Genome-Wide Identification and Characterization of Chemosensory Gene Families in the Mayfly Parafronurus youi (Ephemeroptera: Heptageniidae)

Background: Chemoreception plays a central role in how insects perceive environmental chemical cues and regulate essential behaviors. Mayflies (Ephemeroptera), among the earliest-diverging lineages of winged insects, are important bioindicators of freshwater ecosystems, yet their chemosensory gene repertoire remains poorly characterized. Methods: Using the high-quality genome of Parafronurus youi, we conducted a genome-wide identification and comparative analysis of six major chemosensory gene families. Results: We identified 72 candidate chemosensory genes, including 10 OBPs, 15 CSPs, 9 ORs, 14 GRs, 23 IRs, and 1 SNMP. These gene families differed markedly in physicochemical properties, conserved motifs, domain architecture, gene structure, chromosomal distribution, and phylogenetic relationships. Chemosensory genes were unevenly distributed across the 11 chromosomes, with several families showing clustered organization, whereas no obvious collinear relationships were detected. Compared with representative terrestrial insects, P. youi possesses a comparatively compact chemosensory repertoire, although this comparison should be interpreted cautiously because data sources and annotation strategies differ among studies. Different gene families showed distinct evolutionary patterns, including structurally conserved soluble binding proteins and co-receptors, limited diversification in several receptor families, and a comparatively well-represented IR repertoire. Conclusions: This study provides the first genome-wide overview of chemosensory genes in Ephemeroptera and offers a basis for future functional and evolutionary studies of chemoreception in palaeopteran insects.