Transcriptome analyses provide insights into the phylogeny and adaptive evolution of the mangrove fern genus Acrostichum

The mangrove fern genusAcrostichumgrows in the extremely unstable marine intertidal zone under harsh conditions, such as high salt concentrations, tidal rhythms and long-term climate changes. To explore the phylogenetic relationships and molecular mechanisms underlying adaptations in this genus, we sequenced the transcriptomes of two species ofAcrostichum,A. aureumandA. speciosum, as well as a species in the sister genus,Ceratopteris thalictroides. We obtained 47,517, 36,420 and 60,823 unigenes for the three ferns, of which 24.39–45.63% were annotated using public databases. The estimated divergence time revealed thatAcrostichumadapted to the coastal region during the late Cretaceous, whereas the two mangrove ferns from the Indo West-Pacific (IWP) area diverged more recently. Two methods (the modified branch-site model and the Khmethod) were used to identify several positively selected genes, which may contribute to differential adaptation of the twoAcrostichumspecies to different light and salt conditions. Our study provides abundant transcriptome data and new insights into the evolution and adaptations of mangrove ferns in the inhospitable intertidal zone.