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

Pristionchus pacificus has been established as a nematode model system in evolutionary developmental biology and evolutionary ecology. Field studies in North and South America, Asia, Africa and Europe indicated that nematodes of the genus Pristionchus live in association with scarab beetles. Here, we describe the first account of soil‐ and beetle‐associated nematodes on an island setting by investigating the island of Réunion in the Indian Ocean. Réunion has high numbers of endemic insects and is one among several attractive islands for biodiversity studies. Being of volcanic origin, Réunion is 2–3 million years old, making it the youngest of the Mascareigne islands. We show that beetle‐ and soil‐derived nematodes on Réunion are nearly exclusively hermaphroditic, suggesting that selfing is favoured over gonochorism (outcrossing) during island colonization. Among members of four nematode genera observed on Réunion, Pristionchus pacificus was the most prevalent species. A total of 76 isolates, in association with five different scarab beetles, has been obtained for this cosmopolitan nematode. A detailed mitochondrial haplotype analysis indicates that the Réunion isolates of P. pacificus cover all four worldwide clades of the species. This extraordinary haplotype diversity suggests multiple independent invasions, most likely in association with different scarab beetles. Together, we establish Réunion as a case study for nematode island biogeography, in which the analysis of nematode population genetics and population dynamics can provide insight into evolutionary and ecological processes. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 170–179.     

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.     

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.     

Species-specific recognition of beetle cues by the nematode Pristionchus maupasi

SUMMARY The environment has a strong effect on development as is best seen in the various examples of phenotypic plasticity. Besides abiotic factors, the interactions between organisms are part of the adaptive forces shaping the evolution of species. To study how ecology influences development, model organisms have to be investigated in their environmental context. We have recently shown that the nematode Pristionchus pacificus and its relatives are closely associated with scarab beetles with a high degree of species specificity. For example, P. pacificus is associated with the oriental beetle Exomala orientalis in Japan and the northeastern United States, whereas Pristionchus maupasi is primarily isolated from cockchafers of the genus Melolontha in Europe. Here, we investigate how Pristionchus nematodes identify their specific insect hosts by using chemotaxis studies originally established in Caenorhabditis elegans . We observed that P. maupasi is exclusively attracted to phenol, one of the sex attractants of Melolontha beetles, and that attraction was also observed when washes of adult beetles were used instead of pure compounds. Furthermore, P. maupasi chemoattraction to phenol synergizes with plant volatiles such as the green leaf alcohol and linalool, demonstrating that nematodes can integrate distinct chemical senses from multiple trophic levels. In contrast, another cockchafer-associated nematode, Diplogasteriodes magnus , was not strongly attracted to phenol. We conclude that interception of the insect communication system might be a recurring strategy of Pristionchus nematodes but that different nematodes use distinct chemical cues for finding their beetle hosts.     

The bacterial community of entomophilic nematodes and host beetles

Insects form the most species‐rich lineage of Eukaryotes and each is a potential host for organisms from multiple phyla, including fungi, protozoa, mites, bacteria and nematodes. In particular, beetles are known to be associated with distinct bacterial communities and entomophilic nematodes. While entomopathogenic nematodes require symbiotic bacteria to kill and reproduce inside their insect hosts, the microbial ecology that facilitates other types of nematode–insect associations is largely unknown. To illuminate detailed patterns of the tritrophic beetle–nematode–bacteria relationship, we surveyed the nematode infestation profiles of scarab beetles in the greater Los Angeles area over a five‐year period and found distinct nematode infestation patterns for certain beetle hosts. Over a single season, we characterized the bacterial communities of beetles and their associated nematodes using high‐throughput sequencing of the 16S rRNA gene. We found significant differences in bacterial community composition among the five prevalent beetle host species, independent of geographical origin. Anaerobes Synergistaceae and sulphate‐reducing Desulfovibrionaceae were most abundant in Amblonoxia beetles, while Enterobacteriaceae and Lachnospiraceae were common in Cyclocephala beetles. Unlike entomopathogenic nematodes that carry bacterial symbionts, insect‐associated nematodes do not alter the beetles' native bacterial communities, nor do their microbiomes differ according to nematode or beetle host species. The conservation of Diplogastrid nematodes associations with Melolonthinae beetles and sulphate‐reducing bacteria suggests a possible link between beetle–bacterial communities and their associated nematodes. Our results establish a starting point towards understanding the dynamic interactions between soil macroinvertebrates and their microbiota in a highly accessible urban environment.     

Phylogeny of the nematode genus <Emphasis Type="Italic">Pristionchus</Emphasis> and implications for biodiversity,biogeography and the evolution of hermaphroditism

Background  

The nematode Pristionchus pacificus has originally been developed as a satellite organism for comparison to Caenorhabditis elegans. A 10X coverage of the whole genome of P. pacificus is available, making P. pacificus the first non- Caenorhabditis nematode with a fully sequenced genome. The macroevolutionary comparison between P. pacificus and C. elegans has been complemented by microevolutionary studies of closely related strains and species within the genus Pristionchus. In addition, new understanding of the biology of Pristionchus from field studies, demonstrating a close association with various scarab beetles and the Colorado potato beetle, supports consideration of this nematode in studies of ecosystems. In the course of field studies on four continents more than 1,200 isolates were established from 15,000 beetle specimens representing 18 Pristionchus species. Two remarkable features of the Pristionchus – beetle association are the high species specificity of the interaction and the interception of the beetle's sex communication system for host recognition by the nematodes, as suggested by chemotaxis studies. Evolutionary interpretations of differences in developmental, behavioral and ecological patterns require a phylogenetic framework of the genus Pristionchus.     

A subset of naturally isolated Bacillus strains show extreme virulence to the free‐living nematodes Caenorhabditis elegans and Pristionchus pacificus

The main food source of free‐living nematodes in the soil environment is bacteria, which can affect nematode development, fecundity and survival. In order to occupy a reliable source of bacterial food, some nematodes have formed specific relationships with an array of invertebrate hosts (where bacteria proliferate once the hosts dies), thus forming a tritrophic system of nematode, bacteria and insect or other invertebrates. We isolated 768 Bacillus strains from soil (from Germany and the UK), horse dung and dung beetles and fed them to the genetically tractable free‐living nematodes Caenorhabditis elegans and Pristionchus pacificus to isolate nematocidal strains. While C. elegans is a bacteriovorous soil nematode, P. pacificus is an omnivorous worm that is often found in association with scarab beetles. We found 20 Bacillus strains (consisting of B. cereus, B. weihenstephanensis, B. mycoides and Bacillus sp.) that were pathogenic to C. elegans and P. pacificus causing 70% to 100% mortality over 5 days and significantly affect development and brood size. The most pathogenic strains are three B. cereus‐like strains isolated from dung beetles, which exhibit extreme virulence to C. elegans in less than 24 h, but P. pacificus remains resistant. C. elegans Bre mutants were also highly susceptible to the B. cereus‐like strains indicating that their toxins use a different virulence mechanism than B. thuringiensis Cry 5B toxin. Also, mutations in the daf‐2/daf‐16 insulin signaling pathway do not rescue survival. We profiled the toxin genes (bcet, nhe complex, hbl complex, pcpl, sph, cytK, piplc, hly2, hly3, entFM and entS) of these three B. cereus‐like strains and showed presence of most toxin genes but absence of the hbl complex. Taken together, this study shows that the majority of naturally isolated Bacillus from soil, horse dung and Geotrupes beetles are benign to both C. elegans and P. pacificus. Among 20 pathogenic strains with distinct virulence patterns against the two nematodes, we selected three B. cereus‐like strains to investigate resistance and susceptibility immune responses in nematodes.     

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.     

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.     

Transmission of Bursaphelenchus xylophilus through Oviposition Wounds of Monochamm carolinensis (Coleoptera: Cerambycidae)

Transmission of pinewood nematode through Monochamus carolinensis oviposition wounds was documented. Nematode transmission was measured as the average number of nematodes isolated per oviposition wound excavated and also as the percentage of oviposition wounds from which nematodes were isolated. The influence of three factors that might affect nematode transmission was investigated: age of the beetle vector, number of nematodes carried per beetle, and egg deposition in the oviposition wound. Only the number of nematodes carried by the beetle was found to have a significant effect on transmission. Nematodes were transmitted more frequently and in slightly greater numbers by beetles carrying more nematodes. The influence of pinewood on nematode exit from beetles were investigated by comparing nematode exit from beetles placed over pine chips with those placed over distilled water. Nematodes exited in greater numbers and at a higher frequency from beetles over pine chips than from beetles over distilled water. Apparently, the nematodes are able to detect a factor from the pine chips that promotes their exit from the beetles.     

Chemoattraction in Pristionchus nematodes and implications for insect recognition

Nematodes and insects are the two dominant animal taxa in species numbers, and nematode-insect interactions constitute a significant portion of interspecies associations in a diversity of ecosystems. It has been speculated that most insects represent mobile microhabitats in which nematodes can obtain food, mobility, and shelter. Nematode-insect associations can be classified as phoretic (insects used for transportation, not as food), necromenic (insect used for transportation, then carcass as food), and entomopathogenic (insect is killed and used as food). To determine how nematodes target their hosts, we analyzed the chemosensory response and behavioral parameters of closely related Pristionchus nematodes that form species-specific necromenic associations with scarab beetles and the Colorado potato beetle. We found that all four studied Pristionchus species displayed unique chemoattractive profiles toward insect pheromones and plant volatiles with links to Pristionchus habitats. Moreover, chemoattraction in P. pacificus differs from that of C. elegans not only in the types of attractants, but also in its tempo, mode, and concentration response range. We conclude that Pristionchus olfaction is highly diverse among closely related species and is likely to be involved in shaping nematode-host interactions.     

Susceptibility of lady beetles (Coleoptera: Coccinellidae) to entomopathogenic nematodes

We investigated differential susceptibility of lady beetles to entomopathogenic nematodes, for two reasons: (1) to estimate potential nontarget effects on natural lady beetle populations, (2) to compare the susceptibility of exotic versus native lady beetle species. We hypothesize that successful establishment of some exotically introduced arthropods may be due, in part, to a lower susceptibility relative to competing native species. In laboratory studies, we compared the pathogenicity, virulence, and reproductive capacity of Heterorhabditis bacteriophora and Steinernema carpocapsae among two native (Coleomegilla maculata and Olla v-nigrum) and two successfully established exotic (Harmonia axyridis and Coccinella septempunctata) lady beetles, and a known susceptible lepidopteran host, Agrotis ipsilon. After 1 and 2 days of exposure to either nematode species, mortality of A. ipsilon was higher than in all lady beetles. Thus, we predict that nematode field applications would have significantly less impact on lady beetle populations than on a susceptible target pest. Additionally, the impact of soil-applied nematodes may be lower on lady beetles than on soil-dwelling hosts because the former spends relatively less time on the soil. Exotic lady beetles were less susceptible to nematode infection than native species. Reproductive capacity data also indicated lower host suitability in H. axyridis, but not in C. septempunctata. Overall, the hypothesis that low susceptibility to pathogens in certain exotic lady beetles may have contributed to competitive establishment was supported (especially for H. axyridis). Additional studies incorporating different hosts and pathogens from various geographic locations will be required to further address the hypothesis.     

Fitness costs of phoretic nematodes in the burying beetle,Nicrophorus vespilloides

Nicrophorusvespilloides is a social beetle that rears its offspring on decomposing carrion. Wild beetles are frequently associated with two types of macrobial symbionts, mites, and nematodes. Although these organisms are believed to be phoretic commensals that harmlessly use beetles as a means of transfer between carcasses, the role of these symbionts on N. vespilloides fitness is poorly understood. Here, we show that nematodes have significant negative effects on beetle fitness across a range of worm densities and also quantify the density‐dependent transmission of worms between mating individuals and from parents to offspring. Using field‐caught beetles, we provide the first report of a new nematode symbiont in N. vespilloides, most closely related to Rhabditoides regina, and show that worm densities are highly variable across individuals isolated from nature but do not differ between males and females. Next, by inoculating mating females with increasing densities of nematodes, we show that worm infections significantly reduce brood size, larval survival, and larval mass, and also eliminate the trade‐off between brood size and larval mass. Finally, we show that nematodes are efficiently transmitted between mating individuals and from mothers to larvae, directly and indirectly via the carcass, and that worms persist through pupation. These results show that the phoretic nematode R. regina can be highly parasitic to burying beetles but can nevertheless persist because of efficient mechanisms of intersexual and intergenerational transmission. Phoretic species are exceptionally common and may cause significant harm to their hosts, even though they rely on these larger species for transmission to new resources. However, this harm may be inevitable and unavoidable if transmission of phoretic symbionts requires nematode proliferation. It will be important to determine the generality of our results for other phoretic associates of animals. It will equally be important to assess the fitness effects of phoretic species under changing resource conditions and in the field where diverse interspecific interactions may exacerbate or reduce the negative effects of phoresy.     

Walking Response of the Mediterranean Pine Engraver, <Emphasis Type="Italic">Orthotomicus erosus</Emphasis>, to Novel Plant Odors in a Laboratory Olfactometer

When an herbivorous insect enters a new geographic area, it will select host plants based on short and long distance cues. A conifer-feeding bark beetle that has been recently introduced to North America, the Mediterranean pine engraver, Orthotomicus erosus (Wollaston), has a potentially wide host range, especially among members of the Pinaceae. The long-distance response of the beetles to tree odors may be a key feature of the mechanism of host recognition and selection. We used a laboratory olfactometer to study the walking response of 1,440 O. erosus to odor cues from the bark and phloem of six North American tree species. The beetle moved toward the angiosperm non-host Betula papyrifera more than would be expected by chance, but had a neutral response to odors of two tree species that support reproduction and three species that do not. These results suggest that tree odors alone may not be adequate for O. erosus to recognize novel hosts.     

Molecular phylogeny of beetle associated diplogastrid nematodes suggests host switching rather than nematode-beetle coevolution

Background  

Nematodes are putatively the most species-rich animal phylum. They have various life styles and occur in a variety of habitats, ranging from free-living nematodes in aquatic or terrestrial environments to parasites of animals and plants. The rhabditid nematode Caenorhabditis elegans is one of the most important model organisms in modern biology. Pristionchus pacificus of the family of the Diplogastridae has been developed as a satellite model for comparison to C. elegans. The Diplogastridae, a monophyletic clade within the rhabditid nematodes, are frequently associated with beetles. How this beetle-association evolved and whether beetle-nematode coevolution occurred is still elusive. As a prerequisite to answering this question a robust phylogeny of beetle-associated Diplogastridae is needed.     

Locating host plants via orientation to standing visual targets has dispersal benefits for the monophagous leaf beetle Ambrostoma quadriimpressum

The highly specialized chrysomelid Ambrostoma quadriimpressum Motschulsky (Coleoptera: Chrysomelidae) is strictly monophagous on elm trees, Ulmus pumila L. (Ulmaceae) and a few of its close relatives. In order to elucidate how the adult beetles find their host plants, we performed both choice and no‐choice arena field experiments to examine their spatial maneuvering under seminatural conditions. In the no‐choice experiments, three treatments were tested in which individual beetles in thanatosis were released in the vicinity of a host plant, in which they were released facing (1) toward the host, (2) away from the host but toward a black paper column, or (3) away from the host with no standing target in view. Videorecordings of the beetles’ walking tracks showed that a standing visual target in field of vision strongly influenced their behavior. The host plant and black paper column equally attracted beetles released facing toward them. In choice experiments, the beetles randomly moved to both host and non‐host plants, as well as to the host plant and the black paper column, indicating that they do not discriminate host plants from a distance. The results suggest that this monophagous beetle locates host plants via orientation to standing visual targets. The results also indicate that the beetle encounters hosts by chance. The benefit of such a host‐finding mechanism in light of dispersion and emigration of the beetle is discussed.     

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.     

Burying Beetle Larvae Discriminate Between Individual Parents and Between Some Classes of Adults

Offspring begging can be triggered by a variety of acoustic, visual or chemical cues from the parents. In many birds, nestlings use information derived from these cues to discriminate between individual parents or different classes of adults. Although begging occurs in some insects, we know very little about discrimination between adults by insect larvae. Here, we examine whether begging larvae in the burying beetle Nicrophorus vespilloides can discriminate between individual parents or different classes of adults. We found that larvae showed no discrimination between male and female beetles, but that they begged more towards breeding beetles than towards non‐breeding ones. These results were robust regardless of whether larvae had been reared in presence or absence of adult beetles, thus suggesting that larval discrimination is based on an innate template that requires no prior exposure to adult beetles. We also found that larvae begged more towards unfamiliar beetles than towards familiar ones, suggesting that they can learn to discriminate between individual parents based on cues about familiarity. We conclude that insect larvae may benefit from discriminating between different classes of adult beetles, as it allows them to lower the costs associated with begging in response to irrelevant environmental cues (costly in terms of wasted effort) and with not begging in response to the presence of caring parents (costly in terms of lost feeding opportunities).     

Brief Review of the Associations of Xylobiont Nematodes with Bark Beetles (Coleoptera,Curculionidae: Scolytinae)

Nematode pathogens cause wilt diseases in conifers and deciduous trees. The longhorn beetles (Coleoptera: Cerambycidae) and bark beetles (Coleoptera: Curculionidae: Scolytinae) act as nematode vectors spreading the invasive juvenile stages during their maturation feeding or during oviposition on the plant hosts. There are numerous reviews of nematodes associated with bark beetles on conifers, while little attention has been paid to the nematodes of deciduous trees. The development of Dutch elm disease and ash dieback is mainly caused by fungal pathogens transmitted by bark beetles; the latter act as vectors of not only fungi but also nematodes enclosed in nematangia under their elytra, and also in the tracheae and Malpighian canals. Apart from phytopathogenic nematodes, bark beetles transmit mycophagous and bacterivorous nematodes and own parasites of bark beetles. The ecological groups of nematodes associated with Scolytinae are reviewed; the known records of associations of nematodes with bark beetles are listed for coniferous host plants of Russia and neighboring countries; the world-wide list of these associations for deciduous plant hosts is given.

     

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.