Domestication history of a hexaploid, the sweet potato (Ipomoea batatas (L.) Lam.)

Despite the importance of sweet potato as a food crop, its evolutionary history has been poorly investigated. The geographical and botanical origins of sweet potato remain unclear. Sweet potato is in the section Batatas of the genus, which also includes 13 wild relatives, almost all endemic to the Americas. I. batatas is not known in the wild state. Morphological and genetic analyses indicate that I. trifida is sweet potato¿s closest wild relative, but the genomic composition of I. batatas is still debated. It is still unclear whether this hexaploid is auto-, allo- or auto-allopolyploid. The range of I. trifida extends from northern Peru to Mexico, and the assumed region of origin of I. batatas is somewhere within this vast geographical area. I. trifida forms a complex of ecotypes of differing ploidy levels (diploids to hexaploids), but the distribution and origin of populations of varying ploidy levels are not documented. Morever, no genetic studies have been conducted to determine the relationships between different wild populations of I. trifida and the cultivated I. batatas, which could allow inference of sweet potato¿s region(s) of origin. Finally, a major domesticated trait of I. batatas is its capacity to produce edible storage roots. Some I. trifida are known to form small tuberous roots, but these have not been studied in any depth. The purpose of our study is to investigate the origin of sweet potato and particularly the role of polyploidization in its domestication history. A set of 180 I. trifida populations and 450 sweet potato landraces, distributed from Peru to Mexico, were chosen from the collection of the International Potato Center (CIP, Lima, Peru). Morphological characterisation of these plants is in progress, as well as genetic analyses using neutral chloroplast markers. We plan to evaluate ploidy levels of these different wild and cultivated samples by flow cytometry and study the genome composition of representatives of ploidy groups by genomic in situ hybridization. These analyses should lead to advances in the reconstruction of sweet potato¿s evolutionary history. (Texte intégral

Sorghum, social links and genetic diversity in Northern Cameroon

Sorghum is the Duupa 'cultural supercrop' : it is a main subject of concern for the people and the backbone of the reproduction, on a material and a symbolic way, of the community. More than forty landraces are named and recognized, and widely exchanged between all cultivators in the community. Each year, every cultivator selects panicles from his own previous crop but often mix it with exchanged seeds. Fluxes of seeds follow the lines of kinship or affinal relations, but they can also come from commercial transactions outside the community. An outstanding feature of the Duupa seed system is the institution of free access to sorghum seeds during the collective threshing work parties. Anyone attending these parties can pick a few panicles for his own seeds on the bulk of the crop to be threshed. Strong moral values underline this safety net, which makes the diversity of landraces, seeds a common property. However, these institutionalized exchanged do not account for all the gene flux and people also rely on non-institutionalized, transfers sometimes including grains for current food use rather than properly selectionized panicles. All these transfers, public as well as private, even if difficult to quantify, should be taken into account for a proper understanding of the shaping of sorghum varietal and genetic diversity. (Résumé d'auteur

Inherited biotic protection in a Neotropical pioneer plant

Chelonanthus alatus is a bat-pollinated, pioneer Gentianaceae that clusters in patches where still-standing, dried-out stems are interspersed among live individuals. Flowers bear circum-floral nectaries (CFNs) that are attractive to ants, and seed dispersal is both barochorous and anemochorous. Although, in this study, live individuals never sheltered ant colonies, dried-out hollow stems - that can remain standing for 2 years - did. Workers from species nesting in dried-out stems as well as from ground-nesting species exploited the CFNs of live C. alatus individuals in the same patches during the daytime, but were absent at night (when bat pollination occurs) on 60.5% of the plants. By visiting the CFNs, the ants indirectly protect the flowers - but not the plant foliage - from herbivorous insects. We show that this protection is provided mostly by species nesting in dried-out stems, predominantly Pseudomyrmex gracilis. That dried-out stems remain standing for years and are regularly replaced results in an opportunistic, but stable association where colonies are sheltered by one generation of dead C. alatus while the live individuals nearby, belonging to the next generation, provide them with nectar; in turn, the ants protect their flowers from herbivores. We suggest that the investment in wood by C. alatus individuals permitting stillstanding, dried-out stems to shelter ant colonies constitutes an extended phenotype because foraging workers protect the flowers of live individuals in the same patch. Also, through this process these dried-out stems indirectly favor the reproduction (and so the fitness) of the next generation including both their own offspring and that of their siblings, alladding up to a potential case of inclusive fitness in plants

Des confusions entre espèces préjudiciables à la gestion durable des essences forestières : l'exemple des acajous d'Afrique (Khaya, Meliaceae)

Les espèces du genre Khaya procurent l'un des bois les plus prisés du continent africain. Elles sont commercialisées sous l'appellation " acajou d'Afrique " depuis environ deux siècles. En Afrique continentale, on reconnaît actuellement quatre espèces morphologiquement proches : Khaya anthotheca, K. grandifoliola, K. ivorensis et K. senegalensis. Le nombre de taxons du genre Khaya a varié au cours du temps et la délimitation de ces taxons n'est pas sans avoir des implications sur les stratégies de conservation et de gestion durable des espèces. L'objectif du travail présenté ici est de s'assurer de la délimitation de ces espèces sur la base d'un bilan des connaissances actuelles en taxonomie, génétique, écologie et chimiotaxonomie. Les données disponibles ne permettent pas toujours de séparer sans ambiguïté les quatre acajous d'Afrique continentale. Elles permettent toutefois d'avancer que K. ivorensis, K. grandifoliola et K. senegalensis seraient des espèces à part entière mais dont les limites taxonomiques doivent encore être précisées. Khaya nyasica serait également une espèce qui devrait être séparée de K. anthotheca. Au sein de cette dernière, plusieurs chémotypes ont été identifiés, ce qui suggère que ce taxon pourrait recouvrir un ensemble plus complexe d'au moins deux autres taxons dont le rang spécifique ou sous-spécifique reste à préciser. Des recherches morphométriques, génétiques et écologiques doivent être développées afin de clarifier le statut systématique et l'histoire évolutive de ces divers taxons, en vue de formuler des recommandations appropriées pour la gestion durable des acajous d'Afrique

Gestion des ressources génétiques du sorgho (Sorghum bicolor) chez les Duupa (Nord Cameroun)

Le système de diffusion des semences a un impact direct sur la structure génétique des populations de plantes domestiquées et sur son évolution. Ce système peut être caractérisé par ses acteurs, mais aussi par la nature, le sens et l'intensité des échanges. En décrivant les pratiques de sélection et la circulation des semences de sorgho dans la société duupa au Nord Cameroun, notre étude vise à comprendre la dynamique de la diversité génétique. Les agriculteurs duupa maintiennent une importante diversité variétale (40 variétés) qu'ils cultivent en mélange polyvariétal. Chaque année, ils sélectionnent des panicules pour reconduire leur culture, mais peuvent également utiliser des graines non sélectionnées. Les échanges de semences sont importants, et 56 % des agriculteurs se pourvoient auprès de leur famille ou de leurs voisins. Le battage est le moment privilégié des échanges de semences en panicules, mais les agriculteurs duupa introduisent également dans leurs semences des graines à vocation alimentaire échangées à l'occasion de visites et de cérémonies. Les liens sociaux qu'entretiennent les Duupa influencent directement la circulation des semences et sont au coeur de la dynamique de la diversité génétique. (Résumé d'auteur