Infection success in novel hosts: an experimental and phylogenetic study of Drosophila-parasitic nematodes

Surprisingly little is known about what determines a parasite's host range, which is essential in enabling us to predict the fate of novel infections. In this study, we evaluate the importance of both host and parasite phylogeny in determining the ability of parasites to infect novel host species. Using experimental lab assays, we infected 24 taxonomically diverse species of Drosophila flies (Diptera: Drosophilidae) with five different nematode species (Tylenchida: Allantonematidae: Howardula, Parasitylenchus), and measured parasite infection success, growth, and effects on female host fecundity (i.e., virulence). These nematodes are obligate parasites of mushroom-feeding Drosophila, particularly quinaria and testacca group species, often with severe fitness consequences on their hosts. We show that the potential host ranges of the nematodes are much larger than their actual ranges, even for parasites with only one known host species in nature. Novel hosts that are distantly related from the native host are much less likely to be infected, but among more closely related hosts, there is much variation in susceptibility. Potential host ranges differ greatly between the related parasite species. All nematode species that successfully infected novel hosts produced infective juveniles in these hosts. Most novel infections did not result in significant reductions in the fecundity of female hosts, with one exception: the host specialist Parasitylenchus nearcticus sterilized all quinaria group hosts, only one of which is a host in nature. The large potential host ranges of these parasites, in combination with the high potential for host colonization due to shared mushroom breeding sites, explain the widespread host switching observed in comparisons of nematode and Drosophila phylogenies.     

Defensive endosymbionts: a cryptic trophic level in community ecology

Maternally transmitted endosymbionts are widespread among insects, but how they are maintained within host populations is largely unknown. Recent discoveries show that some endosymbionts protect their hosts from pathogens or parasites. Spiroplasma, an endosymbiont of Drosophila neotestacea, protects female hosts from the sterilizing effects of parasitism by the nematode Howardula aoronymphium. Here, we show that Spiroplasma spreads rapidly within experimental populations of D. neotestacea subject to Howardula parasitism, but is neither strongly favored nor selected against in the absence of Howardula. In a reciprocal experiment, Howardula declined steadily to extinction in populations of Spiroplasma-infected flies, whereas in populations of uninfected flies, the prevalence of Howardula parasitism increased to c. 100%. Thus, Spiroplasma and Howardula exhibit effectively consumer-resource trophic dynamics. The recent spread of Spiroplasma in natural populations of D. neotestacea coincides with a decline in the prevalence of Howardula parasitism in the wild.     

On the capacity of macroparasites to control insect populations

A graphical model of the population dynamics of macroparasites and their hosts is developed. Three principal means by which the parasites can be regulated are considered: reduction in host density as a result of parasite-induced host mortality, reduction in host density as a result of parasite-induced host sterility, and competition among parasites within multiply-infected hosts. The means by which parasites are regulated has a major effect on the degree to which they can depress host population densities. In particular, a parasite that sterilizes its host is expected to reduce host density more than one that causes an equivalent decline in host fitness through increased mortality. A special case of the model is developed for herbivorous insects that, in the absence of parasites, are limited by larval food resources. Parasites that are regulated via parasite-induced host sterility will control the insect populations below the level set by larval resources if the threshold host density for the parasites (N(T)) is less than the ratio of carrying capacity to net reproductive rate of the insects (K/R). Data are presented showing that all three means of parasite regulation, but especially parasite-induced host sterility, can operate in Howardula aoronymphium, a nematode parasite of mycophagous Drosophila flies. Data from a field cage experiment show that, if these nematodes are regulated primarily via reductions in host density due to this sterility, the parameters N(T), K, and R are such that Howardula is likely to play an important role in controlling Drosophila populations. However, this conclusion must be tempered by the fact that these nematodes also cause increased host mortality and experience within-host competition, making the conditions for parasite control of the flies more stringent.     

Effectively vertical transmission of a Drosophila‐parasitic nematode: mechanism and consequences

1. Long‐term control of insects by parasites is possible only if the parasite populations persist. Because parasite transmission rate depends on host density, parasite populations may go extinct during periods of low host density. Vertical transmission of parasites, however, is independent of host density and may therefore provide a demographic bridge through times when their insect hosts are rare. 2. The nematode Howardula aoronymphium, which parasitises mycophagous species of Drosophila, can experience both horizontal and effectively vertical transmission, relative rates of which depend, in theory at least, on the density of hosts at breeding sites. 3. A nine‐generation experiment was carried out in which nematodes were transmitted either exclusively vertically or primarily horizontally. This experiment revealed that these parasites can persist and exhibit positive population growth even when there is only vertical transmission. 4. Assays at the end of the experiment revealed that the vertically transmitted nematodes had suffered no inbreeding depression and that they were similar to the horizontally transmitted nematodes in terms of virulence, infectivity, within‐host growth rate, and fecundity. Thus, vertical transmission of H. aoronymphium did not appear to compromise the ability of these parasites to control Drosophila populations.     

Ecological genetics of abdominal pigmentation in Drosophila falleni: a pleiotropic link to nematode parasitism

Drosophila falleni belongs to the quinaria species group, whose species vary considerably in patterns of wing and abdominal pigmentation. Drosophila falleni itself exhibits substantial variation among wild flies in abdominal spotting patterns. A selection experiment revealed that natural populations of D. falleni harbor high levels of genetic variation for spot number: in 10 generations of selection modal spot number within populations declined from 18 (the modal number in wild-caught females) to as low as zero. Rearing flies at different temperatures shows that some of the variation among wild flies is likely to reflect variation in the environmental conditions under which they developed. Fitness assays did not reveal any cost of reduced spot number with respect to development time, adult survival, or female fecundity. However, spotless flies were almost twice as susceptible to infection by the nematode parasite Howardula aoronymphium. Thus, selection exerted by nematode parasites may influence pigmentation patterns and other, genetically correlated traits in natural populations D. falleni.     

Evolution of multiple components of virulence in Drosophila-nematode associations

Abstract.— Virulence is of central importance in host-parasite interactions, yet little is known about how it changes over extended evolutionary periods. In this study, all four species in the testacea species group of Drosophila were experimentally infected with sympatric and allopatric nematodes in the Howardula aoronymphium species complex, and the effect of parasite infection on three components of host fitness was determined. The Drosophila species show striking differences in their responses to infection, with reductions reaching 80% in adult lifespan, 100% in female fertility, and 90% in male fertility. Female sterility appears to be determined by the host; species that are sterilized by their local nematodes are also sterilized by the other allopatric nematodes in the H. aoronymphium complex. Host species that are not sterilized by their local parasite are not sterilized by other nematodes in the complex. In contrast, reductions in host adult lifespan and male fertility depend on both the host and the parasite. Whereas all nematodes reduced the survival of their local host species equally (about 40–45%), survival of two host species was drastically reduced (about 80%) when infected with an allopatric parasite. Thus, virulence is evolutionarily labile in associations between Drosophila testacea group species and their Howardula parasites. The data suggest that changes in the sterility component of virulence are due primarily to host evolution, whereas changes in the host mortality component are due in large part to parasite evolution.     

Ecology and evolution of host-parasite associations: mycophagous Drosophila and their parasitic nematodes

Associations between mycophagous Drosophila and nematode parasites occur throughout the temperate and boreal regions of North America, Europe, and Asia. The nematode Howardula aoronymphium has substantial adverse effects on host survival and fertility on North American Drosophila. Long-term data show that rainy summers lead to a high prevalence of parasitism in the fall and the following spring, resulting in up to a 1-yr time lag between present rainfall and increased prevalence of H. aoronymphium parasitism. A biogeographic analysis of the relative abundance of different Drosophila species has shown that H. aoronymphium may facilitate the coexistence of different species of Drosophila that compete for larval food resources. The actual host range of parasites in nature is determined by the intrinsic suitability of potential hosts for parasite infection and reproduction and various ecological factors. For H. aoronymphium in eastern North America, intrinsically suitable hosts fall within a restricted clade within the genus Drosophila. However, the temperature sensitivity of H. aoronymphium prevents it from using several host species that occur outside the geographical range of the nematodes. Finally, the host range, virulence, and geographical range of Drosophila-parasitic nematodes appear to be highly dynamic over evolutionary timescales.     

Evolution of Parasitism in Insect-transmitted Plant Nematodes

Nematode-insect associations have evolved many times in the phylum Nematoda, but these lineages involve plant parasitism only in the Secernentean orders Aphelenchida and Tylenchida. In the Aphelenchida (Aphelenchoidoidea), Bursaphelenchus xylophilus (Pine wood nematode), B. cocophilus (Red ring or Coconut palm nematode) (Parasitaphelenchidae), and the many potential host-specific species of Schistonchus (fig nematodes) (Aphelenchoididae) nematode-insect interactions probably evolved independently from dauer-forming, mycophagous ancestors that were phoretically transmitted to breeding sites of their insect hosts in plants. Mycophagy probably gave rise to facultative or obligate plant-parasitism because of opportunities due to insect host switches or peculiarities in host behavior. In the Tylenchida, there is one significant radiation of insect-associated plant parasites involving Fergusobia nematodes (Fergusobiinae: Neotylenchidae) and Fergusonina (Fergusoninidae) flies as mutualists that gall myrtaceous plant buds or leaves. These dicyclic nematodes have different phases that are parasitic in either the insect or the plant hosts. The evolutionary origin of this association is unclear.     

Molecular phylogenetics and the evolution of host plant associations in the nematode genus Fergusobia (Tylenchida: Fergusobiinae)

Fergusobia nematodes (Tylenchida: Fergusobiinae) and Fergusonina flies (Diptera: Fergusoninidae) are putative mutualists that develop together in galls formed in meristematic tissues of many species of the plant family Myrtaceae in Australasia. Fergusobia nematodes were sampled from a variety of myrtaceous hosts and gall types from Australia and one location in New Zealand between 1999 and 2006. Evolutionary relationships of these isolates were inferred from phylogenetic analyses of the DNA sequences of the nuclear ribosomal DNA near-full length small subunit (up to 1689bp for 21 isolates), partial large subunit D2/D3 domain (up to 889bp for 87 isolates), partial mitochondrial cytochrome oxidase subunit I (618 bp for 82 isolates), and combined D2/D3 and mtCOI (up to 1497bp for 66 isolates). The SSU data supported a monophyletic Fergusobia genus within a paraphyletic Howardula. A clade of Drosophila-associated Howardula, including Howardula aoronymphium, was the closest sequenced sister group. Phylogenetic analysis of sequences from D2/D3 and mtCOI, separately and combined, revealed many monophyletic clades within Fergusobia. The relationships inferred by D2/D3 and mtCOI were congruent with some exceptions. Well-supported clades were generally consistent with host plant species and gall type. However, phylogenetic analysis suggested host switching or putative hybridization events in many groups, except the lineage of shoot bud gallers on the broad-leaved Melaleuca species complex.     

An emerging example of tritrophic coevolution between flies (Diptera: Fergusoninidae) and nematodes (Nematoda: Neotylenchidae) on Myrtaceae host plants

A unique obligate mutualism occurs between species of Fergusonina Malloch flies (Diptera: Fergusoninidae) and nematodes of the genus Fergusobia Currie (Nematoda: Neotylenchidae). These mutualists together form different types of galls on Myrtaceae, mainly in Australia. The galling association is species‐specific, and each mutualism in turn displays host specificity. This tritrophic system represents a compelling arena to test hypotheses about coevolution between the host plants, parasitic nematodes and the fergusoninid flies, and the evolution of these intimate mutualisms. We have a basic knowledge of the interactions between the host plant, fly and nematode in this system, but a more sophisticated understanding will require a much more intensive and coordinated research effort. Summaries of the known Fergusonina/Fergusobia species associations and gall type terminology are presented. This paper identifies the key advantages of the system and questions to be addressed, and proposes a number of predictions about the evolutionary dynamics of the system given our understanding of the biology of the mutualists. Future research will profitably focus on (1) gall cecidogenesis and phenology, (2) the interaction between the fly larva and the nematode in the gall, and between the adult female fly and the parasitic nematode, (3) the means by which the fly and nematode life cycles are coordinated, (4) a targeted search of groups in the plant family Myrtaceae that have not yet been identified as gall hosts, and (5) establishment and comparison of the phylogenetic relationships of the host plants, fly species and nematodes. Recently derived phylogenies and divergence time estimation studies of the Diptera and the Myrtaceae show that the fly family Fergusoninidae is less than half the age of the Myrtaceae, discounting the hypothesis of cospeciation and coradiation of the fly/nematode mutualism and the plants at the broadest levels. However, cospeciation may have occurred at shallower levels in the phylogeny, following the establishment of the fly/nematode mutualism on the Myrtaceae. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 699–718.     

Nematode parasitism in a Danish drosophilid community: further evaluation of the disproportionate parasitism hypothesis

The nematodes Parasitylenchus diplogenus and Howardula aoronymphium (Allantonematidae) are parasites of drosophilids (Diptera). Nematodes were found in 3/14 drosophilid species sampled in Danish woodlands: Drosophila phalerata Meigen (3.5% parasitism) was parasitized by H. aoronymphium and D. obscura Fallén (0.5%) and D. subobscura Collin (2.1%) by P. diplogenus. Parasitism was generally rare, and few drosophilid species parasitized, compared to elsewhere in Europe. Parasitism was positively correlated with relative host abundance both within Denmark and across communities, suggesting that species diversity is promoted by a disproportionately high parasitism of more common host species. The prevalence of H. aoronymphium, but not P. diplogenus, parasitism is positively related to temperature across communities.     

When mutualists are pathogens: an experimental study of the symbioses between Steinernema (entomopathogenic nematodes) and Xenorhabdus (bacteria)

In this paper, we investigate the level of specialization of the symbiotic association between an entomopathogenic nematode (Steinernema carpocapsae) and its mutualistic native bacterium (Xenorhabdus nematophila). We made experimental combinations on an insect host where nematodes were associated with non-native symbionts belonging to the same species as the native symbiont, to the same genus or even to a different genus of bacteria. All non-native strains are mutualistically associated with congeneric entomopathogenic nematode species in nature. We show that some of the non-native bacterial strains are pathogenic for S. carpocapsae. When the phylogenetic relationships between the bacterial strains was evaluated, we found a clear negative correlation between the effect a bacterium has on nematode fitness and its phylogenetic distance to the native bacteria of this nematode. Moreover, only symbionts that were phylogenetically closely related to the native bacterial strain were transmitted. These results suggest that co-evolution between the partners has led to a high level of specialization in this mutualism, which effectively prevents horizontal transmission. The pathogenicity of some non-native bacterial strains against S. carpocapsae could result from the incapacity of the nematode to resist specific virulence factors produced by these bacteria.     

Phylogenetic analysis of nuclear and mitochondrial genes supports species groups for Columbicola (Insecta: Phthiraptera)

The dove louse genus Columbicola has become a model system for studying the interface between microevolutionary processes and macroevolutionary patterns. This genus of parasitic louse (Phthiraptera) contains 80 described species placed into 24 species groups. Samples of Columbicola representing 49 species from 78 species of hosts were obtained and sequenced for mitochondrial (COI and 12S) and nuclear (EF-1alpha) genes. We included multiple representatives from most host species for a total of 154 individual Columbicola, the largest molecular phylogenetic study of a genus of parasitic louse to date. These sequences revealed considerable divergence within several widespread species of lice, and in some cases these species were paraphyletic. These divergences correlated with host association, indicating the potential for cryptic species in several of these widespread louse species. Both parsimony and Bayesian maximum likelihood phylogenetic analyses of these sequences support monophyly for nearly all the non-monotypic species groups included in this study. These trees also revealed considerable structure with respect to biogeographic region and host clade association. These patterns indicated that switching of parasites between host clades is limited by biogeographic proximity.     

Generalist nematodes dominate the nemabiome of roe deer in sympatry with sheep at a regional level

The growth of livestock farming and the recent expansion of wild ungulate populations in Europe favor opportunities for direct and/or indirect cross-transmission of pathogens. Comparatively few studies have investigated the epidemiology of gastro-intestinal nematode parasites, an ubiquitous and important community of parasites of ungulates, at the wildlife/livestock interface. In this study, we aimed to assess the influence of livestock proximity on the gastrointestinal nematode community of roe deer in a rural landscape located in southern France. Using ITS-2 rDNA nemabiome metabarcoding on fecal larvae, we analysed the gastrointestinal nematode communities of roe deer and sheep. In addition, we investigated Haemonchus contortus nad4 mtDNA diversity to specifically test parasite circulation among domestic and wild host populations. The dominant gastrointestinal nematode species found in both the roe deer and sheep were generalist species commonly found in small ruminant livestock (e.g. H. contortus), whereas the more specialised wild cervid nematode species (e.g. Ostertagia leptospicularis) were only present at low frequencies. This is in marked contrast with previous studies that found the nemabiomes of wild cervid populations to be dominated by cervid specialist nematode species. In addition, the lack of genetic structure of the nad4 mtDNA of H. contortus populations between host species suggests circulation of gastrointestinal nematodes between roe deer and sheep. The risk of contact with livestock only has a small influence on the nemabiome of roe deer, suggesting the parasite population of roe deer has been displaced by generalist livestock parasites due to many decades of sheep farming, not only for deer grazing close to pastures, but also at a larger regional scale. We also observed some seasonal variation in the nemabiome composition of roe deer. Overall, our results demonstrate significant exchange of gastrointestinal nematodes between domestic and wild ungulates, with generalist species spilling over from domestic ungulates dominating wild cervid parasite communities.     

Virulence and competitive ability in an obligately killing parasite

Mixed infections are thought to have a major influence on the evolution of parasite virulence. During a mixed infection, higher within‐host parasite growth is favored under the assumption that it is critical to the competitive success of the parasite. As within‐host parasite growth may also increase damage to the host, a positive correlation is predicted between virulence and competitive success. However, when parasites must kill their hosts in order be transmitted, parasites may spend energy on directly attacking their host, even at the cost of their within‐host growth. In such systems, a negative correlation between virulence and competitive success may arise. We examined virulence and competitive ability in three sympatric species of obligately killing nematode parasites in the genus Steinernema. These nematodes exist in a mutualistic symbiosis with bacteria in the genus Xenorhabdus. Together the nematodes and their bacteria kill the insect host soon after infection, with reproduction of both species occurring mainly after host death. We found significant differences among the three nematode species in the speed of host killing. The nematode species with the lowest and highest levels of virulence were associated with the same species of Xenorhabdus, indicating that nematode traits, rather than the bacterial symbionts, may be responsible for the differences in virulence. In mixed infections, host mortality rate closely matched that associated with the more virulent species, and the more virulent species was found to be exclusively transmitted from the majority of coinfected hosts. Thus, despite the requirement of rapid host death, virulence appears to be positively correlated with competitive success in this system. These findings support a mechanistic link between parasite growth and both anti‐competitor and anti‐host factors.     

Nematode parasitism in a northern European drosophilid community

Many factors may influence the structure of invertebrate communities. Among these is the presence of parasites which attack some or all members of a guild and potentially promote coexistence of competitor species. We assessed the prevalence of nematode (Allantonematidae) parasitism in Dutch woodland drosophilids (Diptera). Nematodes were found in 6 of the 18 drosophilid species sampled (percentage parasitism in parentheses): Drosophila phalerata (16%), D. kuntzei (5.1%), D. immigrans (0.5%), D. testacea (1.2%) and D. transversa (2.8%) were all parasitized by Howardula aoronymphium and D. subobscura (3%) was parasitized by Parazitylenchus diplogenus. This is the first report of nematode parasitism of D. immigrans and D. transversa. There were no important seasonal trends in percentage parasitism. We explored the consequences of nematode parasitism for individual drosophilids. Nematodes did not exert an important influence on the wing length (adult body size) of the drosophilids we sampled, but egg loads (fecundities) of female D. phalerata, D. subobscura and D. kuntzei were reduced by nematode parasitism. Parasitism rates were positively correlated with relative host abundance, in Dutch and other communities, suggesting that species diversity is promoted by a disproportionately high parasitism of more common host species.     

基于部分18S rDNA, 28S rDNA和COI基因序列的索科线虫亲缘关系

通过PCR扩增获得我国常见昆虫病原索科线虫6属10种18S rDNA、28S rDNA(D3区)和COI基因序列,结合来自GenBank中6属10种索科线虫的18S rDNA同源序列,用邻接法和最大简约法构建系统进化树。结果显示:12属索科线虫分为三大类群,第一大类群是三种罗索属线虫(Romanomermis)先聚在一起,再与两索属(Amphimermis)和蛛索属(Aranimermis)线虫聚为一支;在第二大类群中,六索属(Hexamermis)、卵索属线虫(Ovomermis)和多索属(Agamermis)亲缘关系最近,先聚在一起,再与八腱索属(Octomyomermis)和Thaumamermis线虫聚为一支。第三大类群由索属(Mermis)和异索属(Allomermis)线虫以显著水平的置信度先聚在一起,再与蠓索属(Heleidomermis)和施特克尔霍夫索属(Strelkovimermis)线虫聚为一支。从遗传距离看,基于3个基因的数据集均显示索科线虫属内种间差异明显小于属间差异,武昌罗索线虫(R.wuchangensis)和食蚊罗索线虫(R.culicivorax)同属蚊幼寄生罗索属线虫,其种间的遗传距离最小。     

Host population density and body mass as determinants of species richness in parasite communities: comparative analyses of directly transmitted nematodes of mammals

Epidemiological theory predicts positive correlations between host population density or body mass and species richness among parasite communities. Here I test these predictions by a comparative study of communities of directly transmitted mammalian parasites, gastrointestinal strongylid nematodes. I use data from 45 species of mammals, representing examination of 17 200 individual hosts. The variable studied was the average number of gastrointestinal strongylid nematode species per host population, and three different methods were used to obtain estimates of parasite species richness that are unbiased by number of host individuals examined. Analyses were done using the phylogenetically independent contrast method. Host population density and parasite species richness were strongly positively correlated when the effects of host body weight had been controlled for. Controlling for other variables did not change this, and the relationship was found regardless of method used to correct for uneven sampling effort among host species. A positive relationship between parasite species richness and host body weight was also found, but the effect of host densities had to be controlled for to see this. These relationships between host traits and species richness of directly transmitted parasites are stronger than patterns found using data on indirectly transmitted mammalian parasites, and suggests that links between host traits and parasite species richness are stronger than previously suggested. The results are consistent with parasite species richness being positively linked to pathogen transmission rates and reductions in transmission rates possibly increasing extinction probabilities in parasite populations. The results also suggest that parasites may exert a cost of increases in rate of population energy usage, and thus show that pathogens may be important in generating independence between body mass and rate of population energy usage among host species.     

Bird hosts, blood parasites and their vectors--associations uncovered by molecular analyses of blackfly blood meals

The level of host specificity of blood-sucking invertebrates may have both ecological and evolutionary implications for the parasites they are transmitting. We used blood meals from wild-caught blackflies for molecular identification of parasites and hosts to examine patterns of host specificity and how these may affect the transmission of avian blood parasites of the genus Leucocytozoon . We found that five different species of ornithophilic blackflies preferred different species of birds when taking their blood meals. Of the blackflies that contained avian blood meals, 62% were infected with Leucocytozoon parasites, consisting of 15 different parasite lineages. For the blackfly species, there was a significant association between the host width (measured as the genetic differentiation between the used hosts) and the genetic similarity of the parasites in their blood meals. The absence of similar parasite in blood meals from blackflies with different host preferences is interpreted as a result of the vector–host associations. The observed associations between blackfly species and host species are therefore likely to hinder parasites to be transmitted between different host-groups, resulting in ecologically driven associations between certain parasite lineages and hosts species.     

Nematodes, bacteria, and flies: a tripartite model for nematode parasitism

More than a quarter of the world's population is infected with nematode parasites, and more than a hundred species of nematodes are parasites of humans [1-3]. Despite extensive morbidity and mortality caused by nematode parasites, the biological mechanisms of host-parasite interactions are poorly understood, largely because of the lack of genetically tractable model systems. We have demonstrated that the insect parasitic nematode Heterorhabditis bacteriophora, its bacterial symbiont Photorhabdus luminescens, and the fruit fly Drosophila melanogaster constitute a tripartite model for nematode parasitism and parasitic infection. We find that infective juveniles (IJs) of Heterorhabditis, which contain Photorhabdus in their gut, can infect and kill Drosophila larvae. We show that infection activates an immune response in Drosophila that results in the temporally dynamic expression of a subset of antimicrobial peptide (AMP) genes, and that this immune response is induced specifically by Photorhabdus. We also investigated the cellular and molecular mechanisms underlying IJ recovery, the developmental process that occurs in parasitic nematodes upon host invasion and that is necessary for successful parasitism. We find that the chemosensory neurons and signaling pathways that control dauer recovery in Caenorhabditis elegans also control IJ recovery in Heterorhabditis, suggesting conservation of these developmental processes across free-living and parasitic nematodes.