Phylogenomic analysis supports the deep and gradual evolution of water-conducting tissues.

The origin of land plants was marked by the evolution of key adaptations to terrestrial environments. The patchy distribution of many of these traits in bryophytes, including water-conducting tissues, raises questions about whether these features share a common evolutionary origin or arose independently. Molecular data from 160 species were analyzed using phylogenomic approaches to investigate bryophyte genome evolution and the origins of water-conducting tissues. Hornworts were identified as the most genomically reduced bryophyte lineage. Many of the gene families absent in hornworts are associated with developmental processes, including those important for water-conducting cells, which are not found in this group. Comparative analyses of xylem developmental gene networks across bryophytes revealed a mosaic of evolutionary mechanisms-gene loss, duplication, and co-option-underpinning the emergence of water conduction. These results indicate that reductive, convergent, and tracheophyte-specific evolutionary processes all contributed to the development of water-conducting tissues in land plants. Together, these findings highlight how multiple genetic mechanisms drove the deep and gradual evolution of water-conducting tissues, a key innovation that enabled the successful establishment of plants in terrestrial environments.