Ultrastructural Description of Amphid Neurons in the Pine Wood Nematode Indicates Concurrent Evolution of the Stylet and Specific Neurons.

Understanding how animals perceive environmental stimuli is essential for reconstructing the evolution of their sensory systems. Nematodes provide a useful model for studying sensory adaptation due to their relatively simple nervous systems and broad ecological diversity. The amphid, the primary sensory organ in nematodes, has been well characterized in Caenorhabditis elegans and other bacterivorous species. However, comparatively little is known about amphid structures in nematodes with different ecological niches.​ In this study, we performed serial section transmission electron microscopy and three-dimensional reconstruction of amphid neurons in Bursaphelenchus xylophilus, a fungal-feeding, plant-parasitic nematode. We identified 13 amphid neurons, five of which showed a distinct morphology and are designated as type V neurons.These neurons were previously described as outer accessory cilia in other stylet-bearing nematodes, and had not been observed in bacterivorous species. Type V neurons exhibited trifurcated cilia that extended toward each lip and were structurally reminiscent of mechanosensory neurons.​ The presence of type V neurons only in stylet-bearing nematodes is consistent with the hypothesis that these neurons may have evolved in association with the stylet. Their trifurcated cilia suggest a potential role in detecting mechanical cues during lip contact with substrates, which could trigger stylet ejection. Alternatively, they may also contribute to other sensory modalities. Our findings reveal that fungal-feeding plant-parasitic nematodes possess amphid sensory architectures that differ markedly from those of bacterivorous species.