Title : Taxonomic insights into Nigerian terminaliinae (combretaceae) based on molecular and morphological evidence: Implications for generic circumscription
Abstract:
The circumscription of the subtribe Terminaliinae (Combretaceae) has been a subject of controversy. Its generic phylogeny has also not been clearly elucidated. In this study, the taxonomic position of the Nigerian Terminaliinae taxa was revised based on the morphology and combined analyses of molecular markers to document taxonomic information on classification of Terminaliinae and ascertain the morphological and phylogenetic relationships among the members. Phylogenetic relationships were inferred using three DNA regions: ITS, rbcL and trnH-psbA sequences, while the morphological studies included the analysis of macromorphological and micromorphological features of all Terminaliinae species in Nigeria. The findings support the inclusion of Anogeissus leiocarpus in Terminalia while also retaining the Conocarpus and Pteleopsis species within Terminaliinae. The relationships between Terminalia and other Terminaliinae taxa remain unresolved based on only one molecular marker. The phylogenetic studies showed that trnH-psbA has better species discrimination efficiency than rbcL and ITS genes due to its highly variable nature. Transition/transversion bias test corroborated this finding as trnH-psbA showed higher bias (2.714) than rbcL (2.225) and ITS (2.126). The outcomes from the morphological studies indicated significant differences between Anogeissus, Conocarpus, Pteleopsis and Terminalia. However, the seed shape, inflorescence position, number of petals, stamens and fruit-wings, extrafloral nectaries, epidermal cells shape and anticlinal walls are useful characters for the identification of some Terminaliinae species. The morphometric analysis and molecular data suggest that Anogeissus and Terminalia could have evolved from a common ancestor. Accumulation of morphological differences and acceleration of the evolution of Anogeissus and Terminalia were more intensive than in other Terminaliinae species and this could probably be synchronized with adaptation to different climatic conditions. The phylogenetic relationships between the taxa were best resolved with Maximum Likelihood (ML) and Neighbour-joining (NJ) in combination with nrDNA ITS and chloroplast (rbcL and trnH-psbA) genes, phylogenetic trees constructed with combined ITS, rbcL and trnH-psbA revealed monophyletic nature of origin for the three genera: Anogeissus, Pteleopsis and Terminalia, while Conocarpus serves as a sister group to all remaining genera and species in the subtribe Terminaliinae. Bootstrap support and Bayesian posterior probability were found to be higher in trnH-psbA topologies than that of rbcL and ITS. The dendogram and phylogeny of Terminaliinae revealed nested grouping of the Anogeissus species within Terminalia, and we suggest that all genera of Terminaliinae, with the exception of Conocarpus and Pteleopsis should be included in an expanded circumscription of Terminalia. Morphological and molecular trees revealed congruence in the number of main clades resolved and an indented dichotomous key was produced for easy identification of species.
Keywords: Terminaliinae, Fruit-wings, Phylogeny, Taxonomic position, Dichotomous key, Nigeria
