Ecogeographic Variation in the Morphology of Two Asian Wild Rice Species, Oryza nivara and Oryza rufipogon

To search for variation patterns and diagnostic features between Asian wild rice species, several numerical methods were applied to phenotypic data obtained from 116 accessions representing sympatric populations of Oryza nivara and Oryza rufipogon from tropical continental Asia and O. rufipogon populations from insular Southeast Asia and Australasia. Ordination and cluster analyses separate O. rufipogon from O. nivara, indicating the presence of two sympatric morphological species occupying different ecological niches. Oryza nivara and O. rufipogon are morphologically more differentiated in South Asia than in mainland Southeast Asia, implying more recent divergence and/or more interspecific gene flow among sympatric populations in the latter region. Oryza nivara exhibits South and Southeast Asian phenotypes while the Australasian populations of O. rufipogon appear as distinct from the rest of the species. Seedling height, culm number, and diameter; leaf length and width; and anther length were significantly correlated to certain geoclimatic factors and displayed contrasting correlation directions for O. nivara and O. rufipogon, implying that the two species respond differently to geographic and climatic gradients. Diagnostic characters are provided to delineate the species morphologically. The results suggest the strong influence of ecology on species morphology, existence of geographic races within species and morphological divergence between O. nivara and O. rufipogon.