Revealing the genetic mechanisms underpinning the parasite-induced water-seeking behaviour of insects through RNA-seq.

Water-seeking behaviour in arthropods infected by mermithid nematodes or nematomorphs is a classic example of host manipulation, yet its molecular basis remains poorly understood. We investigated mechanisms underlying this aberrant behaviour in European earwigs (Forficula auricularia) infected with the nematode Mermis nigrescens using comparative RNA-seq across stages of infection and behavioural manipulation in both host and parasite. We detected 12 876 and 9722 expressed genes in earwigs and nematodes, respectively. Differential expression analyses identified 673 upregulated and 593 downregulated genes in earwigs, and 2672 upregulated and 2293 downregulated genes in nematodes across comparisons of infection stage and parasite size. Temporal clustering revealed six earwig and four nematode gene clusters. Gene ontology enrichment indicated increased signalling and sensory pathways in the host, while the parasite upregulated transport and secretory processes during manipulation, suggesting complementary and coordinated transcriptional changes between parasite and host. Notably, a shared GATA transcription factor motif was enriched among overexpressed genes in both organisms during manipulation. Together, these results identify candidate pathways and regulatory elements in host and parasite, providing new insights into the molecular regulation of parasite-induced water-seeking behaviour.