Molecular phylogeny and evidence for natural hybridization and historical introgression between Ceriops species (Rhizophoraceae)

Ceriops (Rhizophoraceae) is a genus comprised of five species of mangroves distributed in tropical and subtropical coastal regions. In this study, sequences from nuclear ribosomal ITS and the plastid trnL intron are used to construct molecular phylogenies of this genus revealing two species complexes, the C. tagalcomplex (C. tagal and C. australis), and the C. decandra complex (C. decandra, C. pseudodecandra and C.zippeliana), each forming a distinct clade. All five species, including the newly designated species C. pseudodecandra, are well supported. However, natural hybridization and historical introgression betweenCeriops species are also demonstrated. The ITS sequences of Ceriops species, in contrast to their plastidtrnL intron sequences, show a great amount of homoplasy during evolution. Historical introgression originating from natural hybridization was demonstrated based on the additivity of ITS sequences from putative parents. Of the five Ceriops species, C. pseudodecandra is a relatively isolated species.C.decandra and C. zippeliana show mutual introgression in most populations. According to both the nuclear ITS sequences and the plastid trnL intron, an intermediate form from Darwin is likely a natural hybrid, with C. tagal and C. australis respectively the maternal and paternal parents.