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.

Developmental transcriptomics in Pristionchus reveals the logic of a plasticity gene regulatory network.

Developmental plasticity enables the production of alternative phenotypes in response to different environmental conditions. While significant advances in understanding the ecological and evolutionary implications of plasticity have been made, understanding its genetic basis has lagged. However, a decade of genetic screens in the model nematode Pristionchus pacificus has culminated in 30 genes which affect mouth-form plasticity. We also recently reported the critical window of environmental sensitivity, and therefore have clear expectations for when differential gene expression should matter. Here, we collated previous data into a gene-regulatory network (GRN), and performed developmental transcriptomics across different environmental conditions, genetic backgrounds, and mouth-form mutants to assess the regulatory logic of plasticity. We found that only two genes in the GRN ( eud-1 and seud-1/sult-1 ) are sensitive to the environment during the critical window. Interestingly, the time points of their sensitivity differ, suggesting that they act as sequential checkpoints. We also observed temporal constraint upon the transcriptional effects of mutating the GRN and revealed unexpected feedback between mouth-form genes. Surprisingly, expression of seud-1/sult-1 , but not eud-1 , correlated with mouth form biases across different strains and species. Finally, a comprehensive analysis of all samples identified metabolism as a shared pathway for regulating mouth-form plasticity. These data are presented in a Shiny app to facilitate gene-expression comparisons across development in up to 14 different conditions. Collectively, our results suggest that mouth-form plasticity evolved a constrained, two-tiered logic to integrate environmental information leading up to the final developmental decision.

Histone 4 lysine 5/12 acetylation enables developmental plasticity of Pristionchus mouth form.

Development can be altered to match phenotypes with the environment, and the genetic mechanisms that direct such alternative phenotypes are beginning to be elucidated. Yet, the rules that govern environmental sensitivity vs. invariant development, and potential epigenetic memory, remain unknown. Here, we show that plasticity of nematode mouth forms is determined by histone 4 lysine 5 and 12 acetylation (H4K5/12ac). Acetylation in early larval stages provides a permissive chromatin state, which is susceptible to induction during the critical window of environmental sensitivity. As development proceeds deacetylation shuts off switch gene expression to end the critical period. Inhibiting deacetylase enzymes leads to fixation of prior developmental trajectories, demonstrating that histone modifications in juveniles can carry environmental information to adults. Finally, we provide evidence that this regulation was derived from an ancient mechanism of licensing developmental speed. Altogether, our results show that H4K5/12ac enables epigenetic regulation of developmental plasticity that can be stored and erased by acetylation and deacetylation, respectively.

Adult Influence on Juvenile Phenotypes by Stage-Specific Pheromone Production.

Many animal and plant species respond to population density by phenotypic plasticity. To investigate if specific age classes and/or cross-generational signaling affect density-dependent plasticity, we developed a dye-based method to differentiate co-existing nematode populations. We applied this method to Pristionchus pacificus, which develops a predatory mouth form to exploit alternative resources and kill competitors in response to high population densities. Remarkably, adult, but not juvenile, crowding induces the predatory morph in other juveniles. High-performance liquid chromatography-mass spectrometry of secreted metabolites combined with genetic mutants traced this result to the production of stage-specific pheromones. In particular, the P. pacificus-specific di-ascaroside#1 that induces the predatory morph is induced in the last juvenile stage and young adults, even though mouth forms are no longer plastic in adults. Cross-generational signaling between adults and juveniles may serve as an indication of rapidly increasing population size, arguing that age classes are an important component of phenotypic plasticity.

Environmental influence on Pristionchus pacificus mouth form through different culture methods.

Environmental cues can impact development to elicit distinct phenotypes in the adult. The consequences of phenotypic plasticity can have profound effects on morphology, life cycle, and behavior to increase the fitness of the organism. The molecular mechanisms governing these interactions are beginning to be elucidated in a few cases, such as social insects. Nevertheless, there is a paucity of systems that are amenable to rigorous experimentation, preventing both detailed mechanistic insight and the establishment of a generalizable conceptual framework. The mouth dimorphism of the model nematode Pristionchus pacificus offers the rare opportunity to examine the genetics, genomics, and epigenetics of environmental influence on developmental plasticity. Yet there are currently no easily tunable environmental factors that affect mouth-form ratios and are scalable to large cultures required for molecular biology. Here we present a suite of culture conditions to toggle the mouth-form phenotype of P. pacificus. The effects are reversible, do not require the costly or labor-intensive synthesis of chemicals, and proceed through the same pathways previously examined from forward genetic screens. Different species of Pristionchus exhibit different responses to culture conditions, demonstrating unique gene-environment interactions, and providing an opportunity to study environmental influence on a macroevolutionary scale.