Genomic exaptation and regulatory landscape shifts as key mechanisms enabling flatworm terrestrialization

Understanding the genomic toolkit that facilitated animal terrestrialization - the transition from aquatic to terrestrial environments - is crucial for unravelling the evolutionary processes behind the origin and diversification of terrestrial biodiversity. Despite its significance, the genomic foundations driving the physiological and metabolic adaptations required for life on land remain largely unexplored across most terrestrial animal phyla. Planarians (phylum Platyhelminthes) represent an ideal model for studying terrestrialization, as only one terrestrial lineage, the family Geoplanidae (order Tricladida) is known to exist. Here, we used an integrative approach combining genomics, transcriptomics, and proteomics to investigate the genetic underpinnings potentially facilitating adaptation to terrestrial environments. Our analysis revealed a significant burst of gene gain preceding the diversification of terrestrial planarians and their split from freshwater relatives, in the branch leading to Tricladida. Upon exposure to abiotic stress, terrestrial and freshwater planarians exhibited distinct genetic toolkits: most differentially expressed genes emerged in orthologous groups gained specifically in the branch leading to Tricladida, over half of which showed signs of strong directional selection in terrestrial flatworms, indicating their adaptive importance for land colonization. Transcriptomic analyses further revealed contrasting stress responses: terrestrial planarians upregulated ancient genes whose origin predates the Geoplanidae lineage to cope with abiotic stress, while freshwater planarians downregulated a separate set of ancestral genes. Our genomic, transcriptomic, and proteomic data consistently show that the genetic toolkit for abiotic stress response in terrestrial planarians is highly differentiated from that of their freshwater counterparts, with significant regulatory shifts as well. Overall, our findings suggest a burst of gene gain in the Tricladida lineage, with co-option of these genes, rather than clade-specific innovations, playing a critical role in the origin and diversification of terrestrial flatworms. This underscores genomic exaptation and regulatory landscape shifts as key mechanisms enabling terrestrialization within Platyhelminthes. This study offers the first genome-wide insight into the genetic toolkit underlying flatworm terrestrialization and contributes broadly to understanding the genomic basis of animal terrestrialization.