Distribution, adaptation environnementale et diversité génétique de dacryodes buettneri (Engl.) H.J. Lam et dacryodes edulis (G. Don) H.J. Lam (burséracées) en Afrique centrale

Abstract

The rainforest of Central Africa belongs to that of the Congo basin. It is the habitat to more than half the wildlife of the continent and is an extremely useful and valuable ecosystem for humanity to feed, shelter, heal, get energy, etc. Unfortunately, this forest wealth is increasingly threatened by abusive exploitation of vast wooded areas before even explore it scientifically and develop effective strategies of conservation and sustainable use. The conservation in situ of species requires prior knowledge of the ecology and genetic data of these species. Two species: Dacryodes buettneri and Dacryodes edulis were chosen for this study. The overall objective of this study is to contribute at the sustainable management of both species. The specific objectives are: to describe the geographical distributions of D. buettneri and D. edulis from the geographic coordinates of available observations and potential distributions modeled with nineteen climate parameters; to characterize the ecological conditions based on the cover of vegetation, climate (precipitation and temperature) and the altitude ; to isolate and to characterize the D. edulis microsatellite loci ; to describe the genetic diversity within populations and between populations of D. buettneri and D. edulis across the lower Guinea endemism (Cameroon and Gabon) and finally to describe the genetic structure of D. buettneri and D. edulis and the genetic relation between these species. The actual distributions based on the coordinates assigned to each observation of D. buettneri and of D. edulis have been established using the GIS software (Diva-Gis). The potential distributions of each species were modeled with 19 climatic parameters. The study of the ecological niches was based on vegetation cover, 19 climatic parameters and altitudes. Graphs describing the ecological requirements from the altitude, rainfall and temperature data have been made using the descriptive statistics by Statistica 6 software. The nuclear DNA of 694 individuals across 29 populations (23 populations of D. edulis) was extracted from cambium and leaflets. Five microsatellite loci were isolated from D. edulis and genetic analyses were based on isolated microsatellite polymorphism. Intra-population and inter-population diversity and genetic structure within species were studied. D. buettneri is a species of wet forests. It can be seen in destroyed vegetation because it is protected by farmers. The centre of Gabon offer more favorable climatic parameters conditions to its growth. The confidence interval of the precipitation ranges from 1500 to 2550 mm/year. Optimal average annual temperatures are between 23.8 to 25.8 °C. The species grows from the level of the sea up to 750 m altitude. D. edulis is a species of humid forests. South of Cameroon and Gabon provide more favorable climatic parameters conditions to its natural growth naturally. The optimal annual precipitation ranges from 1600 to 2600 mm/year. Optimal average annual temperatures range from 23.5 to 25.5 °C while the confidence interval is between 21.8 and 25.8 °C. The species grows naturally from the level of sea up to 750 m altitude. It is domesticated and has widely adapted to ecological conditions. The environmental conditions of the individuals of the plantations represent the fundamental ecological niche while those of wild individuals represent the real or realized ecological niche. Genetic analysis of populations of D. buettneri has identified populations of Chaillu (P = 100% and Hatt = 0.47), of Casterville (P = 80% and Hatt = 0.36) and of Makoukou (P = 80% and Hatt = 0.36) as the most polymorphic populations. All studied populations were exhibited positive coefficients of consanguinity, ranging from 0.15 (Mt Crystal population) and 0.36 (Casterville population). Regarding D. edulis, the average of intra-population genetic parameters are A = 5.36; P = 90% and Hatt = 0.47. The average of coefficient of consanguinity is F = 0.12. Five populations have showed coefficient of consanguinity significantly less than zero. The establishment of plantations of D. edulis by farmers therefore maintained genetic diversity of the species. The study of the genetic structure has not showed different genetic groups within species that could be associated with varietal demarcations. The study of the ecological niches of D. buettneri and D. edulis combined with analyses of population genetics parameters is a coherent contribution to optimize the in situ conservation and the sustainable use of these species.