Whole Genome Duplication Events Likely Contributed to the Aquatic Adaptive Evolution of Parkerioideae

As the only aquatic lineage of Pteridaceae, Parkerioideae is distinct from many xeric-adapted species of the family and consists of the freshwater Ceratopteris species and the only mangrove ferns from the genus Acrostichum. Previous studies have shown that whole genome duplication (WGD) has occurred in Parkerioideae at least once and may have played a role in their adaptive evolution; however, more in-depth research regarding this is still required. In this study, comparative and evolutionary transcriptomics analyses were carried out to identify WGDs and explore their roles in the environmental adaptation of Parkerioideae. Three putative WGD events were identified within Parkerioideae, two of which were specific to Ceratopteris and Acrostichum, respectively. The functional enrichment analysis indicated that the lineage-specific WGD events have played a role in the adaptation of Parkerioideae to the low oxygen concentrations of aquatic habitats, as well as different aquatic environments of Ceratopteris and Acrostichum, such as the adaptation of Ceratopteris to reduced light levels and the adaptation of Acrostichum to high salinity. Positive selection analysis further provided evidence that the putative WGD events may have facilitated the adaptation of Parkerioideae to changes in habitat. Moreover, the gene family analysis indicated that the plasma membrane H+-ATPase (AHA), vacuolar H+-ATPase (VHA), and suppressor of K+ transport growth defect 1 (SKD1) may have been involved in the high salinity adaptation of Acrostichum. Our study provides new insights into the evolution and adaptations of Parkerioideae in different aquatic environments.