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Dynamics of genetic diversity in populations of African walnut, Coula edulis Baill. (Coulaceae): Influences of natural processes and human activities

par Kamdem, Narcisse
Président du jury Noret, Nausicaa
Promoteur Hardy, Olivier J. ;Sonke, Bonaventure
Publication Non publié, 2025-10-09

Thèse de doctorat

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Résumé :

The genetic diversity of canopy tree species from tropical rainforests is increasingly known, but little knowledge exists about small-stature trees. However, these species can provide additional information on dynamics of genetic diversity due to their biological traits. In this context, this thesis examines the historical, evolutionary, and ecological processes that shape genetic variability in a tree species found in African tropical rainforests. The study focuses on Coula edulis Baill. (Coulaceae), commonly known as "African walnut", as a model species. It is a lower canopy tree species, endemic and widely distributed in the Guineo-Congolian forest, whose seeds are an important source of food and income for local communities. Genetic diversity was studied using 21 nuclear microsatellite markers (SSRs) developed during this study, at two spatial scales to highlight the impacts of different processes acting at different scales.At large scale, a phylogeographic study revealed a strong genetic differentiation (FST = 0.39 to 0.59) between forest populations from West Africa (Upper Guinea, UG) and Central Africa (Lower Guinea, LG). Morphological differences between samples from these populations raise questions about taxonomic delimitation within the species. This genetic differentiation possibly resulted from an ancient divergence during the climatic fluctuations in the Quaternary, which led to a bottleneck in the UG population. Within LG, several genetic discontinuities (FST = 0.08 to 0.53) were revealed, leading to six distinct populations (genetic clusters). These clusters reflect both ancient and more recent divergence events, resulting in the formation of three parapatric genetic clusters in the northwestern part of LG, one allopatric cluster located to the east of these three, and two additional parapatric genetic clusters in the southern part of LG. Noteworthy discontinuities were also observed, one near 1°N around Equatorial Guinea and another around 2°S, as well as a relict population of Coula edulis in southeastern Cameroon.At fine scale, two major studies were conducted: (i) Pollen and seed dispersal were studied at three sites in Cameroon, subject to varying intensities of anthropogenic disturbances, primarily including defaunation, as well as other pressures such as illegal logging, shifting slash-and-burn agriculture, and fruit harvesting. Short-distance dispersal events and low site-to-site variability were obtained for pollen (173-358 m) and seeds (105-219 m). Nearly all juveniles resulted from outcrossing events while about 30% of the seeds collected in one site resulted from self-fertilization, suggesting a mixed mating system with a strong, early-acting, inbreeding depression. There was evidence of assortative mating between related individuals, as well as strong fine-scale spatial genetic structure (FSGS) within sites and cohorts. Pollen and seed dispersal distances, as well as FSGS, appear unaffected by defaunation. The analysis of regeneration dynamics suggests that the species shows strong resilience to the disappearance of large mammals. (ii) Coula edulis seeds are primarily dispersed by small mammals that are resilient to defaunation, such as rodents, Cricetomys emini "giant pouched rat", Atherurus africanus "brush-tailed porcupine", as well as squirrels. In contrast, large mammals, such as Mandrillus sphinx "mandrill" and Loxodonta cyclotis "forest elephant", which are more vulnerable to defaunation, mainly act as seed predators.This thesis contributes to a better understanding of the processes driving the current distribution and genetic variability of Coula edulis populations in African tropical rainforests. The results highlight the species’ remarkable resilience to anthropogenic disturbances, especially defaunation. This capacity for resilience, reflected in both its genetic diversity and regeneration dynamics, positions C. edulis as a relevant model for studying the mechanisms that maintain biodiversity and ecosystem stability in forests under increasing pressure.