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Forest refugia revisited: nSSRs and cpDNA sequences support historical isolation in a wide-spread African tree with high colonization capacity, Milicia excelsa (Moraceae)

Authors

  • KASSO DAÏNOU,

    1. Laboratory of Tropical and Subtropical Forestry, Unit of Forest and Nature Management, Gembloux Agro-Bio Tech, University of Liege, 2 Passage des déportés, 5030 Gembloux, Belgium
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  • JEAN-PHILIPPE BIZOUX,

    1. Biodiversity and Landscape Unit, Gembloux Agro-Bio Tech, University of Liege, 2 Passage des déportés, 5030 Gembloux, Belgium
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  • JEAN-LOUIS DOUCET,

    1. Laboratory of Tropical and Subtropical Forestry, Unit of Forest and Nature Management, Gembloux Agro-Bio Tech, University of Liege, 2 Passage des déportés, 5030 Gembloux, Belgium
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  • GRÉGORY MAHY,

    1. Biodiversity and Landscape Unit, Gembloux Agro-Bio Tech, University of Liege, 2 Passage des déportés, 5030 Gembloux, Belgium
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  • OLIVIER J. HARDY,

    1. Evolutionary Biology and Ecology – CP 160/12, Faculté des Sciences, Université Libre de Bruxelles, 50 Av. F. Roosevelt, 1050 Brussels, Belgium
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  • MYRIAM HEUERTZ

    1. Evolutionary Biology and Ecology – CP 160/12, Faculté des Sciences, Université Libre de Bruxelles, 50 Av. F. Roosevelt, 1050 Brussels, Belgium
    2. Real Jardín Botánico, Consejo Superior de Investigaciones Científicas, Plaza de Murillo 2, E-28014 Madrid, Spain
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    • Present address: Forest Ecology and Genetics, Forest Research Centre (CIFOR-INIA), carretera de La Coruña km 7.5, E-28040 Madrid, Spain.


Kasso Daïnou, Fax: +32 81622342; E-mail: kdainou@ulg.ac.be

Abstract

The impact of the Pleistocene climate oscillations on the structure of biodiversity in tropical regions remains poorly understood. In this study, the forest refuge theory is examined at the molecular level in Milicia excelsa, a dioecious tree with a continuous range throughout tropical Africa. Eight nuclear microsatellites (nSSRs) and two sequences and one microsatellite from chloroplast DNA (cpDNA) showed a deep divide between samples from Benin and those from Lower Guinea. This suggests that these populations were isolated in separate geographical regions, probably for several glacial cycles of the Pleistocene, and that the nuclear gene pools were not homogenized despite M. excelsa’s wind-pollination syndrome. The divide could also be related to seed dispersal patterns, which should be largely determined by the migration behaviour of M. excelsa’s main seed disperser, the frugivorous bat Eidolon helvum. Within Lower Guinea, a north–south divide, observed with both marker types despite weak genetic structure (nSSRs: FST = 0.035, cpDNA: GST = 0.506), suggested the existence of separate Pleistocene refugia in Cameroon and the Gabon/Congo region. We inferred a pollen-to-seed dispersal distance ratio of c. 1.8, consistent with wide-ranging gene dispersal by both wind and bats. Simulations in an Approximate Bayesian Computation framework suggested low nSSR and cpDNA mutation rates, but imprecise estimates of other demographic parameters, probably due to a substantial gene flow between the Lower Guinean gene pools. The decline of genetic diversity detected in some Gabonese populations could be a consequence of the relatively recent establishment of a closed canopy forest, which could negatively affect M. excelsa’s reproductive system.

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