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This study demonstrates that phasmid sensory neurons and caudal glands coexist in Mononchus aquaticus, an early-branching aquatic nematode, challenging the long-held assumption that these structures are mutually exclusive. Using light and electron microscopy, researchers refute the hypothesis that phasmid neurons evolved from caudal glands during the transition from aquatic to terrestrial habitats. The findings suggest that phasmid sensory organs may have been present in ancestral nematodes rather than being a derived trait associated with terrestrial adaptation.
Why it matters
This discovery fundamentally changes our understanding of nematode evolution and cell type evolution more broadly, demonstrating that evolutionary transitions may be more complex than previously assumed. It also has implications for the classification of nematodes, as the historical division into Aphasmidia and Phasmidia based on phasmid presence appears to be incorrect.
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⚠️ Preprint – Noch nicht peer-reviewed
Dieser Artikel wurde noch nicht von unabhängigen Experten begutachtet. Die Ergebnisse sind vorläufig und sollten mit Vorsicht interpretiert werden.
The highly conserved body plan of nematodes makes members of this phylum excellent models to study cell type evolution. Early branching nematode lineages, mostly occupying aquatic habitats, usually contain caudal glands deployed for underwater attachment to a substrate, but have been thought to lack phasmid sensory organs, resulting in their historical classification as Aphasmidia. With the transition to a terrestrial environment, nematodes lost caudal glands and gained phasmid sensory neurons. The supposed mutually exclusive existence of caudal glands and phasmids has led to the suggestion that phasmid neurons may have evolved from caudal glands. Here, we rule out this possibility through light and electron microscopical analysis of Mononchus aquaticus, a member of the early branching Dorylaimia lineage, showing that phasmid sensory neurons and caudal glands do coexist. This observation not only argues against a proposed cell type evolution scenario accompanying aquatic-to-terrestrial transitions but also indicates that the presence of phasmid sensory organs may have been an ancestral trait of the nematode phylum.