducted to identify possible subgroup associations with ecological, physiological, or geographic factors. A dataset of 132 M. xenoplax specimens was used to generate sequences of a 712 bp region of the cytochrome oxidase subunit I gene. Maximum-likelihood and Bayesian phylogenies recognized seven COI HG (≥99/0.99 posterior probability/bootstrap value). Species delimitation metrics largely supported the genetic integrity of the HG. Discriminant function analysis of HG morphological traits identified stylet length, total body length, and stylet knob width as the strongest distinguishing features among the seven groups, with stylet length as the strongest single distinguishing morphological feature. Multivariate analysis identified land cover, ecoregion, and maximum temperature as predictors of 53.6% of the total variation (P = 0.001). Within land cover, HG categorized under “herbaceous,” “woody wetlands,” and “deciduous forest” were distinct in DAPC and RDA analyses and were significantly different (analysis of molecular variance P = 0.001). These results provide empirical evidence for molecular, morphological, and ecological differentiation associated with HG within the monophyletic clade that represents the species Mesocriconema xenoplax.

Flexible reprogramming of Pristionchus pacificus motivation for attacking Caenorhabditis elegans in predator-prey competition

Animals with diverse diets must adapt their food priorities to a wide variety of environmental conditions. This diet optimization problem is especially complex for predators that compete with prey for food. Although predator-prey competition is widespread and ecologically critical, it remains difficult to disentangle predatory and competitive motivations for attacking competing prey. Here, we dissect the foraging decisions of the omnivorous nematode Pristionchus pacificus to reveal that its seemingly failed predatory attempts against Caenorhabditis elegans are actually motivated acts of efficacious territorial aggression. While P. pacificus easily kills and eats larval C. elegans with a single bite, adult C. elegans typically survives and escapes bites. However, non-fatal biting can provide competitive benefits by reducing access of adult C. elegans and its progeny to bacterial food that P. pacificus also eats. We show that the costs and benefits of both predatory and territorial outcomes influence how P. pacificus decides which food goal, prey or bacteria, should guide its motivation for biting. These predatory and territorial motivations impose different sets of rules for adjusting willingness to bite in response to changes in bacterial abundance. In addition to biting, predatory and territorial motivations also influence which search tactic P. pacificus uses to increase encounters with C. elegans. When treated with an octopamine receptor antagonist, P. pacificus switches from territorial to predatory motivation for both biting and search. Overall, we demonstrate that P. pacificus assesses alternate outcomes of attacking C. elegans and flexibly reprograms its foraging strategy to prioritize either prey or bacterial food.

Nematode Interactions on Beetle Hosts Indicate a Role of Mouth-Form Plasticity in Resource Competition

Competition is rampant across kingdoms, arising over potential mates, food resources, and space availability. When faced with opponents, phenotypic plasticity proffers organisms indispensable advantageous strategies to outcompete rivals. This tactic is especially crucial on decaying insect hosts as myriad microbes and numerous nematodes struggle to establish thriving populations and ensure resource availability for future generations. Scarab beetles and their associated nematode symbionts on La Réunion Island have provided exceptional systems to study complicated cross-phylum interactions in soil, and recently we have identified a previously unexplored beetle host, Gymnogaster bupthalma, to be reliably co-infested with diplogastrids Pristionchus mayeri and Acrostichus spp. These nematodes maintain the capacity to plastically respond to environmental conditions by developing disparate mouth forms, a strict bacterial-feeding morph or an omnivorous morph that enables predation on other nematodes. In addition, under stressful settings these worms can enter an arrested development stage called dauer, non-feeding dispersal larvae that resume development into reproducing adults when conditions improve. By investigating this beetle-nematode system in a natural context, we uncovered a novel Pristionchus strategy, wherein dauer dispersal from the carcass is gradual and a reproducing population is sustained. Remarkably, usually preferential-bacterial morph P. mayeri develop as predators in populations dense with competitors.

Pristionchus trametes n. sp. (Diplogastridae) isolated from the mushroom Trametes orientalis in Kyoto, Japan

A new species of Pristionchus was isolated from fruiting bodies of the wood-decaying fungus Trametes orientalis collected from Kyoto, Japan. Attempts to culture it using bacteria, yeast, and freeze-killed wax moth larvae as food or substrate failed. The eurystomatous form of the species was not found in the collected material, and the species is typologically characterized by: its ‘small’ stoma with thin, membrane-like cheilostomatal plates, a small triangular right subventral tooth, thorn-like dorsal tooth, and small left subventral denticles; a short, blunt male tail spike; and a short, conical female tail. Although the posterior probability support was not high (66%), phylogenetic analysis of both small and large ribosomal RNA gene subunits suggests that the species is closely related to P. elegans and P. bucculentus. The new species can be distinguished from those two by its diagnostic characters comprising the stomatal morphology and male and female tail characters.

Nematode biphasic 'boom and bust' dynamics are dependent on host bacterial load while linking dauer and mouth-form polyphenisms

Cross-kingdom interactions involve dynamic processes that shape terrestrial ecosystems and represent striking examples of co-evolution. The multifaceted relationships of entomopathogenic nematodes with their insect hosts and symbiotic bacteria are well-studied cases of co-evolution and pathogenicity. In contrast, microbial interactions in soil after the natural death of insects and other invertebrates are minimally understood. In particular, the turnover and succession of nematodes and bacteria during insect decay have not been well documented - although it represents a rich ecological niche with multiple species interactions. Here, we utilize developmentally plastic nematode Pristionchus pacificus and its associated scarab beetles as models. On La Réunion Island, we collected rhinoceros beetle Oryctes borbonicus, induced death, and placed carcasses in cages both on the island and in a mock-natural environment in the laboratory controlling for high spatial and temporal resolution. Investigating nematode population density and dispersal dynamics, we were able to connect two imperative plasticities, dauer and mouth form. We observed a biphasic 'boom and bust' dispersal dynamic of dauer larvae that corresponds to bacterial load on carcasses but not bacterial type. Strikingly, all post-dauer adults have the predatory mouth form, demonstrating novel intricate interactions on decaying insect hosts. Thus, ecologically relevant survival strategies incorporate critical plastic traits.

Four Pristionchus species associated with two mass-occurring Parafontaria laminata populations

Phoretic nematodes associated with two mass-occurring populations of the millipede Parafontaria laminata were examined, focusing on Pristionchus spp. The nematodes that propagated on dissected millipedes were genotyped using the D2-D3 expansion segments of the 28S ribosomal RNA gene. Four Pristionchus spp. were detected: P. degawai, P. laevicollis, P. fukushimae, and P. entomophagus. Of the four, P. degawai dominated and it was isolated from more than 90% of the millipedes examined. The haplotypes of partial sequences of mitochondrial cytochrome oxidase subunit I examined for Pristionchus spp. and P. degawai showed high haplotype diversity.

Evidence of multiple colonizations as a driver of black fly diversification in an oceanic island

True oceanic islands typically host reduced species diversity together with high levels of endemism, which make these environmental set-ups ideal for the exploration of species diversification drivers. In the present study, we used black fly species (Diptera: Simuliidae) from Reunion Island as a model to highlight the main drivers of insect species diversification in this young and remote volcanic island located in the Southwestern Indian Ocean. Using local and regional (Comoros and Seychelles archipelagos) samples as well as specimens from continental Africa, we tested the likelihood of two distinct scenarios, i.e. multiple colonizations vs. in-situ diversification. For this, posterior odds were used to test whether species from Reunion did form a monophyletic group and we estimated divergence times between species. Three out of the four previously described Reunion black fly species could be sampled, namely Simulium ruficorne, Simulium borbonense and Simulium triplex. The phylogenies based on nuclear and mitochondrial markers showed that S. ruficorne and S. borbonense are the most closely related species. Interestingly, we report a probable mitochondrial introgression between these two species although they diverged almost six million years ago. Finally, we showed that the three Reunion species did not form a monophyletic group, and, combined with the molecular datation, the results indicated that Reunion black fly diversity resulted from multiple colonization events. Thus, multiple colonizations, rather than in-situ diversification, are likely responsible for an important part of black fly diversity found on this young Darwinian island.

Pristionchus nematodes occur frequently in diverse rotting vegetal substrates and are not exclusively necromenic, while Panagrellus redivivoides is found specifically in rotting fruits

The lifestyle and feeding habits of nematodes are highly diverse. Several species of Pristionchus (Nematoda: Diplogastridae), including Pristionchus pacificus, have been reported to be necromenic, i.e. to associate with beetles in their dauer diapause stage and wait until the death of their host to resume development and feed on microbes in the decomposing beetle corpse. We review the literature and suggest that the association of Pristionchus to beetles may be phoretic and not necessarily necromenic. The view that Pristionchus nematodes have a necromenic lifestyle is based on studies that have sought Pristionchus only by sampling live beetles. By surveying for nematode genera in different types of rotting vegetal matter, we found Pristionchus spp. at a similar high frequency as Caenorhabditis, often in large numbers and in feeding stages. Thus, these Pristionchus species may feed in decomposing vegetal matter. In addition, we report that one species of Panagrellus (Nematoda: Panagrolaimidae), Panagrellus redivivoides, is found in rotting fruits but not in rotting stems, with a likely association with Drosophila fruitflies. Based on our sampling and the observed distribution of feeding and dauer stages, we propose a life cycle for Pristionchus nematodes and Panagrellus redivivoides that is similar to that of C. elegans, whereby they feed on the microbial blooms on decomposing vegetal matter and are transported between food patches by coleopterans for Pristionchus spp., fruitflies for Panagrellus redivivoides and isopods and terrestrial molluscs for C. elegans.

Succession and dynamics of Pristionchus nematodes and their microbiome during decomposition of Oryctes borbonicus on La Réunion Island

Insects and nematodes represent the most species-rich animal taxa and they occur together in a variety of associations. Necromenic nematodes of the genus Pristionchus are found on scarab beetles with more than 30 species known from worldwide samplings. However, little is known about the dynamics and succession of nematodes and bacteria during the decomposition of beetle carcasses. Here, we study nematode and bacterial succession of the decomposing rhinoceros beetle Oryctes borbonicus on La Réunion Island. We show that Pristionchus pacificus exits the arrested dauer stage seven days after the beetles´ deaths. Surprisingly, new dauers are seen after 11 days, suggesting that some worms return to the dauer stage after one reproductive cycle. We used high-throughput sequencing of the 16S rRNA genes of decaying beetles, beetle guts and nematodes to study bacterial communities in comparison to soil. We find that soil environments have the most diverse bacterial communities. The bacterial community of living and decaying beetles are more stable but one single bacterial family dominates the microbiome of decaying beetles. In contrast, the microbiome of nematodes is relatively similar even across different families. This study represents the first characterization of the dynamics of nematode-bacterial interactions during the decomposition of insects.

Variation in rates of spontaneous male production within the nematode species Pristionchus pacificus supports an adaptive role for males and outcrossing

Background

The nematode species Pristionchus pacificus has an androdioecious mating system in which populations consist of self-fertilizing hermaphrodites and relatively few males. The prevalence of males in such a system is likely to depend on the relative pros and cons of outcrossing. While outcrossing generates novel allelic combinations and can therefore increase adaptive potential, it may also disrupt the potentially beneficial consequences of repeated generations of selfing. These include purging of deleterious alleles, inheritance of co-adapted allele complexes, improved hermaphrodite fitness and increased population growth. Here we use experimental and population genetic approaches to test hypotheses relating to male production and outcrossing in laboratory and natural populations of P. pacificus sampled from the volcanic island of La Réunion.

Results

We find a significant interaction between sampling locality and temperature treatment influencing rates of spontaneous male production in the laboratory. While strains isolated at higher altitude, cooler localities produce a higher proportion of male offspring at 25 °C relative to 20 or 15 °C, the reverse pattern is seen in strains isolated from warmer, low altitude localities. Linkage disequilibrium extends across long physical distances, but fails to approach levels reported for the partially selfing nematode species Caenorhabditis elegans. Finally, we find evidence for admixture between divergent genetic lineages.

Conclusions

Elevated rates of laboratory male generation appear to occur under environmental conditions which differ from those experienced by populations in nature. Such elevated male generation may result in higher outcrossing rates, hence driving increased effective recombination and the creation of potentially adaptive novel allelic combinations. Patterns of linkage disequilibrium decay support selfing as the predominant reproductive strategy in P. pacificus. Finally, despite the potential for outcrossing depression, our results suggest admixture has occurred between distinct genetic lineages since their independent colonization of the island, suggesting outcrossing depression may not be uniform in this species.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-017-0873-7) contains supplementary material, which is available to authorized users.

Sudhausia floridensis n. sp. (Nematoda: Diplogastridae) isolated from Onthophagus tuberculifrons (Coleoptera: Scarabaeidae) from Florida, USA

Sudhausia floridensis n. sp., a phoretic associate of a dung beetle, Onthophagus tuberculifrons, is described based on its typological characters and molecular profiles of part of the small subunit ribosomal RNA (rRNA) gene, D2-D3 expansion segments of the large subunit of rRNA gene, as well as part of the mitochondrial cytochrome oxidase subunit I (mtCOI) gene. Adult beetles collected from the cattle unit at the University of Florida, Gainesville, FL, USA were dissected and placed on a water agar plate to allow nematode propagation. The new species was isolated from a culture plate several weeks later, and subcultured to establish a laboratory strain. Sudhausia floridensis n. sp. is typologically very similar to S. aristotokia, but could be distinguished from it by the shape of the tail tip of the male, and presence vs absence of a terminal notch in the bursal velum. Molecularly, S. floridensis n. sp. is close to S. crassa, regardless of its morphological similarity to S. aristotokia, but it is distinguished from its African congeners S. aristotokia and S. crassa by 28 and 12 bp differences in SSU, respectively.

Hybrid crosses and the genetic basis of interspecific divergence in lifespan in Pristionchus nematodes

Characterizing the genetic basis of among-species variation in lifespan is a major goal of evolutionary gerontology research, but the very feature that defines separate species - the inability to interbreed - makes achieving this goal impractical, if not impossible, for most taxa. Pristionchus nematodes provide an intriguing system for tackling this problem, as female lifespan varies among species that can be crossed to form viable (although infertile) hybrids. By conducting reciprocal crosses among three species - two dioecious (long-lived Pristionchus exspectatus and short-lived Pristionchus arcanus) and one androdioecious (short-lived Pristionchus pacificus) - we found that female lifespan was long for all hybrids, consistent with the hypothesis that the relatively short lifespans seen for P. pacificus hermaphrodites and P. arcanus females are caused by independent, recessive alleles that are masked in hybrid genomes. Cross-direction had a small effect on survivorship for crosses involving P. exspectatus, indicating that nuclear-mitochondrial interactions may also influence Pristionchus longevity. Our findings suggest that long lifespan in P. exspectatus reflects the realization of an ancestral potential for extended longevity in the P. pacificus species complex. This work demonstrates the utility of interspecific hybrids for ageing research and provides a foundation for future work on the genetic architecture of interspecific lifespan variation.

Life History Responses and Gene Expression Profiles of the Nematode Pristionchus pacificus Cultured on Cryptococcus Yeasts

Nematodes, the earth's most abundant metazoa are found in all ecosystems. In order to survive in diverse environments, they have evolved distinct feeding strategies and they can use different food sources. While some nematodes are specialists, including parasites of plants and animals, others such as Pristionchus pacificus are omnivorous feeders, which can live on a diet of bacteria, protozoans, fungi or yeast. In the wild, P. pacificus is often found in a necromenic association with beetles and is known to be able to feed on a variety of microbes as well as on nematode prey. However, in laboratory studies Escherichia coli OP50 has been used as standard food source, similar to investigations in Caenorhabditis elegans and it is unclear to what extent this biases the obtained results and how relevant findings are in real nature. To gain first insight into the variation in traits induced by a non-bacterial food source, we study Pristionchus-fungi interactions under laboratory conditions. After screening different yeast strains, we were able to maintain P. pacificus for at least 50-60 generations on Cryptococcus albidus and Cryptococcus curvatus. We describe life history traits of P. pacificus on both yeast strains, including developmental timing, survival and brood size. Despite a slight developmental delay and problems to digest yeast cells, which are both reflected at a transcriptomic level, all analyses support the potential of Cryptococcus strains as food source for P. pacificus. In summary, our work establishes two Cryptococcus strains as alternative food source for P. pacificus and shows change in various developmental, physiological and morphological traits, including the transcriptomic profiles.

A locus in Pristionchus pacificus that is responsible for the ability to give rise to fertile offspring at higher temperatures

Temperature is a stress factor that varies temporally and spatially, and can affect the fitness of cold-blooded organisms, leading to a loss of reproductive output; however, little is understood about the genetics behind the long-term response of organisms to temperature. Here, we approach this problem in the model nematode Pristionchus pacificus by utilising a large collection of natural isolates with diverse phenotypes. From this collection we identify two strains, one from California that can give rise to fertile offspring up to 28°C and one from Japan that is fertile up to 30°C. We show that the optimum temperature and the upper temperature limit for fertility is shifted higher in the Japanese strain suggesting that there is a mechanism that controls the temperature response of fertility across a range of temperatures. By crossing the two strains, and using genetic mapping, we identify a region on chromosome V that is responsible for maintaining fertility at higher temperatures. Thus, we conclude that fitness of P. pacificus at high temperature is under genetic control, suggesting that it could be subject to natural selection.

Phenotypic plasticity and developmental innovations in nematodes

Developmental plasticity has been implicated as a facilitator for phenotypic diversification, but the molecular mechanisms controlling it are largely unknown. We review recent comparative analyses in non-Caenorhabditis nematodes that display polyphenisms in larval development, mouth morphology and reproductive mode. Some of the challenges ahead will be to connect how these phenotypic traits are linked to each other at the molecular level, and at the ecological level. This will require sampling of several nematode species, the characterization of their ecology and the employment of both classical genetics and recently developed technological advances, such as genome editing.

Functional Conservation and Divergence of daf-22 Paralogs in Pristionchus pacificus Dauer Development

Small-molecule signaling in nematode dauer formation has emerged as a major model to study chemical communication in development and evolution. Developmental arrest as nonfeeding and stress-resistant dauer larvae represents the major survival and dispersal strategy. Detailed studies in Caenorhabditis elegans and Pristionchus pacificus revealed that small-molecule communication changes rapidly in evolution resulting in extreme structural diversity of small-molecule compounds. In C. elegans, a blend of ascarosides constitutes the dauer pheromone, whereas the P. pacificus dauer pheromone includes additional paratosides and integrates building blocks from diverse primary metabolic pathways. Despite this complexity of small-molecule structures and functions, little is known about the biosynthesis of small molecules in nematodes outside C. elegans Here, we show that the genes encoding enzymes of the peroxisomal β-oxidation pathway involved in small-molecule biosynthesis evolve rapidly, including gene duplications and domain switching. The thiolase daf-22, the most downstream factor in C. elegans peroxisomal β-oxidation, has duplicated in P. pacificus, resulting in Ppa-daf-22.1, which still contains the sterol-carrier-protein (SCP) domain that was lost in C. elegans daf-22, and Ppa-daf-22.2. Using the CRISPR/Cas9 system, we induced mutations in both P. pacificus daf-22 genes and identified an unexpected complexity of functional conservation and divergence. Under well-fed conditions, ascaroside biosynthesis proceeds exclusively via Ppa-daf-22.1 In contrast, starvation conditions induce Ppa-daf-22.2 activity, resulting in the production of a specific subset of ascarosides. Gene expression studies indicate a reciprocal up-regulation of both Ppa-daf-22 genes, which is, however, independent of starvation. Thus, our study reveals an unexpected functional complexity of dauer development and evolution.

Rhabditidoides humicolus n. sp. associated with arthropods from rotting palm tissue in Florida, USA

Rhabditidoides humicolus n. sp. is described and illustrated from arthropods associated with decaying tissue from the crown shaft of a living spindle palm, Hyophorbe verschaffeltii, in southern Florida, USA. In addition to its generic character, i.e., the arrangement of male genital papillae, ⟨v1, v2, v3d, CO, v4, (ad, v5, ph, v6), (pd, v7)⟩, the new species is characterised by its small stomatal flaps, a secretory pore-like opening, a pair of deirids, two pairs of post-deirids and small subventral vulval papillae located just anterior to the vulva. Besides those newly found characters, there are only a few typological differences between R. humicolus n. sp. and several previously described species in the genus. However, based on the biological characters, e.g., gonochoristic reproduction, association as dauers with the crane fly, Limonia (Rhipidia) schwarzi (Diptera: Limoniidae), millipedes, an immature cockroach, and staphylinid beetle adults, and distribution in southern Florida, the new species was considered to be different from others in the genus.

Characterisation of, and entomopathogenic studies on, Pristionchus aerivorus (Cobb in Merrill & Ford, 1916) Chitwood, 1937 (Rhabditida: Diplogastridae) from North Carolina, USA

During a survey of entomopathogenic nematodes in North Carolina, USA, aPristionchusspecies was recovered using theGalleriabait method. Morphological studies with light microscopy and scanning electron microscopy, mating tests with reference strains, as well as molecular analyses of the near-full-length small subunit rRNA gene (18S) and D2-D3 expansion segments of the large subunit rRNA gene (28S) identified this isolate asPristionchus aerivorus. ExposedGallerialarvae were killed within 48 h and high numbers of nematodes were recovered from the cadavers about 5 days later. Preliminary tests revealed that this nematode is capable of infecting at least two other insect species (Helicoverpa zeaandTenebrio molitor) under laboratory conditions. The status of the genusChroniodiplogasteris discussed and confirmed as a junior synonym ofPristionchusbased on morphological observation and molecular phylogenetic analysis.

A review of the taxonomy, phylogeny, distribution and co-evolution of Schistonchus Cobb, 1927 with proposal of Ficophagus n. gen. and Martininema n. gen. (Nematoda: Aphelenchoididae)

The purposes of this paper are to clarify the taxonomic status of the fig-pollinating wasp associateSchistonchussensu lato(Nematoda: Aphelenchoididae) and to suggest directions for future research on the systematics, life history and ecology of the group. Molecular phylogenetic analyses suggest thatSchistonchus s.l.is polyphyletic, and the composition of the three major clades is outlined, together with information on nematode morphology, plant host species, associated pollinating wasp species, and distribution. Biological information and collection data is presented forSchistonchus s.l.fromFicussycones (Moracea) in Africa, Australia, Asia and Central America, and its putative phylogeny is discussed based on molecular and morphological evidence. Both wasps and figs are millions of years old and have worldwide distribution in tropical areas,i.e., opportunities forSchistonchus s.l.-like nematodes to have evolved could have occurred more than once. In addition, figs and their pollinating wasps have variable life histories, which could have provided opportunities forSchistonchus s.l.to also develop different life histories. However, these histories occur inside fig sycones and in association with wasps, which has apparently led to evolutionary convergence and extreme morphological conservatism. Diagnostic characters and their states, derived from examination of described species and morphospecies ofSchistonchus s.l.and informed by molecular phylogenetic inferences, are discussed and illustrated.Schistonchus sensu strictois redefined, andFicophagusn. gen. andMartinineman. gen. are proposed.Schistonchus s.s.is morphologically characterised by having the excretory pore opening in the region of, or posterior to, the metacorpus;Ficophagusn. gen. by having the excretory pore opening very near the cephalic region; andMartinineman. gen. by having it opening at the anterior end of the metacorpus. Several species ofSchistonchus s.s.have a labial disc, but there is no evidence of this in eitherFicophagusn. gen. orMartinineman. gen.

How to become a parasite without sex chromosomes: a hypothesis for the evolution of Strongyloides spp. and related nematodes

Parasitic lifestyles evolved many times independently. Just within the phylum Nematoda animal parasitism must have arisen at least four times. Switching to a parasitic lifestyle is expected to lead to changes in various life history traits including reproductive strategies. Parasitic nematode worms of the genus Strongyloides represent an interesting example to study these processes because they are still capable of forming facultative free-living generations in between parasitic ones. The parasitic generation consists of females only, which reproduce parthenogenetically. The sex in the progeny of the parasitic worms is determined by environmental cues, which control a, presumably ancestral, XX/XO chromosomal sex determining system. In some species the X chromosome is fused with an autosome and one copy of the X-derived sequences is removed by sex-specific chromatin diminution in males. Here I propose a hypothesis for how today's Strongyloides sp. might have evolved from a sexual free-living ancestor through dauer larvae forming free-living and facultative parasitic intermediate stages.

Preferential host switching and codivergence shaped radiation of bark beetle symbionts, nematodes of Micoletzkya (Nematoda: Diplogastridae)

Host-symbiont systems are of particular interest to evolutionary biology because they allow testable inferences of diversification processes while also providing both a historical basis and an ecological context for studies of adaptation. Our investigations of bark beetle symbionts, predatory nematodes of the genus Micoletzkya, have revealed remarkable diversity of the group along with a high level of host specificity. Cophylogenetic analyses suggest that evolution of the nematodes was largely influenced by the evolutionary history of beetles. The diversification of the symbionts, however, could not be attributed to parallel divergence alone; our results indicate that adaptive radiation of the nematodes was shaped by preferential host shifts among closely related beetles along with codivergence. Whereas ecological and geographic isolation have played a major role in the diversification of Micoletzkya at shallow phylogenetic depths, adaptations towards related hosts have played a role in shaping cophylogenetic structure at a larger evolutionary scale.

Environmental variables explain genetic structure in a beetle-associated nematode

The distribution of a species is a complex expression of its ecological and evolutionary history and integrating population genetic, environmental, and ecological data can provide new insights into the effects of the environment on the population structure of species. Previous work demonstrated strong patterns of genetic differentiation in natural populations of the hermaphroditic nematode Pristionchus pacificus in its La Réunion Island habitat, but gave no clear understanding of the role of the environment in structuring this variation. Here, we present what is to our knowledge the first study to statistically evaluate the role of the environment in shaping the structure and distribution of nematode populations. We test the hypothesis that genetic structure in P. pacificus is influenced by environmental variables, by combining population genetic analyses of microsatellite data from 18 populations and 370 strains, with multivariate statistics on environmental data, and species distribution modelling. We assess and quantify the relative importance of environmental factors (geographic distance, altitude, temperature, precipitation, and beetle host) on genetic variation among populations. Despite the fact that geographic populations of P. pacificus comprise vast genetic diversity sourced from multiple ancestral lineages, we find strong evidence for local associations between environment and genetic variation. Further, we show that significantly more genetic variation in P. pacificus populations is explained by environmental variation than by geographic distances. This supports a strong role for environmental heterogeneity vs. genetic drift in the divergence of populations, which we suggest may be influenced by adaptive forces.

A developmental switch coupled to the evolution of plasticity acts through a sulfatase

Developmental plasticity has been suggested to facilitate phenotypic diversity, but the molecular mechanisms underlying this relationship are little understood. We analyzed a feeding dimorphism in Pristionchus nematodes whereby one of two alternative adult mouth forms is executed after an irreversible developmental decision. By integrating developmental genetics with functional tests in phenotypically divergent populations and species, we identified a regulator of plasticity, eud-1, that acts in a developmental switch. eud-1 mutations eliminate one mouth form, whereas overexpression of eud-1 fixes it. EUD-1 is a sulfatase that acts dosage dependently, is necessary and sufficient to control the sexual dimorphism of feeding forms, and has a conserved function in Pristionchus evolution. It is epistatic to known signaling cascades and results from lineage-specific gene duplications. EUD-1 thus executes a developmental switch for morphological plasticity in the adult stage, showing that regulatory pathways can evolve by terminal addition of new genes.

Opposing forces of A/T-biased mutations and G/C-biased gene conversions shape the genome of the nematode Pristionchus pacificus

Base substitution mutations are a major source of genetic novelty and mutation accumulation line (MAL) studies revealed a nearly universal AT bias in de novo mutation spectra. While a comparison of de novo mutation spectra with the actual nucleotide composition in the genome suggests the existence of general counterbalancing mechanisms, little is known about the evolutionary and historical details of these opposing forces. Here, we correlate MAL-derived mutation spectra with patterns observed from population resequencing. Variation observed in natural populations has already been subject to evolutionary forces. Distinction between rare and common alleles, the latter of which are close to fixation and of presumably older age, can provide insight into mutational processes and their influence on genome evolution. We provide a genome-wide analysis of de novo mutations in 22 MALs of the nematode Pristionchus pacificus and compare the spectra with natural variants observed in resequencing of 104 natural isolates. MALs show an AT bias of 5.3, one of the highest values observed to date. In contrast, the AT bias in natural variants is much lower. Specifically, rare derived alleles show an AT bias of 2.4, whereas common derived alleles close to fixation show no AT bias at all. These results indicate the existence of a strong opposing force and they suggest that the GC content of the P. pacificus genome is in equilibrium. We discuss GC-biased gene conversion as a potential mechanism acting against AT-biased mutations. This study provides insight into genome evolution by combining MAL studies with natural variation.

Natural variation in chemosensation: lessons from an island nematode

All organisms must interact with their environment, responding in behavioral, chemical, and other ways to various stimuli throughout their life cycles. Characterizing traits that directly represent an organism's ability to sense and react to their environment provides useful insight into the evolution of life-history strategies. One such trait for the nematode Pristionchus pacificus, chemosensation, is involved in navigation to beetle hosts. Essential for the survival of the nematode, chemosensory behavior may be subject to variation as nematodes discriminate among chemical cues to complete their life cycle. We examine this hypothesis using natural isolates of P. pacificus from La Réunion Island. We select strains from a variety of La Réunion beetle hosts and geographic locations and examine their chemoattraction response toward organic compounds, beetle washes, and live beetles. We find that nematodes show significant differences in their response to various chemicals and are able to chemotax to live beetles in a novel assay. Further, strains can discriminate among different cues, showing more similar responses toward beetle washes than to organic compounds in cluster analyses. However, we find that variance in chemoattraction response is not significantly associated with temperature, location, or beetle host. Rather, strains show a more concerted response toward compounds they most likely directly encounter in the wild. We suggest that divergence in odor-guided behavior in P. pacificus may therefore have an important ecological component.

Two new species of Pristionchus (Nematoda: Diplogastridae) support the biogeographic importance of Japan for the evolution of the genus Pristionchus and the model system P. pacificus

Pristionchus pacificus Sommer, Carta, Kim, and Sternberg, 1996 is an important model organism in evolutionary biology that integrates developmental biology with ecology and population genetics. This species is part of a sub-complex of the genus Pristionchus that is considered to have originated in East Asia. Here, we describe two new species of Pristionchus, P. maxplancki and P. quartusdecimus, which were isolated from beetles in Japan, supporting the hypothesis that a region including Japan is the origin of diversification of the P. pacificus species complex. Phytogeny inferred from a partial small subunit rRNA gene and 25 ribosomal protein genes shows P. maxplancki to be the closest known outgroup to a triad of sibling species, including P. pacificus. Pristionchus quartusdecimus is a putative outgroup to the P. pacificus species complex, supporting a more ancient origin of Pristionchus species in the region. Species diagnoses are based on morphological and molecular characters, in addition to reproductive isolation for P. maxplancki. Members of the P. pacificus species complex as well as P. quartusdecimus are distinguished by stegostomatal structures, male genital papilla arrangement, and gubernaculum shape. The discovery of a new member of the P. pacificus species complex allows greater precision in polarizing and reconstructing ancestral states in the comparative model system centering on P. pacificus. Together with previous reports, these findings support an important biogeographic role of Japan in the evolution of the genus Pristionchus and the P. pacificus species complex, especially the associated phenotypic evolution of mouth morphology.

Feeding plasticity in the nematode Pristionchus pacificus is influenced by sex and social context and is linked to developmental speed

The increasing evidence for a role of developmental plasticity in evolution offers exciting prospects for testing interactions between ecological and developmental genetic processes. Recent advances with the model organism Pristionchus pacificus have provided inroads to a mechanistic understanding of a developmental plasticity. The developmental plasticity of P. pacificus comprises two discontinuous adult mouth-forms, a stenostomatous ("narrow mouthed") and a eurystomatous ("wide mouthed") form, the latter of which is structurally more complex and associated with predatory feeding. Both forms are consistently present in populations, but fundamental properties guiding fluctuations in their appearance have been poorly understood. Here, we provide a systematic characterization of the mouth plasticity in P. pacificus, quantifying a strong sexual dimorphism and revealing that, in an inbred genetic background, maternal phenotype is linked to that of male offspring. Furthermore, cues from conspecifics influenced the developmental decision in juvenile nematodes. Separating individuals from a population resulted in a lower eurystomatous frequency, which decreased incrementally with earlier isolation. Finally, the time to the reproductively mature stage was, in the presence of an abundant bacterial food supply, less for stenostomatous than for eurystomatous individuals, suggesting the potential for a fitness trade-off between developmental time and breadth of diet. This study provides a baseline understanding of the mouth dimorphism in P. pacificus as a necessary reference point for comparative analysis.

New role for DCR-1/dicer in Caenorhabditis elegans innate immunity against the highly virulent bacterium Bacillus thuringiensis DB27

Bacillus thuringiensis produces toxins that target invertebrates, including Caenorhabditis elegans. Virulence of Bacillus strains is often highly specific, such that B. thuringiensis strain DB27 is highly pathogenic to C. elegans but shows no virulence for another model nematode, Pristionchus pacificus. To uncover the underlying mechanisms of the differential responses of the two nematodes to B. thuringiensis DB27 and to reveal the C. elegans defense mechanisms against this pathogen, we conducted a genetic screen for C. elegans mutants resistant to B. thuringiensis DB27. Here, we describe a B. thuringiensis DB27-resistant C. elegans mutant that is identical to nasp-1, which encodes the C. elegans homolog of the nuclear-autoantigenic-sperm protein. Gene expression analysis indicated a substantial overlap between the genes downregulated in the nasp-1 mutant and targets of C. elegans dcr-1/Dicer, suggesting that dcr-1 is repressed in nasp-1 mutants, which was confirmed by quantitative PCR. Consistent with this, the nasp-1 mutant exhibits RNA interference (RNAi) deficiency and reduced longevity similar to those of a dcr-1 mutant. Building on these surprising findings, we further explored a potential role for dcr-1 in C. elegans innate immunity. We show that dcr-1 mutant alleles deficient in microRNA (miRNA) processing, but not those deficient only in RNAi, are resistant to B. thuringiensis DB27. Furthermore, dcr-1 overexpression rescues the nasp-1 mutant's resistance, suggesting that repression of dcr-1 determines the nasp-1 mutant's resistance. Additionally, we identified the collagen-encoding gene col-92 as one of the downstream effectors of nasp-1 that play an important role in resistance to DB27. Taken together, these results uncover a previously unknown role for DCR-1/Dicer in C. elegans antibacterial immunity that is largely associated with miRNA processing.

The nematode Pristionchus pacificus as a model system for integrative studies in evolutionary biology

Comprehensive studies of evolution have historically been hampered by the division among disciplines. Now, as biology moves towards an '-omics' era, it is more important than ever to tackle the evolution of function and form by considering all those research areas involved in the regulation of phenotypes. Here, we review recent attempts to establish the nematode Pristionchus pacificus as a model organism that allows integrative studies of development and evo-devo, with ecology and population genetics. Originally developed for comparative study with the nematode Caenorhabditis elegans, P. pacificus provided insight into developmental pathways including dauer formation, vulva and gonad development, chemosensation, innate immunity and neurobiology. Its subsequent discovery across a wide geographic distribution in association with scarab beetles enabled its evaluation in a biogeographic context. Development of an evolutionary field station on La Réunion Island, where P. pacificus is present in high abundance across a number of widespread habitat types, allows examination of the microfacets of evolution - processes of natural selection, adaptation and drift among populations can now be examined in this island setting. The combination of laboratory-based functional studies with fieldwork in P. pacificus has the long-term prospective to provide both proximate (mechanistic) and ultimate (evolutionary and ecological) causation and might therefore help to overcome the long-term divide between major areas in biology.

Unraveling the evolutionary history of the nematode Pristionchus pacificus: from lineage diversification to island colonization

The hermaphroditic nematode Pristionchus pacificus is a model organism with a range of fully developed genetic tools. The species is globally widespread and highly diverse genetically, consisting of four major independent lineages (lineages A, B, C, and D). Despite its young age (∼2.1 Ma), volcanic La Réunion Island harbors all four lineages. Ecological and population genetic research studies suggest that this diversity is due to repeated independent island colonizations by P. pacificus. Here, we use model-based statistical methods to rigorously test hypotheses regarding the evolutionary history of P. pacificus. First, we employ divergence analyses to date diversification events among the four “world” lineages. Next, we examine demographic properties of a subset of four populations (“a”, “b”, “c”, and “d”), present on La Réunion Island. Finally, we use the results of the divergence and demographic analyses to inform a modeling-based approximate Bayesian computation (ABC) approach, where we test hypotheses about the order and timing of establishment of the Réunion populations. Our dating estimates place the recent common ancestor of P. pacificus lineages at nearly 500,000 generations past. Our demographic analysis supports recent (<150,000 generations) spatial expansion for the island populations, and our ABC approach supports c>a>b>d as the most likely colonization order of the island populations. Collectively, our study comprehensively improves previous inferences about the evolutionary history of P. pacificus.

Evolutionary history of nematodes associated with sweat bees

Organisms that live in close association with other organisms make up a large part of the world's diversity. One driver of this diversity is the evolution of host-species specificity, which can occur via reproductive isolation following a host-switch or, given the correct circumstances, via cospeciation. In this study, we explored the diversity and evolutionary history of Acrostichus nematodes that are associated with halictid bees in North America. First, we conducted surveys of bees in Virginia, and found six halictid species that host Acrostichus. To test the hypothesis of cospeciation, we constructed phylogenetic hypotheses of Acrostichus based on three genes. We found Acrostichus puri and Acrostichus halicti to be species complexes comprising cryptic, host-specific species. Although several nodes in the host and symbiont phylogenies were congruent and tests for cospeciation were significant, the host's biogeography, the apparent patchiness of the association across the host's phylogeny, and the amount of evolution in the nematode sequence suggested a mixture of cospeciation, host switching, and extinction events instead of strict cospeciation. Cospeciation can explain the relationships between Ac. puri and its augochlorine hosts, but colonization of Halictus hosts is more likely than cospeciation. The nematodes are vertically transmitted, but sexual transmission is also likely. Both of these transmission modes may explain host-species specificity and congruent bee and nematode phylogenies. Additionally, all halictid hosts come from eusocial or socially polymorphic lineages, suggesting that sociality may be a factor in the suitability of hosts for Acrostichus.

Tandem-repeat patterns and mutation rates in microsatellites of the nematode model organism Pristionchus pacificus

Modern evolutionary biology requires integrative approaches that combine life history, population structure, ecology, and development. The nematode Pristionchus pacificus has been established as a model system in which these aspects can be studied in one organism. P. pacificus has well-developed genetic, genomic, and transgenic tools and its ecologic association with scarab beetles is well described. A recent study provided first mutation rate estimates based on mitochondrial genome sequencing and mutation accumulation line experiments that help resolve rather ancient evolutionary branches. Here, we analyzed the tandem-repeat pattern and studied spontaneous mutation rates for microsatellite markers by using the previously generated mutation accumulation lines. We found that 0.59%–3.83% of the genome is composed of short tandem repeats. We developed 41 microsatellite markers, randomly chosen throughout the genome and analyzed them in 82 mutation accumulation lines after 142 generations. A total of 31 mutations were identified in these lines. There was a strong correlation between allele size and mutation rate in P. pacificus, similar to Caenorhabditis elegans. In contrast to C. elegans, however, there is no evidence for a bias toward multistep mutations. The mutation spectrum of microsatellite loci in P. pacificus shows more insertions than deletions, indicating a tendency toward lengthening, a process that might have contributed to the increase in genome size. The mutation rates obtained for individual microsatellite markers provide guidelines for divergence time estimates that can be applied in P. pacificus next-generation sequencing approaches of wild isolates.

Pristionchus uniformis, should I stay or should I go? Recent host range expansion in a European nematode

Pristionchus pacificus has been developed as a model system in evolutionary developmental biology, evolutionary ecology, and population genetics. This species has a well-known ecological association with scarab beetles. Generally, Pristionchus nematodes have a necromenic association with their beetle hosts. Arrested dauer larvae invade the insect and wait for the host's death to resume development. Only one Pristionchus species is known to frequently associate with a non-scarab beetle. Pristionchus uniformis has been isolated from the chrysomelid Leptinotarsa decemlineata, also known as the Colorado potato beetle, in Europe and North America, but is also found on scarab beetles. This unusual pattern of association with two unrelated groups of beetles on two continents requires the involvement of geographical and host range expansion events. Here, we characterized a collection of 81 P. uniformis isolates from North America and Europe and from both scarab beetles and L. decemlineata. We used population genetic and phylogenetic analyses of the mitochondrial gene nd2 to reconstruct the genetic history of P. uniformis and its beetle association. Olfactory tests on beetles chemical extracts showed that P. uniformis has a unique chemoattractive profile toward its beetle hosts. Our results provide evidence for host range expansion through host-switching events in Europe where P. uniformis was originally associated with scarab beetles and the nematode's subsequent invasion of North America.

Description of three Pristionchus species (Nematoda: Diplogastridae) from Japan that form a cryptic species complex with the model organism P. pacificus

Three new species of Pristionchus (P. exspectatus, P. arcanus, and P. japonicus) are described from Japan. They are morphologically similar, with P. exspectatus and P. arcanus being almost indistinguishable from the model organism P. pacificus. Reproductive isolation, namely the inability to produce interfertile F1 hybrids, separates all species pairs in the species complex. Additionally, all three new species are distinguished from P. pacificus Sommer, Carta, Kim, and Sternberg, 1996 by having a gonochoristic instead of hermaphroditic mode of reproduction. In addition to its reproductive isolation, P. japonicus is distinct from other Pristionchus species by its arrangement of genital papillae. All species in the complex are separated from each other by molecular sequence divergence, as indicated by analysis of 27 nuclear protein-coding genes and unique sequences of the small subunit ribosomal RNA gene. The identification of a species complex that includes P. pacificus is invaluable for studies of population genetics, speciation, and macroevolution, particularly the evolution of hermaphroditism in the genus.

Hormone signaling and phenotypic plasticity in nematode development and evolution

Phenotypic plasticity refers to the ability of an organism to adopt different phenotypes depending on environmental conditions. In animals and plants, the progression of juvenile development and the formation of dormant stages are often associated with phenotypic plasticity, indicating the importance of phenotypic plasticity for life-history theory. Phenotypic plasticity has long been emphasized as a crucial principle in ecology and as facilitator of phenotypic evolution. In nematodes, several examples of phenotypic plasticity have been studied at the genetic and developmental level. In addition, the influence of different environmental factors has been investigated under laboratory conditions. These studies have provided detailed insight into the molecular basis of phenotypic plasticity and its ecological and evolutionary implications. Here, we review recent studies on the formation of dauer larvae in Caenorhabditis elegans, the evolution of nematode parasitism and the generation of a novel feeding trait in Pristionchus pacificus. These examples reveal a conserved and co-opted role of an endocrine signaling module involving the steroid hormone dafachronic acid. We will discuss how hormone signaling might facilitate life-history and morphological evolution.

Nematode model systems in evolution and development

The free-living nematode Caenorhabditis elegans is one of the most important model organisms in all areas of modern biology. Using the knowledge about C. elegans as a baseline, nematodes are now intensively studied in evolution and development. Evolutionary developmental biology or for short, 'evo-devo' has been developed as a new research discipline during the last two decades to investigate how changes in developmental processes and mechanisms result in the modification of morphological structures and phenotypic novelty. In this article, we review the concepts that make nematode evo-devo a successful approach to evolutionary biology. We introduce selected model systems for nematode evo-devo and provide a detailed discussion of four selected case studies. The most striking finding of nematode evo-devo is the magnitude of developmental variation in the context of a conserved body plan. Detailed investigation of early embryogenesis, gonad formation, vulva development, and sex determination revealed that molecular mechanisms evolve rapidly, often in the context of a conserved body plan. These studies highlight the importance of developmental systems drift and neutrality in evolution.

Multi locus analysis of Pristionchus pacificus on La Réunion Island reveals an evolutionary history shaped by multiple introductions, constrained dispersal events and rare out-crossing

Pristionchus pacificus, recently established as a model organism in evolutionary biology, is a cosmopolitan nematode that has a necromenic association with scarab beetles. The diverse array of host beetle species and habitat types occupied by P. pacificus make it a good model for investigating local adaptation to novel environments. Presence of P. pacificus on La Réunion Island, a young volcanic island with a dynamic geological history and a wide variety of ecozones, facilitates such investigation in an island biogeographic setting. Microsatellite data from 20 markers and 223 strains and mitochondrial sequence data from 272 strains reveal rich genetic diversity among La Réunion P. pacificus isolates, shaped by differentially timed introductions from diverse sources and in association with different beetle species. Distinctions between volcanic zones and between arid western and wet eastern climatic zones have likely limited westward dispersal of recently colonized lineages and maintained a genetic distinction between eastern and western clades. The highly selfing lifestyle of P. pacificus contributes to the strong fine-scale population structure detected, with each beetle host harbouring strongly differentiated assemblages of strains. Periodic out-crossing generates admixture between genetically diverse lineages, creating a diverse array of allelic combinations likely to increase the evolutionary potential of the species and facilitate adaptation to local environments and beetle hosts.

Host-finding behaviour in the nematode Pristionchus pacificus

Costs and benefits of foraging have been studied in predatory animals. In nematodes, ambushing or cruising behaviours represent adaptations that optimize foraging strategies for survival and host finding. A behaviour associated with host finding of ambushing nematode dauer juveniles is a sit-and-wait behaviour, otherwise known as nictation. Here, we test the function of nictation by relating occurrence of nictation in Pristionchus pacificus dauer juveniles to the ability to attach to laboratory host Galleria mellonella. We used populations of recently isolated and mutagenized laboratory strains. We found that nictation can be disrupted using a classical forward genetic approach and characterized two novel nictation-defective mutant strains. We identified two recently isolated strains from la Réunion island, one with a higher proportion of nictating individuals than the laboratory strain P. pacificus PS312. We found a positive correlation between nictation frequencies and host attachment in these strains. Taken together, our combination of genetic analyses with natural variation studies presents a new approach to the investigation of behavioural and ecological functionality. We show that nictation behaviour in P. pacificus nematodes serves as a host-finding behaviour. Our results suggest that nictation plays a role in the evolution of new life-history strategies, such as the evolution of parasitism.

Reproductive mode evolution in nematodes: insights from molecular phylogenies and recently discovered species

The Phylum Nematoda has long been known to contain a great diversity of species that vary in reproductive mode, though our understanding of the evolutionary origins, causes and consequences of nematode reproductive mode change have only recently started to mature. Here we bring together and analyze recent progress on reproductive mode evolution throughout the phylum, resulting from the application of molecular phylogenetic approaches and newly discovered nematode species. Reproductive mode variation is reviewed in multiple free-living, animal-parasitic and plant-parasitic nematode groups. Discussion ranges from the model nematode Caenorhabditis elegans and its close relatives, to the plant-parasitic nematodes of the Meloidogyne genus where there is extreme variation in reproductive mode between and even within species, to the vertebrate-parasitic genus Strongyloides and related genera where reproductive mode varies across generations (heterogony). Multiple evolutionary transitions from dioecous (obligately outcrossing) to hermaphroditism and parthenogenesis in the phylum are discussed, along with one case of an evolutionary transition from hermaphroditism to doioecy in the Oscheius genus. We consider the roles of underlying genetic mechanisms in promoting reproductive plasticity in this phylum, as well as the potential evolutionary forces promoting transitions in reproductive mode.