TAXONOMIC REVISION OF THE GENUS Phyllanthus LINN. IN NIGERIA

Abstract

The genus Phyllanthus (Phyllanthaceae) is of economic importance in herbal medicine and owing to its diversity, grows in similar habitats and shares common local names in Nigeria. These common local names give rise to misidentifications and taxonomic confusion. The only available taxonomic information on Phyllanthus is the Flora of West Tropical Africa which is not current. Therefore, this study was conducted to provide revised taxonomic information on Phyllanthus species and a reliable key for their identification. One hundred and forty two specimens comprising 55 field collections covering major ecological zones in Nigeria and 87 representative herbaria materials from Forest Herbarium Ibadan, University of Ibadan herbarium and Obafemi Awolowo University herbarium were assessed for morphological characters using standard taxonomic methods. Light and Scanning Electron Microscopes were used to examine leaf epidermal and pollen morphology. Genomic DNA from 20 fresh and field collected young leaf samples was extracted, amplified using chloroplast rbcL primer and the product was sequenced using standard techniques. Phylogenetic trees were constructed using maximum composite likelihood based method. Principal Component Analysis (PCA) and cladograms of the assessed characters were used to establish taxonomic relationships among the taxa. Data were subjected to descriptive statistics. Nineteen Phyllanthus species were recognised from the herbarium materials out of which 16 species were validated because Phyllanthus floribundus, P. fraternus and P. physocarpus were synonyms of P. muellerianus, P. amarus and P. acidus, respectively. The length and width of leaves ranged from 0.45 - 11.70 cm and 0.15 - 5.40 cm, respectively. The leaves were entire with alternate arrangement and the leaf shapes were mainly oblong, lanceolate and linear. Only P. muellerianus had recurved stipular spines at the nodal points. Perianth lobes ranged from 4-6 and were mainly green. Fruits were green except in P. acidus (yellow), P. muellerianus (red), P. reticulatus (black) and P. urinaria (reddish-brown). Epidermal cells were mostly irregular/polygonal on adaxial and/or abaxial surfaces but rectangular in P. muellerianus and P. floribundus. Stomata iii were anisocytic, anomocytic, laterocyclic and paracytic. Epicuticular wax deposits, sessile multicellular scales, unicellular trichomes, oil droplets, crystal sand and druses were recorded in all except P. maderaspatensis, P. mannianus, P. nigericus, P. niruroides, P. pentandrus, P. sublanatus and P. urinaria. Pollen was 3-colporate, finely reticulate, prolate, subprolate and oblate-spheroidal, while the size ranged from small (12.40 x 13.00 µm) to medium (31.50 x 23.25 µm). Four clusters and clades each were delineated from the PCA and cladograms, respectively indicating that the genus is paraphyletic, contrary to the presumed monophyletic relationship of the species. A dichotomous key was produced for easy identification of the species. Sixteen Phyllanthus species were established and revised taxonomic information revealed clusters useful in understanding the relationships among the species of Phyllanthus in Nigeria.