Transcriptional variation and divergence of host-finding behaviour in Steinernema carpocapsae infective juveniles

Abstract

BackgroundSteinernema carpocapsae is an entomopathogenic nematode that employs nictation and jumping behaviours to find potential insect hosts. Here we aimed to investigate the transcriptional basis of variant host-finding behaviours in the infective juvenile (IJ) stage of three S. carpocapsae strains (ALL, Breton and UK1), with a focus on neuronal genes known to influence behaviour in other nematode species. Identifying gene expression changes that correlate with variant host-finding behaviours will further our understanding of nematode biology.ResultsRNA-seq analysis revealed that whilst up to 28% of the S. carpocapsae transcriptome was differentially expressed (P < 0.0001) between strains, remarkably few of the most highly differentially expressed genes (> 2 log2 fold change, P < 0.0001) were from neuronal gene families. S. carpocapsae Breton displays increased chemotaxis toward the laboratory host Galleria mellonella, relative to the other strains. This correlates with the up-regulation of four srsx chemosensory GPCR genes, and a sodium transporter gene, asic-2, relative to both ALL and UK1 strains. The UK1 strain exhibits a decreased nictation phenotype relative to ALL and Breton strains, which correlates with co-ordinate up-regulation of neuropeptide like protein 36 (nlp-36), and down-regulation of an srt family GPCR gene, and a distinct asic-2-like sodium channel paralogue. To further investigate the link between transcriptional regulation and behavioural variation, we sequenced microRNAs across IJs of each strain. We have identified 283 high confidence microRNA genes, yielding 321 predicted mature microRNAs in S. carpocapsae, and find that up to 36% of microRNAs are differentially expressed (P < 0.0001) between strains. Many of the most highly differentially expressed microRNAs (> 2 log2 fold, P < 0.0001) are predicted to regulate a variety of neuronal genes that may contribute to variant host-finding behaviours. We have also found evidence for differential gene isoform usage between strains, which alters predicted microRNA interactions, and could contribute to the diversification of behaviour.ConclusionsThese data provide insight to the transcriptional basis of behavioural variation in S. carpocapsae, supporting efforts to understand the molecular basis of complex behaviours in nematodes.

Highly Divergent Neuropeptide - non-coding RNA regulatory networks underpin variant host-finding behaviours inSteinernemaspecies infective juveniles

“…Expression levels of ten neuropeptides genes correlated with behaviour across the three species, of which three, nlp-36, flp-11 and flp-25 showed the best evidence across the study of playing a potentially functional role. nlp-36 was previously shown to correlate with decreased nictation behaviour across three S. carpocapsae strains (Warnock et al, 2019); the current finding that expression of this gene also correlates with decreased nictation behaviour across three species of Steinernema reinforces the evidence that nlp-36 is crucial to these differences. In the model nematode Caenorhabditis elegans there is evidence that a cyclic nucleotide-gated channel subunit, TAX-2, which positively regulates NLP-36, is involved in thermosensation, olfaction, chemosensation, and axon guidance (Coburn and Bargmann, 1996; Coburn et al, 1998); differences in host-seeking behaviour could be mediated via nlp-36 expression by any of these pathways.…”

“…The IJs display qualitatively different host-finding strategies between species (Spence et al, 2008). However we have also reported substantial behavioural variation between populations of Steinernema carpocapsae (Warnock et al, 2019). S. carpocapsae is classed as an ‘ambusher’, and will nictate and jump directionally in response to host-specific volatiles and mechanosensory input.…”

“…In recent work, we demonstrated that behavioural variation between populations of S. carpocapsae correlated with variation in the expression of genes and microRNAs. In particular, a range of neuronal genes were implicated, including neuropeptides (J. S. Lee et al, 2017; Morris et al, 2017; Warnock et al, 2019). Neuropeptide gene expression is enriched within the IJ stage of numerous parasitic nematode species (J. S. Lee et al, 2017), suggesting a critical role in behavioural diversification, which is supported by functional investigation in S. carpocapsae (Morris et al, 2017).…”

“…A previous study of the genetic basis of behavioural difference in S. carpocapsae strains found evidence of an interaction between isoform variation and attendant microRNA targeting influencing behaviour (Warnock et al, 2019). Here for the first time long non-coding isoforms of neuropeptide transcripts are shown to be the targets of microRNAs associated with different host-finding behaviours in three Steinernema species.…”

Highly divergent neuropeptide – non-coding RNA regulatory networks underpin variant host-finding behaviours in Steinernema species infective juveniles

“…The IJs display qualitatively different host-finding strategies between species (Spence et al, 2008). However, we have also reported substantial behavioural variation between populations of Steinernema carpocapsae (Warnock et al, 2019). Steinernema carpocapsae is classed as an 'ambusher', and will nictate and jump directionally in response to host-specific volatiles and mechanosensory input.…”