Gene Expression Analysis Provides Insights Into the Functional and Developmental Differentiations of Pleopodal Lungs in a Terrestrial Isopod Crustacean, <i>Porcellio scaber</i>

Acquisition of air-breathing organs was one of the key events in the diversification of arthropods. Among terrestrial arthropods, isopod crustaceans have evolved a unique air-breathing structure called the pleopodal lung, which is located in their abdominal appendages (pleopods), while retaining pleopodal gills. These lungs offer an intriguing model for studying the evolution of respiratory organs during arthropod terrestrialization. However, the molecular mechanisms underlying lung function or development in isopods remain poorly understood. In this study, we conducted comparative transcriptomic analyzes using the common rough woodlouse, Porcellio scaber, in which pleopods with and without lungs are adjacent to each other. The results revealed distinct gene expression profiles linked to the structure and function of pleopods, including genes involved in morphogenesis. In particular, candidate lung development regulatory genes that were expressed specifically in the exopods of the second pleopods during the manca 1 stage were identified. Transcriptome analysis and immunohistochemistry suggested that the Hox gene abdominal-A is involved in lung formation. However, the two genes previously implicated in respiratory organ formation in pancrustaceans, trachealess and ventral veins lacking, did not show lung-related expression. Our comparison of gene expression patterns between exopods with and without lungs suggest that the function of gas exchange in the pleopodal lungs may be influenced by structural differences resulting from changes in developmental processes. Overall, this study provides essential insights into the molecular mechanisms underlying pleopodal lung development and sets the foundation for future evolutionary research.