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Diversity of African fruit tree species of the genus Parkia: evolutionary history and potential for genetic improvement

par Ahossou, Enangnon Oscar Doré
Président du jury Flot, Jean-François
Promoteur Hardy, Olivier J.
Co-Promoteur Daïnou, Kasso
Publication Non publié, 2022-03-02

Thèse de doctorat

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Résumé :

Interest in the conservation and domestication of tropical fruit trees has grown with increasing understanding of their socio-economic, nutritional and medicinal potentials for local societies. Good knowledge of the candidate trees’ genetics, ecology, physiology and productivity traits are important for their long-term conservation. In the present thesis, we initiated a multidisciplinary research project on the taxonomy, uses, biogeography and phylogeny of the pantropical tree genus Parkia, focussing on genetic diversity, conservation, domestication and improvement of its African species. This research mainly questions (i) the evolutionary processes leading to the pantropical distribution of the genus Parkia and the exact number of its African species and (ii) the existance of evolutionary units presenting interesting traits for conservation and domestication within African populations of Parkia. Bibliographic reviews on the genus Parkia reveal diversely used species with higher richness in the Neotropics (20 species), followed by the Indo-Pacific region (12 species), Africa (three species) and Madagascar (one species). However, morphological diversity could suggest more taxonomic entities in Africa.Phylogenetic analyses point to the Neotropics as the original cradle of the genus Parkia which subsequently dispersed to the Indo-Pacific, Africa and Madagascar during the Miocene. Parkia madagascariensis belongs to the Indo-Pacific clade, suggesting different diversification trajectories between Madagascar and continental Africa. Phylogeographic analyses in mainland Africa based on nuclear microsatellites uncovered six, four and four parapatric genetic clusters within P. biglobosa, P. bicolor and P. filicoidea, respectively. The six clusters identified within P. biglobosa in the Sudano-Guinean savanna show relatively low genetic differentiation, except for the extreme eastern and western clusters. The clusters within each of the forest species (P. bicolor and P. filicoidea) are well differentiated genetically, occur in adjacent phytogeographic entities, and are largely congruent with previously described taxonomic entities. Genomic data show that P. bicolor is paraphyletic and that plastid introgression occurred between P. bicolor and P. filicoidea in the Congo basin. Pods and seeds traits were compared between P. biglobosa genetic clusters within three climatic zones in West Africa. Results show that morphometric traits in both natural and planted populations were significantly different between genetic clusters while no significant differences were observed between climatic zones. By contrast, nutritional traits showed no significant difference between genetic clusters or climatic zones in either natural or planted populations.Our results globaly suggest a denser taxonomic sampling to further characterize evolutionary trends within the genus Parkia. Although, additional (sub)species could be recognized within P. bicolor and P. filicoidea, there is a need for further fine scale genetic analyses. Domestication projects on P. biglobosa should focus on the identification of additional traits.