Expanding the view on the evolution of the nematode dauer signalling pathways: refinement through gene gain and pathway co-option
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Signalling pathways underlie development, behaviour and pathology. To understand patterns in the evolution of signalling pathways, we undertook a comprehensive investigation of the pathways that control the switch between growth and developmentally quiescent dauer in 24 species of nematodes spanning the phylum.
Our analysis of 47 genes across these species indicates that the pathways and their interactions are not conserved throughout the Nematoda. For example, the TGF-β pathway was co-opted into dauer control relatively late in a lineage that led to the model species Caenorhabditis elegans. We show molecular adaptations described in C. elegans that are restricted to its genus or even just to the species. Similarly, our analyses both identify species where particular genes have been lost and situations where apparently incorrect orthologues have been identified.
Our analysis also highlights the difficulties of working with genome sequences from non-model species as reliance on the published gene models would have significantly restricted our understanding of how signalling pathways evolve. Our approach therefore offers a robust standard operating procedure for genomic comparisons.
KeywordsGenome Assembly Mass Drug Administration Nematode Species Infective Stage Versus Species
An animal’s phenotype, whether developmental or behavioral in response to stimuli, is mediated through signalling pathways. While there are a limited number of pathway types, they can vary in terms of membership and can be strung together to form complex interactions . Given their role in development and in human disease, understanding how these pathways evolve is an important, outstanding question. The surge of metazoan genomes has created the temptation to search for orthologous genes involved in a pathway of interest and then to offer some biological interpretations. Such comparisons are a routine feature of publications announcing new genomes. However, genomes are often published at a relatively early draft stage, with errors in the assembly leading to incorrect or absent gene models. This is an important pitfall for comparative genomics and molecular biology analyses; the non-detection of an orthologue is not necessarily biologically relevant, but can be the consequence of technical issues. Here, we have investigated the utility and viability of genomic comparisons in the context of the evolution of signalling pathways. As a model pathway, we have analyzed the conservation of the Caenorhabditis elegans dauer larva development pathways across the Nematoda.
The phylum Nematoda is an excellent system for the study of pathway evolution for two reasons. Firstly, from a free-living ancestral state, parasitism of plants and animals has evolved independently at least 15 times . These transitions were made possible through a range of molecular adaptations , which must include changes in the architecture of signalling pathways as they respond to new stimuli . Secondly, constituents of these pathways are likely a promising source of new anti-parasitics, possibly through repurposing already licensed drugs . Multi-drug resistance is a growing problem in veterinary medicine and an emerging threat to the efficacy of human-based mass drug administration programs . The search for new control strategies has motivated sequencing projects for many species of parasitic nematodes . Morphological and behavioral similarities between the C. elegans dauer and the infective stage of some parasitic species has led to the dauer hypothesis, in which pre-adaptations in free-living ancestors led to the multiple independent transitions to parasitism as a life strategy [9, 10]. This hypothesis is gaining support through biochemical manipulation of the pathways in parasitic species . Understanding the conservation of genes that control this important stage could therefore prioritise targets for new anthelmintics.
Here, we have identified homologues – putative orthologues – of 47 genes across C. elegans and 23 other nematode species, both free-living and parasitic. While the pathways controlling the dauer transition are generally well-conserved, we identify an evolutionary path of gene duplication and pathway co-option that leads from the ancestral nematode to C. elegans. Further, we highlight the issues encountered and solutions implemented when working with draft genomes.
Identifying potential homologues for dauer signalling genes
Homologues for 14 C. elegans genes, daf-38, tax-2, tax-4, daf-2, age-1, pdk-1, pptr-1, akt-1, ftt-2, daf-16, daf-1, pdp-1, egl-4, ncr-1 were present in all 23 species (Fig. 2). If we removed species of Clade I from the count, then homologues for a further eight genes are present in all remaining species. Below, we consider each of the four pathways and the conservation of homologues.
The genes tax-2 and tax-4 encode subunits of a sensory transduction nucleotide-gated channel and are conserved throughout the species examined. The daf-11 encoded transmembrane guanylate cyclase, whose activity powers the channel, is found in Clade III, IV and V species. The set of G-protein subunits and the G-protein coupled receptors (GPCRs) are less well conserved. Their proteins are implicated as sensors of environmental cues and control C. elegans’ dauer. The GPCR daf-38 is ubiquitous, with gpa-3 lost in the Trichinella/Trichuris lineage. The other components are likely restricted to the Caenorhabditis species, with srg-37 found only in C. elegans.
The C. elegans genome is predicted to contain 40 genes that encode for insulin-like proteins . Five have been implicated in the regulation of dauer [38, 39, 40]. Proteins encoded by ins-4, ins-6 and daf-28 are agonists and are restricted to Caenorhabditis species. The antagonists, ins-1 and ins-18, are more widely conserved.
Dauer TGF − β signalling
Broadly conserved across the phylum were genes that encode the TGF − β ligand, DAF-7, the subunits of its receptor, DAF-1 and DAF-4, and the receptor’s regulator BRA-1. Genes encoding the Smad transcription factors were increasingly restricted in their phylogenetic distribution. daf-8, an inducer of transcription, and daf-3, a repressor, were found in Clade IV and V species. Another inducer, daf-14, was found throughout Clade V. Homologues to another repressor, egl-4, were found throughout, while daf-5 was restricted to Clade V species. In the Clade I species, we were unable to find homologues for any of the Smad transcription factors, daf-5, scd-1 or scd-2.
Phylogenetic reconstruction of dauer TGF-β ligand
Notable from the TGF-β pathway was the absence of a homologue for its ligand, daf-7, in Strongyloides ratti. Several studies have proposed a putative orthologue for daf-7; the candidate gene has been cloned from S. ratti (AY672707), the sister species Strongyloides stercoralis (AAV84743) and Parastrongyloides trichosuri (ABQ10586) [42, 43]. While we could not find a significant alignment between AY672707 and any S. ratti gene model, we did successfully recover a splice-aware alignment from the raw genome assembly [33, 34]. However, even the inclusion of this model in the S. ratti annotation did not generate a FRB match to C. elegans daf-7.
Steroid hormone signalling
This pathway was conserved throughout species in Clades III, IV and V, with ncr-2 restricted to Caenorhabditis species. High sequence similarity between the sterol transport proteins NCR-1 and NCR-2 (BLASTP; Expect = 0) indicates a gene duplication event early in the Caenorhabditis lineage. Both the nuclear hormone receptor, daf-12, and the cytochrome P450 daf-9 are essential in the regulation of dauer development [51, 52]. Searches of the gene models and genome assembly failed to return a match for daf-12 in either H. contortus isolates. However, a likely candidate daf-12 was found when C. elegans proteins were aligned to DNA sequence reads generated as part of the H. contortus genome projects [14, 15, 37]. The same approach was necessary to identify daf-9 in N. americanus. Homologues to neither could be robustly identified in T. spiralis and T. suis. However, it is important to note that both these species have significantly reduced complements of nhr and cyp gene classes (D. Curran pers. comm.).
The ability of C. elegans to undergo dauer arrest is one of the most renowned postembryonic developmental transitions in any species. Here, we present the evolution of the signalling pathways that control dauer throughout the nematode phylum. There are four points for discussion: pathway-level evolution in dauer, gene-level evolution in dauer, the analogy of free-living dauer and parasitic infective stages, and the quality of draft genomes.
The decision to enter dauer is controlled in C. elegans by at least four pathways, with Insulin/IGF-1 and TGF-β acting synergistically. With compelling comparisons drawn between the dauer arrested state of C. elegans and the infective stage of some parasitic nematodes , the question here was whether common genetic pathways existed across the phylum that would likely control this transition. The answer is yes for species of Clades III, IV and V, and unlikely for Clade I.
Of the four pathways the cGMP and Insulin/IGF-1 pathways are broadly conserved. In the cGMP pathway, the nucleotide-gated channel (subunits: tax-2 and tax-4) is ubiquitous. However, the identity of the genes involved in the detection of environmental cues, the GPCRs and G-proteins is less clear. Our approach has been to take a simple assumption and look for the homologues – putative orthologues – of C. elegans proteins. However, genes that sample the environment are frequently members of large gene families that display complex patterns of gene birth and death [53, 54, 55]. This makes transference of function difficult, even between closely related species, as observed in Caenorhabditis pheromone receptors . In non-Caenorhabditis species, it is likely that several GPCRs are involved in sensing environmental cues. However, preliminary evidence points to a reduced complement in the parasites, H. contortus, S. ratti and A. suum (data not shown). Whether this is a consequence of the more restricted environments of parasitic nematodes remains to be determined. While the antagonist of the insulin receptor DAF-2 appear to be broadly conserved in nematodes, putative orthologues to known agonists in C. elegans were more restricted. It is probable that DAF-2 receptors in non-Caenorhabditis species bind other insulin-like peptides. A comprehensive genomic survey and phylogenetic reconstruction of these relatively short – approximately 110 amino acids – is necessary.
Single gene studies have proposed homology in the Insulin/IGF-1 pathway in several parasitic species, with functional assays for daf-2 , age-1  and daf-16 [41, 59, 60, 61]. The conservation of the DAF-7–TGF-β pathway is less clear. The pattern of gene homology in Clade IV and V species is indicative of a DAF-7-TGF-β pathway that acts synergistic to Insulin/IGF-1. The antagonist pair, daf-8 and daf-5, arose in the MRCA of Clade IV and V. The activation and repression of this pathway with respect to the dauer phenotype has been fine-tuned with daf-14 and daf-5 in the MRCA of the Rhabditida (H. contortus and N. americanus) and Rhabditoidea (Caenorhabditis). From a broader perspective, the patterns of gene homology indicate a more fundamental function for the DAF-7 induced pathway and later co-option in the Clade IV/V MRCA to the regulation of life stage transition. Support for this hypothesis is provided by the phenotypes exhibited by mutant strains and RNA interference assays. Knock-out of the genes early in the pathway display defects related to related to egg-laying in addition to the altering likelihood of dauer development. The phenotypes associated with daf-3 and daf-5 are restricted to dauer. However, even within the Clade V nematodes, the expression of daf-7 is confounding. In C. elegans, daf-7 expression peaks in L2, reducing significantly in a dauer arrested animal . A reduction of daf-7 expression likely leads to dauer entry. However, inspection of the RNA-Seq data from H. contortus and N. americanus shows that daf-7 expression is maximal in the infective L3 stage, which is supported by real-time PCR in H. contortus [14, 16, 46]. One proposition is that the DAF-7–TGF-β pathway has flipped in parasitic nematodes to maintain the infective stage and promotes transition to the adult . An assumption there is that the role of the pathway in C. elegans is the ancient characteristic, with the parasitic one being derived. Interestingly, the daf-7 orthologues of Clade III B. malayi and Clade I T. suis have expression patterns that match the Clade V parasites [26, 45], but in Clade III T. canis expression in the adult is significantly greater than the infective L3s (iL3) . Therefore, we propose an alternative evolutionary path, in which the iL3 to adult transition through DAF-7–TGF-β mediation is basal, and the behavior observed in C. elegans is derived. Detailed gene expression profiles across the entire life-cycle for the free-living P. pacificus and P. redivius will, we hope, provide enlightenment.
The final pathway, steroid hormone biosynthesis, is well conserved in Clade III, IV and V species. In the final step, dafachronic acids, a class of steroid hormones generated by daf-9, bind to daf-12, a nuclear hormone receptor. For Strongyloides stercoralis, a facultative parasite (Clade IV), the application of exogeneous dafachronic acid to L1 worms led to the development of free-living adults, rather than iL3s . The cytochrome P450 daf-9 was previously reported to be lost in parasitic lineages [63, 64]. Searches for functional equivalents in cytochrome P450s will be difficult in such a large gene family . Here, we have provided promising candidates for validation by functional rescue.
For species of Clade I, the most striking absences are for daf-11 in all three species and daf-9 in T. spiralis and T. muris. The daf-11 gene encodes a receptor that defines the cGMP pathway, which controls daf-2 transcription in response to environmental stimuli. The gene daf-9 encodes for a cytochrome P450 that generates the sterol ligands for DAF-12, which leads to reproductive growth. The phylogenetic distribution of daf-9 is most parsimoniously explained by gene loss, a consequence of a major reduction in genome size and gene count. Our current work shows that T. spiralis and T. muris likely only have two or three cytochrome P450 genes (unpublished). These must have broad specificity and are unlikely to carry out the precise role required of daf-9 in C. elegans. Similarly, there are few other guanylate cyclases encoded in the three Clade I species to carry out the function of daf-11. The role of the cGMP pathway in transmitting signal to the insulin pathway through daf-11 may therefore have arisen after the split of Clade I nematodes. Alternatively, daf-11 may be present in other, as yet unsampled, Clade I species and has been lost in two distinct lineages. A functional analogue of daf-11 might be unnecessary in the species surveyed. Their infective stage is L1 for T. spiralis and T. suis and L2 in R. culicivorax. In all these species the transition from infective larva to adult is through a series of rapid molts, the control of which may not be in response to external stimuli.
The refinement of signalling pathways transducing a stimulus to a response are often considered at a macro-level, with the co-option of new pathway types ; here, DAF-7–TGF-β in Clade V nematodes. Equally important are micro-level refinements. We observed gene duplications restricted to Caenorhabditis and to just C. elegans. The products interact and provide functional redundancy. There were instances of putative duplications for other genes in other nematodes, though genome assembly error through allelic polymorphism cannot be ruled out for some.
Finally, it is important to make a comment on the use of draft genome assemblies in non-model organism genomes. The assignment of gene homology between C. elegans and another species relies on careful consideration of sequence similarity shared between two gene models or a model and a region of an assembled genome [33, 34, 35, 36]. In some instances, this required aligning short genomic reads against seed proteins . We recognize the limitations in transferring functional annotation based on sequence similarity, but emphasize the use of multiple alignment algorithms and the manual inspection of each alignment. This gene-by-gene consideration proved critical, as no single alignment score threshold would have sufficed. Importantly, the sole reliance on gene models would have resulted in the homologous relationships for over 30 % of the genes going unfound. Central to this is the new crop of protein-to-genome aligners that are refining long established genome annotations [33, 66].
Genome assemblies and predicted gene models were downloaded from Wormbase  for all species, except Romanomermis culicivorax, which was downloaded from http://nematodes.org/genomes/romanomermis_culicivorax/index.html. The gene expression data for each species was taken from the supplementary information of each genome publication and is cited in the text.
Seven search strategies were used to identify potential homologues. The first is our own implementation of the fuzzy reciprocal BLASTP hits first used across 12 species of Drosophila . The remaining six methods used C. elegans protein sequences to search the sequence of the genome assemblies: TBLASTN , Figmop , spaln  exonerate , genBLAST  and DIAMOND . We have previously used Figmop to annotate cytochrome P450 genes missed by standard gene finding protocols . All search results were inspected manually with the help of Kablammo . Gene models generated by the six genome-based search strategies often overlapped gene models published as part of that species’ genome paper. In these instances, both gene models were aligned to the C. elegans protein. Our newer gene models were considered a minor improvement if their alignment had a coverage 5 % greater than the published gene model, and a major improvement if the alignment has a coverage 15 % greater. Where our predictions overlapped multiple published gene models or no gene model, the new model was considered a major improvement. The presence and absence matrix (Fig. 2) was generated using the ETE Toolkit . The alignments generated by DIAMOND were processed using samtools  and Fig. 3 was generated using Geneious version 9 (http://www.geneious.com, ).
TGF-β phylogenetic reconstruction
In addition to the putative homologues to DAF-7, searches were run using C. elegans DBL-1 (Wormbase: T25F10.2), TIG-2 (F39G3.8), TIG-3 (Y46E12BL.1) and UNC-129 (C53D6.2) as the seeds. Individually cloned sequences were identified from the literature. The alignment was built with MAFFT, using the linsi options . The most appropriate phylogenetic model was LG + G . The phylogenetic tree was reconstructed with PhyML using the following command line options: ‘-c 4 -m LG -v 0.0 -a e -o tlr -f d -d aa -b 1000 -s BEST --rand_start –n_rand_starts 101’ . Nodes with less than 50 % bootstrap support were left unresolved. The final tree figure was generated using TreeGraph 2  and FigTree (http://tree.bio.ed.ac.uk/software/figtree/).
This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) through a Discovery Grant (#06239-2015) to JDW and an Collaborative Research and Training Experience Program (CREATE) program in Host-Parasite Interactions (#413888-2012) to JDW.
AG – Study design, data collection, data analysis, writing of the manuscript. DMC – Data analysis. SCH – Study design, writing of the manuscript. JDW –Study design, data collection, data analysis, writing of the manuscript. All authors have read and approved the final manuscript.
The authors declare that they have no competing interests.
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