Evidence of hermaphroditism and sex ratio distortion in the fungal feeding nematode Bursaphelenchus okinawaensis

Highlighting Potential Physical and Chemical Cues Involved in Conspecific Recognition System in a Predator Nematode, Seinura caverna

Conspecific recognition is the ability to distinguish and respond to individuals of the same species. In nematodes, this behavior can mediate aggregation, feeding behavior, or mating. Here, we investigated whether and how the predatory nematode Seinura caverna recognizes and avoids conspecifics to prey on. In predation assays, S. caverna did not kill conspecifics, but killed nematodes of three heterospecific species. Interestingly, S. caverna did not kill Ektaphelenchoides spondylis nematodes. Seinura caverna did not eject its stylet when encountering conspecifics or E. spondylis. The characterization of the internal cuticle structure of 13 nematode species suggested that the cuticle may play a role in the preying decision, as E. spondylis and S. caverna exhibited similar, type III, cuticle layers. Chemical extracts from S. caverna further repelled conspecifics. We discuss the potential hierarchical use of physical and chemical cues in S. caverna predation behavior and provide insights into the evolutionary adaptations and behavior of this organism.

How did Bursaphelenchus nematodes acquire a specific relationship with their beetle vectors, Monochamus?

For insect-borne pathogens, phoretic ability is important not only to spread more widely and efficiently but also to evolve virulence. Bursaphelenchus xylophilus, the causal agent of pine wilt disease, is transmitted by the cerambycid beetle Monochamus alternatus, which is associated with pine tree host. Their specific phoretic ability to appropriate vectors depending on their life cycle is critical for efficient transfer to the correct host and is expected to enhance virulence. We evaluated how B. xylophilus acquired a specific relationship with M. alternatus with a focus on Bursaphelenchus okinawaensis, a close relative of B. xylophilus that has evolved a relationship with a cerambycid beetle vector. Bursaphelenchus okinawaensis has a single dispersal stage (dauer) larva (third-stage dispersal [DIII] larva), whereas B. xylophilus has two distinct dispersal stages (DIII and fourth-stage dispersal [DIV] larva). Also, the dauer formation in B. okinawaensis is not completely dependent on its beetle vector, whereas DIV larvae of B. xylophilus are induced by volatile from the beetle vector. We investigated the induction conditions of dauer larvae in B. okinawaensis and compared to with B. xylophilus. The dauer percentages of B. okinawaensis significantly increased when the nematode population on the plate increased or when we propagated the nematodes with a crude extract of cultured nematodes, which likely contained dauer-inducing pheromones. In addition, dauer formation tended to be enhanced by the crude extract at high temperatures. Furthermore, when we propagated the nematodes with M. alternatus pupae until the beetles eclosed, B. okinawaensis significantly developed into dauer larvae. However, only 1.3% of dauer larvae were successfully transferred to M. alternatus, the rate lower than that of B. xylophilus. DIII and DIV of B. xylophilus were induced by increasing the nematode population and the presence of the beetle vector, respectively. These results suggest that B. okinawaensis has acquired specificity for the cerambycid beetle through dauer formation, which is efficiently induced in the presence of the beetle, and the DIV larval stage, exclusive to the xylophilus group, may be crucial for high transfer ability to the beetle vector.

Possible stochastic sex determination in Bursaphelenchus nematodes

Sex determination mechanisms evolve surprisingly rapidly, yet little is known in the large nematode phylum other than for Caenorhabditis elegans, which relies on chromosomal XX-XO sex determination and a dosage compensation mechanism. Here we analyze by sex-specific genome sequencing and genetic analysis sex determination in two fungal feeding/plant-parasitic Bursaphelenchus nematodes and find that their sex differentiation is more likely triggered by random, epigenetic regulation than by more well-known mechanisms of chromosomal or environmental sex determination. There is no detectable difference in male and female chromosomes, nor any linkage to sexual phenotype. Moreover, the protein sets of these nematodes lack genes involved in X chromosome dosage counting or compensation. By contrast, our genetic screen for sex differentiation mutants identifies a Bursaphelenchus ortholog of tra-1, the major output of the C. elegans sex determination cascade. Nematode sex determination pathways might have evolved by “bottom-up” accretion from the most downstream regulator, tra-1.

In most species, sex is determined by genetic or environmental factors. Here, the authors present evidence that sex determination in Bursaphelenchus nematodes is instead likely to be regulated by a random, epigenetic mechanism.

Reproductive plasticity in response to food source in the fungal-feeding nematode Bursaphelenchus okinawaensis

Organisms use various strategies to cope with fluctuating environments. Some organisms express different phenotypes in alternative conditions through a process known as ‘phenotypic plasticity’, which is presumably an evolutionary adaptation to environmental variation. Nematodes adapt to various environments; it has been suggested that phenotypic plasticity is a contributing factor in their high level of environmental adaptability. We investigated the reproductive plasticity in response to a food source in the fungal-feeding nematode, Bursaphelenchus okinawaensis. Bursaphelenchus okinawaensis is known to reproduce primarily as a self-fertilising hermaphrodite on a filamentous fungus and yeast; here, we showed that newly isolated SH3 strain hermaphrodites produced a small number of progenies (⩽9 progenies per hermaphrodite) on the yeast test plate, while they laid similar numbers of eggs to the SH1 strain on the fungus test plate. Subsequent sperm observation by 4′,6-diamidino-2-phenylindole (DAPI) revealed that SH3 hermaphrodites could produce only a small number of sperm on the yeast test plate. Some hermaphrodites did not produce any eggs, indicating that they had become females rather than hermaphrodites. These results showed that the hermaphrodite or female status of SH3 nematodes was a plastic character, dependent on the food stimulus. An intra-strain crossing test between SH1 and SH3 suggested that the reproductive plasticity was controlled by a single recessive gene. This study provided an insight into a novel type of phenotypic plasticity in nematodes.

The entomopathogenic nematode Steinernema hermaphroditum is a self-fertilizing hermaphrodite and a genetically tractable system for the study of parasitic and mutualistic symbiosis

Entomopathogenic nematodes (EPNs), including Heterorhabditis and Steinernema, are parasitic to insects and contain mutualistically symbiotic bacteria in their intestines (Photorhabdus and Xenorhabdus, respectively) and therefore offer opportunities to study both mutualistic and parasitic symbiosis. The establishment of genetic tools in EPNs has been impeded by limited genetic tractability, inconsistent growth in vitro, variable cryopreservation, and low mating efficiency. We obtained the recently described Steinernema hermaphroditum strain CS34 and optimized its in vitro growth, with a rapid generation time on a lawn of its native symbiotic bacteria Xenorhabdus griffiniae. We developed a simple and efficient cryopreservation method. Previously, S. hermaphroditum isolated from insect hosts was described as producing hermaphrodites in the first generation. We discovered that CS34, when grown in vitro, produced consecutive generations of autonomously reproducing hermaphrodites accompanied by rare males. We performed mutagenesis screens in S. hermaphroditum that produced mutant lines with visible and heritable phenotypes. Genetic analysis of the mutants demonstrated that this species reproduces by self-fertilization rather than parthenogenesis and that its sex is determined chromosomally. Genetic mapping has thus far identified markers on the X chromosome and three of four autosomes. We report that S. hermaphroditum CS34 is the first consistently hermaphroditic EPN and is suitable for genetic model development to study naturally occurring mutualistic symbiosis and insect parasitism.

Fisher vs. the Worms: Extraordinary Sex Ratios in Nematodes and the Mechanisms that Produce Them

Parker, Baker, and Smith provided the first robust theory explaining why anisogamy evolves in parallel in multicellular organisms. Anisogamy sets the stage for the emergence of separate sexes, and for another phenomenon with which Parker is associated: sperm competition. In outcrossing taxa with separate sexes, Fisher proposed that the sex ratio will tend towards unity in large, randomly mating populations due to a fitness advantage that accrues in individuals of the rarer sex. This creates a vast excess of sperm over that required to fertilize all available eggs, and intense competition as a result. However, small, inbred populations can experience selection for skewed sex ratios. This is widely appreciated in haplodiploid organisms, in which females can control the sex ratio behaviorally. In this review, we discuss recent research in nematodes that has characterized the mechanisms underlying highly skewed sex ratios in fully diploid systems. These include self-fertile hermaphroditism and the adaptive elimination of sperm competition factors, facultative parthenogenesis, non-Mendelian meiotic oddities involving the sex chromosomes, and environmental sex determination. By connecting sex ratio evolution and sperm biology in surprising ways, these phenomena link two "seminal" contributions of G. A. Parker.

Cytoplasmic Lipases-A Novel Class of Fungal Defense Proteins Against Nematodes

Fungi are an attractive food source for predators such as fungivorous nematodes. Several fungal defense proteins and their protective mechanisms against nematodes have been described. Many of these proteins are lectins which are stored in the cytoplasm of the fungal cells and bind to specific glycan epitopes in the digestive tract of the nematode upon ingestion. Here, we studied two novel nematotoxic proteins with lipase domains from the model mushroom Coprinopsis cinerea. These cytoplasmically localized proteins were found to be induced in the vegetative mycelium of C. cinerea upon challenge with fungivorous nematode Aphelenchus avenae. The proteins showed nematotoxicity when heterologously expressed in E. coli and fed to several bacterivorous nematodes. Site-specific mutagenesis of predicted catalytic residues eliminated the in-vitro lipase activity of the proteins and significantly reduced their nematotoxicity, indicating the importance of the lipase activity for the nematotoxicity of these proteins. Our results suggest that cytoplasmic lipases constitute a novel class of fungal defense proteins against predatory nematodes. These findings improve our understanding of fungal defense mechanisms against predators and may find applications in the control of parasitic nematodes in agriculture and medicine.

Three Seinura species from Japan with a description of S. shigaensis n. sp. (Tylenchomorpha: Aphelenchoididae)

A preliminary survey of Seinura spp. was conducted in the Kyoto area, Western Japan. The survey yielded four new strains of Seinura spp., including two strains of S. caverna, a strain of S. italiensis, and a strain of an undescribed species. Molecularly, the two strains of S. caverna were nearly identical to the type strain but showed some minor variations, particularly in the mitochondrial cytochrome oxidase subunit I gene. The small subunit and D2-D3 large subunit sequences of the Japanese strain of S. italiensis were nearly identical and identical to its original description, respectively, and the difference in the small subunit was due to mis-reading of the sequences. The new species, S. shigaensis n. sp., was phylogenetically close to S. caverna and S. persica, although these three species were clearly different phylogenetically. The new species was typologically similar or nearly identical to several other Seinura spp., including S. chertkovi, S. christiei, S. italiensis, S. steineri, and S. tenuicaudata, but it can be distinguished from those species by the morphometric values. Because the new species is phylogenetically very close to S. caverna, it could be a good comparative system for S. caverna as a potential satellite model for the predatory nematode.

Male reproductive toxicity involved in spermatogenesis induced by perfluorooctane sulfonate and perfluorooctanoic acid in Caenorhabditis elegans

As a persistent organic pollutant, perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) have gained increasing research attention over recent years because of their potential risk to humans and the environment. In this paper, we investigated the reproductive toxicity of these pollutants using a C. elegans model to evaluate spermatogenesis throughout the entire developmental cycle of him-5 mutant by exposing to 0.001, 0.01, and 0.1 mmol/L PFOS or PFOA for 48 h. Experimental results suggested that PFOS and PFOA exposure led to reductions in brood size, germ cell number, spermatid size, and motility, and increases in rate of malformation spermatids. Analysis of variance (ANOVA) showed that exposure to PFOS resulted in higher levels of damage than PFOA in germ cells only in 0.001 mmol/L exposure group. RT-qPCR was used to further investigate the expression of genes associated with different stages of spermatogenesis, such as mitosis and meiosis, fibrous body-membranous organelles (FB-MOs), and sperm activation. The expression levels of wee-1.3, spe-4, spe-6, and spe-17 genes were increased, while those of puf-8, spe-10, fer-1, swm-1, try-5, and spe-15 genes were decreased. Our results suggesting that PFOS or PFOA may cause spermatogenesis damage by disrupting the mitotic proliferation, meiotic entry, formation of the MOs, fusion of the MOs and plasma membrane (PM), and pseudopods. Loss-of-function studies using puf-8 and spe-10 mutants revealed spe-10 gene was specifically involved in PFOS- or PFOA-induced reproductive toxicity via regulating one or more critical palmitoylation events, while puf-8 gene was not direct target of PFOS and PFOA, and PFOS and PFOA may act on the upstream gene of puf-8, thus affecting reproductive ability. Taken together, these results demonstrate the potential adverse impact of PFOS and PFOA exposure on spermatogenesis and provide valuable data for PFC risk assessment. Grapical abstract.

Major ascaroside pheromone component asc-C5 influences reproductive plasticity among isolates of the invasive species pinewood nematode

Pheromones are communication chemicals and regulatory signals used by animals and represent unique tools for organisms to mediate behaviors and make "decisions" to maximize their fitness. Phenotypic plasticity refers to the innate capacity of a species to tolerate a greater breadth of environmental conditions across which it adapts to improve its survival, reproduction, and fitness. The pinewood nematode, Bursaphelenchus xylophilus, an invasive nematode species, was accidentally introduced from North America into Japan, China, and Europe; however, few studies have investigated its pheromones and phenotypic plasticity as a natural model. Here, we demonstrated a novel phenomenon, in which nematodes under the condition of pheromone presence triggered increased reproduction in invasive strains (JP1, JP2, CN1, CN2, EU1, and EU2), while it simultaneously decreased reproduction in native strains (US1 and US2). The bidirectional effect on fecundity, mediated by presence/absence of pheromones, is henceforth termed pheromone-regulative reproductive plasticity (PRRP). We further found that synthetic ascaroside asc-C5 (ascr#9), the major pheromone component, plays a leading role in PRRP and identified 2 candidate receptor genes, Bxydaf-38 and Bxysrd-10, involved in perceiving asc-C5. These results suggest that plasticity of reproductive responses to pheromones in pinewood nematode may increase its fitness in novel environments following introduction. This opens up a new perspective for invasion biology and presents a novel strategy of invasion, suggesting that pheromones, in addition to their traditional roles in chemical signaling, can influence the reproductive phenotype among native and invasive isolates. In addition, this novel mechanism could broadly explain, through comparative studies of native and invasive populations of animals, a potential underlying factor behind of the success of other biological invasions.

Telomere-to-Telomere Genome Assembly of Bursaphelenchus okinawaensis Strain SH1

Bursaphelenchus okinawaensis is a self-fertilizing, hermaphroditic, fungal-feeding nematode used as a laboratory model for the genus Bursaphelenchus, which includes the important pathogen Bursaphelenchus xylophilus. Here, we report the nearly complete genome sequence of B. okinawaensis. The 70-Mbp assembly contained six scaffolds (>11 Mbp each) with telomere repeats on their ends, indicating complete chromosomes.

Bursaphelenchus okinawaensis is a self-fertilizing, hermaphroditic, fungus-feeding nematode used as a laboratory model for the genus Bursaphelenchus, which includes the important pathogen Bursaphelenchus xylophilus. Here, we report the nearly complete genome sequence of B. okinawaensis. The 70-Mbp assembly contained six scaffolds (>11 Mbp each) with telomere repeats on their ends, indicating complete chromosomes.

Combining microfluidics and RNA-sequencing to assess the inducible defensome of a mushroom against nematodes

BACKGROUND

Fungi are an attractive source of nutrients for predators. As part of their defense, some fungi are able to induce the production of anti-predator protein toxins in response to predation. A previous study on the interaction of the model mushroom Coprinopsis cinerea by the fungivorous nematode Aphelenchus avenae on agar plates has shown that the this fungal defense response is most pronounced in the part of the mycelium that is in direct contact with the nematode. Hence, we hypothesized that, for a comprehensive characterization of this defense response, an experimental setup that maximizes the zone of direct interaction between the fungal mycelium and the nematode, was needed.

RESULTS

In this study, we conducted a transcriptome analysis of C. cinerea vegetative mycelium upon challenge with A. avenae using a tailor-made microfluidic device. The device was designed such that the interaction between the fungus and the nematode was confined to a specific area and that the mycelium could be retrieved from this area for analysis. We took samples from the confrontation area after different time periods and extracted and sequenced the poly(A)+ RNA thereof. The identification of 1229 differentially expressed genes (DEGs) shows that this setup profoundly improved sensitivity over co-cultivation on agar plates where only 37 DEGs had been identified. The product of one of the most highly upregulated genes shows structural homology to bacterial pore-forming toxins, and revealed strong toxicity to various bacterivorous nematodes. In addition, bacteria associated with the fungivorous nematode A. avenae were profiled with 16S rRNA deep sequencing. Similar to the bacterivorous and plant-feeding nematodes, Proteobacteria and Bacteroidetes were the most dominant phyla in A. avenae.

CONCLUSIONS

The use of a novel experimental setup for the investigation of the defense response of a fungal mycelium to predation by fungivorous nematodes resulted in the identification of a comprehensive set of DEGs and the discovery of a novel type of fungal defense protein against nematodes. The bacteria found to be associated with the fungivorous nematode are a possible explanation for the induction of some antibacterial defense proteins upon nematode challenge.

Seinura caverna n. sp. (Tylenchomorpha: Aphelenchoididae), an androdioecious species isolated from bat guano in a calcareous cave

A new species of a predator aphelenchoidid, Seinura caverna n. sp., is described and illustrated. The new species was isolated from bat guano collected from a calcareous cave in Japan. The new species is characterised by its three-lined lateral field, secretory-excretory pore at the level of the posterior two-thirds of the metacorpus, a long post-uterine sac, hermaphrodite tail shape elongate conoid with a filiform tip, and an androdioecious reproductive mode. The new species is typologically and biologically close to S. steineri, but is distinguished by its longer post-uterine sac, slightly longer stylet and slightly larger median bulb. The comparisons with other morphologically similar species, i.e., S. chertkovi, S. tenuicaudata and S. steineri, are discussed. A molecular phylogenetic analysis based on two small and large subunit ribosomal RNA genes revealed that the new species is located at the basal position of clade 3 of the Aphelenchoididae, clearly separate from S. demani, its congener with unclear rectum and anus, which is located at the derived position, suggesting that further generic revision is necessary for the genus. Aphelenchoides lii n. comb. (= Seinura lii) is proposed.

Toxicity of Potential Fungal Defense Proteins towards the Fungivorous Nematodes Aphelenchus avenae and Bursaphelenchus okinawaensis

Our results support the hypothesis that cytoplasmic proteins abundant in fungal fruiting bodies are involved in fungal resistance against predation. The toxicity of these proteins toward stylet-feeding nematodes, which are also capable of feeding on plants, and the abundance of these proteins in edible mushrooms, may open possible avenues for biological crop protection against parasitic nematodes, e.g., by expression of these proteins in crops.

Resistance of fungi to predation is thought to be mediated by toxic metabolites and proteins. Many of these fungal defense effectors are highly abundant in the fruiting body and not produced in the vegetative mycelium. The defense function of fruiting body-specific proteins, however, including cytoplasmically localized lectins and antinutritional proteins such as biotin-binding proteins, is mainly based on toxicity assays using bacteria as a heterologous expression system, with bacterivorous/omnivorous model organisms as predators. Here, we present an ecologically more relevant experimental setup to assess the toxicity of potential fungal defense proteins towards the fungivorous, stylet-feeding nematodes Aphelenchus avenae and Bursaphelenchus okinawaensis. As a heterologous expression host, we exploited the filamentous fungus Ashbya gossypii. Using this new system, we assessed the toxicity of six previously characterized, cytoplasmically localized, potential defense proteins from fruiting bodies of different fungal phyla against the two fungivorous nematodes. We found that all of the tested proteins were toxic against both nematodes, albeit to various degrees. The toxicity of these proteins against both fungivorous and bacterivorous nematodes suggests that their targets have been conserved between the different feeding groups of nematodes and that bacterivorous nematodes are valid model organisms to assess the nematotoxicity of potential fungal defense proteins.

IMPORTANCE Our results support the hypothesis that cytoplasmic proteins abundant in fungal fruiting bodies are involved in fungal resistance against predation. The toxicity of these proteins toward stylet-feeding nematodes, which are also capable of feeding on plants, and the abundance of these proteins in edible mushrooms, may open possible avenues for biological crop protection against parasitic nematodes, e.g., by expression of these proteins in crops.

Let's rise up to unite taxonomy and technology

What do you think of when you think of taxonomy? An 18th century gentlemen in breeches? Or perhaps botany drawings hung on the walls of a boutique hotel? Such old-fashioned conceptions to the contrary, taxonomy is alive today although constantly struggling for survival and recognition. The scientific community is losing valuable resources as taxonomy experts age and retire, and funding for morphological studies and species descriptions remains stagnant. At the same time, organismal knowledge (morphology, ecology, physiology) has never been more important: genomic studies are becoming more taxon focused, the scientific community is recognizing the limitations of traditional “model” organisms, and taxonomic expertise is desperately needed to fight against global biodiversity declines resulting from human impacts. There has never been a better time for a taxonomic renaissance.

Transfer of Bursaphelenchus okinawaensis (Nematoda: Aphelenchoididae) associated with Monochamus maruokai (Coleoptera: Cerambycidae) into M. alternatus (Coleoptera: Cerambycidae) and Psacothea hilaris (Coleoptera: Cerambycidae)

We examined the transfer ofBursaphelenchus okinawaensisassociated withMonochamus maruokaiintoM. alternatusandPsacothea hilarisusing a simple nematode loading method in order:i) to clarify the effects of cerambycid beetles on the formation of dauer juveniles ofB. okinawaensis; andii) to clarify whether dauer juveniles transfer to cerambycid beetles other thanM. maruokai. Dauer juveniles appeared at a high percentage withoutM. alternatusorP. hilarisand these beetles did not have any positive effects on their formation. Dauer juveniles transferred to the adults ofM. alternatusandP. hilaris. We concluded that dauer juveniles ofB. okinawaensisform readily without its vector beetles and may be able to transfer to many kinds of cerambycid beetles, although the numbers carried by a beetle are small.

Mating dynamics in a nematode with three sexes and its evolutionary implications

Nematodes have diverse reproductive strategies, which make them ideal subjects for comparative studies to address how mating systems evolve. Here we present the sex ratios and mating dynamics of the free-living nematode Rhabditis sp. SB347, in which males, females and hermaphrodites co-exist. The three sexes are produced by both selfing and outcrossing, and females tend to appear early in a mother’s progeny. Males prefer mating with females over hermaphrodites, which our results suggest is related to the female-specific production of the sex pheromones ascr#1 and ascr#9. We discuss the parallels between this system and that of parasitic nematodes that exhibit alternation between uniparental and biparental reproduction.