Reproduction et Diversité Génétique chez Parkia biglobosa (Jacq.) G. Don

Parkiabiglobosa(Jacq.)G.Don, African locust bean, is one of the 34 known species of the genusParkiawhose centre of origin isSouth America. It is anagroforestrytree species spread throughout the Africansudaniansavannas, fromSenegalin the West toUgandain the East. P.biglobosa is also an important socio-economic species in high demand by local communities for its multiple amenities and uses (food, traditional medicine and pharmacy, preservation and improvement of soil fertility, rites and customs, etc.). Such an importance engenders several diverse research programmes on this species that have been undertaken inWest Africa. However, the species remains under pressure as a result of its over-exploitation by human population, shortage of rainfall and inappropriate agricultural practices, leading to the degradation and/or lack of regeneration of P .biglobosaparklands in many zones. In this context, the present study was undertaken in order to understand the reproductive biology and the genetic diversity of P.biglobosa, which are essential for the elaboration of an efficient strategy for sustainable use and conservation of genetic resources.We investigated the mating system of the species in the semi-arid zones ofBurkina Faso. Two populations were selected inBissighinandNagaré, with 153 and 209 trees, respectively, and were weekly monitored during two seasonal cycles of fruit formation. The floralphenologicalsurvey revealed relatively high synchronism index values of 0.78 forNagaréand 0.79 forBissighin. This was an indication that although the fruiting period lasts over about 5 months, the majority of the trees flowered synchronously in these two populations. Thephenologicalobservations have highlighted a seasonal profile of P.biglobosa , which mainly occurs in the dry season, like most of thesudaniansavannah species. The high values ofmultilocusoutcrossingfor both, family level and within populations (tm = 0.941 and 0.981), suggest that the species presents partial self-incompatibility, which is also a common feature in many hermaphrodite species. Genetic (protandry, self-incompatibility) and environmental (elevated level of floral synchronism) factors could explain such high values of multi-locusoutcrossing. We demonstrated that there was a weak parental relationship between the trees that were analyzed (low tm-tsvalues, between 0 and 0.086). The paternity correlation (rp= 0.299 forBissighinand 0.502 forNagaré) showed that each tree was pollinated by only two or three pollen donors, probably involving bees as the main pollinators.The study of P.biglobosa genetic diversity using enzymatic electrophoresis allowed us to determine intra- and inter-population variations, the structure and genetic distances between populations. Open pollinated seeds collected from 1658 trees of 64 populations in 11 western and central African countries were analyzed and the genetic control of five enzymatic systems coding for 8 polymorphous locus with 33 alleles was thus elucidated. We obtained high values of intra-population genetic diversity, with mean number of alleles per locus (A = 4.13), with the percentage of polymorphism (P = 100%), and with the expectedheterozygosity(He = 0.34). The great value of gene flow (Nm = 1.61) and the low genetic differentiation between populations (Fst= 0.13) indicate that there was an important rate of gene exchange between the populations and that the genetic diversity of the species was also great within populations. The genetic distances were also low (between 0 and 0.240), indicating that the populations were similar enough to belong to the same genetic group. The actual genetic diversity and genetic structure of P.biglobosa might have resulted from its evolutionary history without a bottleneck sequence, from the reproductive biology with high flowering synchronism andoutcrossingrates, and from the fact that farmers had had developed P.biglobosa parkland systems through years, hence facilitating gene flows between populations.The storage conditions of P.biglobosa seeds in CNSF seed bank were assessed in order to ensure better conservation of these genetic resources. Methods for optimum germination were identified for stored seeds, using both the commercial lots aged between 0 and 20 years that were collected inBurkina Faso, and the seeds of 23 populations ( c. 11 years old) from West andCentral Africathat were selected because of their high levels of genetic diversity. The germination results of these seeds, chemically scarified with sulphuric acid, showed no significant effects of the storage periods. The same optimum pre-treatment and duration, initially identified and recommended by CNSF for this species can be applied independent of the age of seeds between 0 and 20 years.This physiological study revealed variations of seed moisture content (MC) during their storage at 4°C in the CNSF seed bank, partly explained by inappropriate handling of seeds (without any special precautions), i.e. the regular openings of plastic bags during sub-samples withdrawing of seeds within the cold room at > 75% RH. Because of the partial porosity of P.biglobosa seed integuments, the remaining seeds in the bags therefore re-equilibrate within this high RH condition and so increase their actual MC. Subsequently, the seed viability decreases due to elevated metabolic activity, despite the fact that they can maintain great longevity (95% germination after 11 years storage at 4°C) when they are properly dried and conserved in hermetic containers. The constructed water sorption isotherm for this species showed that the recommended level of ≤ 5% MC corresponds to about 10% RH for P.biglobosa . We suggested monitoring and handling procedures for conserving seeds in order to improve their longevity during storage.Analyses of reproductive and genetic parameters, in association with eco-geographical and anthropological factors, allowed us to sample 11 representative «ideal» populations inWest Africafor ex-situ long term conservation of seeds of the species. We have also recommended the association of in-situ conservation of these populations in the parklands, which success will depend on the common efforts of local communities, conservationists, technicians and politicians. Considering the genetic parameters of the species in Burkina Faso, its geographic distribution, thephyto-geographic division of the country, and according to the 'law of diminishing returns', we have selected 10 populations in Burkina Faso that can be used to collect seeds for the purposes of national planting programmes and for their long term conservation in CNSF seed bank, in agreement with the state of the art. In addition, for each population 15 individuals are suitable to capture maximum genetic variation and can be systematically sampled. The urgent need to take coordinated actions for sustainable use and conservation of threatened and important genetic resources of thesudanianandsudano-sahelianspecies, led us to propose sampling guidelines for better seed collections. Because many scientific disciplines are involved in the conservation of genetic resources and species are distributed across national boundaries, we suggest multi-disciplinary teams at national levels and scientific cooperation at regional level in order to generate and share information and experiences that are necessary for elaborating efficient strategies for sustainable management of genetic resources.This thesis offers new perspectives and approaches for the conservation of P.biglobosa resources. It informs on the population characteristics that are important for capturing the maximum genetic diversity of the species. The sustainable use and conservation of genetic resources of the recommended «ideal» populations inBurkina Fasoas well as in the West African region compel a strengthened collaboration between all stakeholders. The application of best practices in seed laboratory provides more chances to maintain viability and prolong longevity of seeds, and so guaranty the long term ex-situ conservation of the species. We hope that all these results will improve and guide conservation programmes, not only forParkiabiglobosa , but also will inspire works on importantsudano-sahelianspecies for their efficient conservation. The sustainability of conservation and uses of plant genetic resources for the actual and future generations will then be guaranteed.