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Limited seed dispersal and microspatial population structure of an agamospermous grass of West African savannahs, Hyparrhenia diplandra (Poaceae)¹

²Fonctionnement et Evolution des Systèmes Ecologiques, UMR 7625-CNRS, Ecole Normale Supérieure, 46, rue d'Ulm, F-75252 Paris Cedex 05, France; ³Fonctionnement et Evolution des Systèmes Ecologiques, UMR 7625-CNRS, bat A, 7^ème étage, case 237, 7 Quai Saint Bernard, 75252 Paris Cedex 05, France

We studied the microspatial population structure of the perennial tussock grass, Hyparrhenia diplandra (Poaceae), a facultative agamospermous species of West African savannahs. The microspatial population structure of H. diplandra was investigated by choosing two 100-m²; quadrats at random from which all individuals were mapped. The genotype of every individual was determined using two highly polymorphic microsatellite loci. A chloroplast locus was also used to investigate the role of seed dispersal on the genetic structure of populations. The genetic diversity index (0.85) was high for a clonal species. Significant genetic differentiation over short distances was detected by F statistics, and spatial autocorrelation analyses within both quadrats showed significant isolation-by-distance patterns, both with the cytoplasmic locus and the nuclear loci. Some clones formed large patches (up to 5 m in diameter) whereas others were more scattered. However, the genetic differentiation between quadrats was much higher when studied with the cytoplasmic locus than with the nuclear loci, indicating that gene flow via pollen, but not seeds, may frequently occur between quadrats. The maintenance of genetic diversity in this facultative agamospermous species most likely results from several factors, such as low seed dispersal ability, nonnegligible gene flow through pollen, and selective pressures induced by regularly occurring fires in this ecosystem.

Key Words: clonal diversity • fire • gene flow • Hyparrhenia diplandra • Poaceae • seed and pollen dispersal • spatial autocorrelation • tropical savannah

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