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. . tagal complex (. 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 between . Ceriops species are also demonstrated. The ITS sequences of . Ceriops species, in contrast to their plastid . trnL 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.