Taxonomy and genetic structure of Meru oak populations, Vitex keniensis Turrill and Vitex fischeri Gürke, in East Africa

Vitex keniensis Turrill (Verbenaceae), the Meru oak, is a popular but threatened indigenous forest tree species in East Africa valued for high quality timber, fast growth, edible fruits, medicine, bee forage (honey). The tree has a high agroforestry potential. However, efforts to promote its propagation and conservation is hampered by lack of information on its biological and ecological characteristics. The taxonomic distinction between its populations in Mt. Kenya and Nyambene hills and those of V. fischeri in Lake Victoria and Mt. Elgon regions has been unclear. The research project investigated the taxonomy and compared the mating systems and genetic structure of the four populations of V. keniensis and V. fischeri .

The Vitex populations showed similarity in their leaf, flower, seed and seedling morphology, phenological pattern, polyploidy (2n=96) and autotetraploidy, homogamy, partial self-incompatibility, bee-pollination, inter-population reproductive compatibility and low inter-population variation 3.5%. These observations differed from the expected pattern of their geographical distance, ecological gradient and the current taxonomic grouping. The low population divergence could be attributed to common ancestry and possibly lack of divergence. V. keniensis and V. fischeri should be considered conspecific. The name V. fischeri has priority, the well-known name, V. keniensis (Meru oak) therefore falls into synonymy, thus Vitex fischeri (= V. keniensis ).

The bulk of genetic variation is within the populations (96.5%), therefore sampling from any of the four populations can capture a sufficiently large gene pool of the species for improvement breeding and conservation. It is a bee-pollinated, autotetraploid, therefore a minimum of a hundred dispersed mature trees at least 300 m apart can be preserved in-situ, and their propagules used for ex-situ conservation, breeding and propagation stock (living gene-bank).