In its natural habitat, the nematode Caenorhabditis elegans encounters a plethora of other organisms, including many that are pathogenic [1, 2]. The study of interactions between C. elegans and various pathogens has contributed to characterizing key mechanisms of innate immunity [2-4]. However, how C. elegans recognizes different pathogens to mount pathogen-specific immune responses remains still largely unknown [3, 5-8]. Expanding the range of known C. elegans-infecting pathogens and characterizing novel pathogen-specific immune responses are key steps toward answering this question. We report here that the oomycete Myzocytiopsis humicola is a natural pathogen of C. elegans, and we describe its infection strategy. We identify a new host immune response to pathogen exposure that involves induction of members of a previously uncharacterized gene family encoding chitinase-like (CHIL) proteins. We demonstrate that this response is highly specific against M. humicola and antagonizes the infection. We propose that CHIL proteins may diminish the ability of the oomycete to infect by hindering pathogen attachment to the host cuticle. This work expands our knowledge of natural eukaryotic pathogens of C. elegans and introduces a new pathosystem to address how animal hosts recognize and respond to oomycete infections.

Biological systems are subject to inherent stochasticity. Nevertheless, development is remarkably robust, ensuring the consistency of key phenotypic traits such as correct cell numbers in a certain tissue. It is currently unclear which genes modulate phenotypic variability, what their relationship is to core components of developmental gene networks, and what is the developmental basis of variable phenotypes. Here, we start addressing these questions using the robust number of Caenorhabditis elegans epidermal stem cells, known as seam cells, as a readout. We employ genetics, cell lineage tracing, and single molecule imaging to show that mutations in lin-22, a Hes-related basic helix-loop-helix (bHLH) transcription factor, increase seam cell number variability. We show that the increase in phenotypic variability is due to stochastic conversion of normally symmetric cell divisions to asymmetric and vice versa during development, which affect the terminal seam cell number in opposing directions. We demonstrate that LIN-22 acts within the epidermal gene network to antagonise the Wnt signalling pathway. However, lin-22 mutants exhibit cell-to-cell variability in Wnt pathway activation, which correlates with and may drive phenotypic variability. Our study demonstrates the feasibility to study phenotypic trait variance in tractable model organisms using unbiased mutagenesis screens.

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 geographic origin. Anaerobes Synergistaceae and sulfate-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 sulfate-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.

Populations often display consistent developmental phenotypes across individuals despite inevitable biological stochasticity. Nevertheless, developmental robustness has limits, and systems can fail upon change in the environment or the genetic background. We use here the seam cells, a population of epidermal stem cells in Caenorhabditis elegans, to study the influence of temperature change and genetic variation on cell fate. Seam cell development has mostly been studied so far in the laboratory reference strain (N2), grown at 20° temperature. We demonstrate that an increase in culture temperature to 25° introduces variability in the wild-type seam cell lineage, with a proportion of animals showing an increase in seam cell number. We map this increase to lineage-specific symmetrization events of normally asymmetric cell divisions at the fourth larval stage, leading to the retention of seam cell fate in both daughter cells. Using genetics and single-molecule imaging, we demonstrate that this symmetrization occurs via changes in the Wnt asymmetry pathway, leading to aberrant Wnt target activation in anterior cell daughters. We find that intrinsic differences in the Wnt asymmetry pathway already exist between seam cells at 20° and this may sensitize cells toward a cell fate switch at increased temperature. Finally, we demonstrate that wild isolates of C. elegans display variation in seam cell sensitivity to increased culture temperature, although their average seam cell number is comparable at 20°. Our results highlight how temperature can modulate cell fate decisions in an invertebrate model of stem cell patterning.

A fundamental question in medical genetics is how the genetic background modifies the phenotypic outcome of mutations. We address this question by focusing on the seam cells, which display stem cell properties in the epidermis of Caenorhabditis elegans. We demonstrate that a putative null mutation in the GATA transcription factor egl-18, which is involved in seam cell fate maintenance, is more tolerated in the CB4856 isolate from Hawaii than the lab reference strain N2 from Bristol. We identify multiple quantitative trait loci (QTLs) underlying the difference in phenotype expressivity between the two isolates. These QTLs reveal cryptic genetic variation that reinforces seam cell fate through potentiating Wnt signalling. Within one QTL region, a single amino acid deletion in the heat shock protein HSP-110 in CB4856 is sufficient to modify Wnt signalling and seam cell development, highlighting that natural variation in conserved heat shock proteins can shape phenotype expressivity.

A fundamental question in medical genetics is how the genetic background modifies the phenotypic outcome of key mutations. We address this question by focusing on the epidermal seam cells, which display stem cell properties in Caenorhabditis elegans. We demonstrate that a null mutation in the GATA transcription factor egl-18, which is involved in seam cell fate maintenance, is more tolerated and thus has lower expressivity in the divergent CB4856 isolate from Hawaii than the lab reference strain N2 from Bristol. We identify multiple quantitative trait loci (QTLs) underlying the difference in mutation expressivity between the two isolates. These QTLs reveal cryptic genetic variation, which acts to reinforce seam cell fate through potentiating Wnt signalling. Within one QTL region, a single amino acid deletion in the heat shock protein HSP-110 in CB4856 lowers egl-18 mutation expressivity. Our work underscores that natural variation in conserved heat shock proteins can shape mutation expressivity.

Developmental patterning in Caenorhabditis elegans is known to proceed in a highly stereotypical manner, which raises the question of how developmental robustness is achieved despite the inevitable stochastic noise. We focus here on a population of epidermal cells, the seam cells, which show stem cell-like behaviour and divide symmetrically and asymmetrically over post-embryonic development to generate epidermal and neuronal tissues. We have conducted a mutagenesis screen to identify mutants that introduce phenotypic variability in the normally invariant seam cell population. We report here that a null mutation in the fusogen eff-1 increases seam cell number variability. Using time-lapse microscopy and single molecule fluorescence hybridisation, we find that seam cell division and differentiation patterns are mostly unperturbed in eff-1 mutants, indicating that cell fusion is uncoupled from the cell differentiation programme. Nevertheless, seam cell losses due to the inappropriate differentiation of both daughter cells following division, as well as seam cell gains through symmetric divisions towards the seam cell fate were observed at low frequency. We show that these stochastic errors likely arise through accumulation of defects interrupting the continuity of the seam and changing seam cell shape, highlighting the role of tissue homeostasis in suppressing phenotypic variability during development.

23Populations often display consistent developmental phenotypes across individuals 24 despite the inevitable biological stochasticity. Nevertheless, developmental robustness 25 has limits and systems can fail upon change in the environment or the genetic 26 background. We use here the seam cells, a population of epidermal stem cells in 27 Caenorhabditis elegans, to study the influence of temperature change and genetic 28 variation on cell fate. Seam cell development has mostly been studied so far in the lab 29 reference strain (N2), grown at 20° temperature. We demonstrate that an increase in 30 culture temperature to 25°, introduces variability in the wild-type seam cell lineage with a 31 proportion of animals showing an increase in seam cell number. We map this increase to 32 lineage-specific symmetrisation events of normally asymmetric cell divisions at the final 33 larval stage, leading to the retention of seam cell fate in both daughter cells. Using 34 genetics and single molecule imaging, we demonstrate that this symmetrisation occurs 35 via changes in the Wnt asymmetry pathway, leading to aberrant Wnt target activation in 36 anterior cell daughters. We find that intrinsic differences in the Wnt asymmetry pathway 37 already exist between seam cells at 20° and this may sensitise cells towards a cell fate 38 switch at increased temperature. Finally, we demonstrate that wild isolates of C. elegans 39 display variation in seam cell sensitivity to increased culture temperature, although seam 40 cell numbers are comparable when raised at 20°. Our results highlight how temperature 41 can modulate cell fate decisions in an invertebrate model of stem cell patterning. 42 43 44 45 46 47 48 During development, organisms must withstand environmental and genetic perturbations 49 to produce consistent phenotypes (Felix and Barkoulas 2012). These phenotypes are 50 often a product of complex developmental events that require a tight balance between cell 51 division and cell differentiation (Soufi and Dalton 2016). A key example is stem cell 52 divisions, consisting of highly controlled asymmetric and symmetric patterns, which are 53 vital for generating cell diversity, as well as maintaining cell numbers in tissues and organs 54 (Morrison and Kimble 2006; Knoblich 2008). Developmental robustness has inherent 55 limits and certain perturbations can push a system outside its buffering zone (Braendle 56 and Felix 2008; Barkoulas et al. 2013). In these cases, it is also important to understand 57 how systems fail by investigating how perturbations precisely modulate developmental 58 processes. Here we address the question of how changes in environmental temperature 59 can affect cell fate outcomes using the nematode C. elegans as a model system. While it 60 is well known that increasing or decreasing environmental temperature can change the 61 development speed in C. elegans, the effect of temperature on specific cell division and 62 fate acquisition events is less well understood. The C. elegans adult hermaphrodite 63 co...