Diversité génétique et dynamique des forêts d Afrique centrale (Une étude multi-échelle de la structure de la diversité génétique d un arbre pionnier, Aucoumea klaineana)

Cette thèse contribue à la compréhension des processus historiques, évolutifs et écologiques liés au maintien de la forte diversité biologique dans les forêts d'Afrique centrale. Notre approche est basée sur l'étude de la structuration de la diversité génétique intraspécifique d'Aucoumea klaineana, arbre pionnier endémique du domaine forestier de Basse Guinée. Plusieurs échelles spatiales ont été considérées afin de découpler les impacts des différents processus (historiques, évolutifs et écologiques) qui agissent à des pas de temps différents sur la diversité génétique. Nous avons montré qu'à l'échelle de l'aire de répartition, une grande part de l'hétérogénéité de la diversité génétique est expliquée par l'isolement historique de populations dans des refuges forestiers au cours du Dernier Maximum Glaciaire. Avec le retour des conditions favorables, les espèces forestières ont recolonisé les savanes pour atteindre leurs aires de distribution actuelles. Pour A. klaineana, les effets de fondation responsable de la perte de diversité génétique associés aux processus de colonisation ont été évités grâce à l'homogénéité du succès reproducteur des individus. A l'échelle des populations, la structure génétique spatiale (SGS) résulte du processus d'isolement par la distance, l'équilibre formé entre flux de gènes et dérive. Une telle structuration est observée chez A. klaineana, caractérisé par des distances de dispersion du pollen (128 m) et des graines (118 m) très limitées. Cet équilibre a été mis en évidence par des analyses de SGS qui laissent supposer que lorsque la densité d'arbre est plus faible et/ou que le milieu est ouvert, les distances de dispersion des gènes sont plus fortes. Ce résultat est lié aux syndromes de dispersion de l'espèce qui contribuent ainsi localement au maintien de la diversité génétique.This work contributes to our knowledge on historical, evolutionary and ecological processes linked with the high biological diversity in central Africa. We studied the structure of the genetic diversity of Aucoumea klaineana, a pioneer tree species, endemic to the Lower Guinea forest domain. Several spatial-scales were considered to infer impacts of the different processes (historical, evolutionary and ecological) acting with distinct time-scales on the genetic diversity. Considering the intraspecific genetic variation in the contemporary distribution of species, we showed that the heterogeneity in the distribution of the genetic diversity is due to the isolation of several source populations in forest refugia during the Last Glacial Maximum. After dry and cold periods, forest species recolonized savannahs and reached their current distribution areas. In A. klaineana, founder effects associated with colonization processes were avoided by the homogeneity in reproductive success in adult trees. At fine-scale, the spatial genetic structure (SGS) generally resulted from the isolation by distance process, the equilibrium between gene dispersal and drift. Such structure is observed for A. klaineana, characterized by both limited pollen and seed dispersal distances (128 and 118 m respectively). Here, SGS analyses may illustrate that reduced density of trees and/or forest opening is compensated by higher gene dispersal distances. This result is linked with dispersal syndromes of the species that locally contribute to the maintenance of the genetic diversity.MONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Ancient and recent evolutionary history of the bruchid beetle, Acanthoscelides obtectus Say, a cosmopolitan pest of beans

International audienceAcanthoscelides obtectus Say is a bruchid species of Neotropical origin, and is specialized on beans of the Phaseolus vulgaris L. group. Since the domestication and diffusion of beans, A. obtectus has become cosmopolitan through human‐mediated migrations and is now a major pest in bean granaries. Using phylogeographic methods applied to mitochondrial DNA (mtDNA) and nuclear microsatellite molecular markers, we show that the origin of this species is probably further south than Mesoamerica, as commonly thought. Our results also indicate that A. obtectus and its Mesoamerican sister species Acanthoscelides obvelatus , two morphologically close species differing principally in voltinism, speciated in allopatry: A. obtectus (multivoltine) arising in Andean America and A. obvelatus (univoltine) in Mesoamerica. In contrast to Mesoamerica where beans fruit once yearly, wild beans in Andean America fruit year‐round, especially in regions showing little or no seasonality. In such habitats where resources are continuously present, multivoltinism is adaptive. According to existing hypotheses, multivoltinism in A. obtectus is a new adaptation that evolved after bean domestication. Our data suggest the alternative hypothesis that multivoltinism is an older trait, adapted to exploit the year‐round fruiting of wild beans in relatively aseasonal habitats, and allowed A. obtectus to become a pest in bean granaries. This trait also permitted this species to disperse through human‐mediated migrations associated with diffusion of domesticated beans. We also show that diversity of Old World A. obtectus populations can be quite well explained by a single colonization event about 500 bp . Human‐mediated migrations appear not to be rare, as our results indicate a second more recent migration event from Andean America to Mexico

What is in a name? That which we call cecropia peltata by any other name would be as invasive?

The recent opinion piece by Sheil and Padmanaba (2011) argues that greater attention is required for invasive species management procedures that are relevant to and realistic for developing countries. They use the example of the Neotropical tree Cecropia as an introduction toWest Java to illustrate their point. In our invited response we contend that the assumptions and data on the dynamics of Cecropia in Java presented in their paper, as well as their review of global Cecropia introductions, are of reduced scientific value. Even so, we agree with the paper’s opinion that the naturalised species of Cecropia in West Java represent a considerable invasion risk and that funding must be improved so that the capacity for invasion ecology research and management of invasive alien species in developing countries is more effective. Unlike Sheil and Padmanaba (2011), however, we conclude that there is already enough evidence to be concerned by the threat of Cecropia to natural ecosystems, but that knowledge of the relevant taxa is currently insufficient to recommend the most appropriate control options not only for Java, but also for other Cecropia introductions elsewhere in the world. © 2011 Botanical Society of Scotland and Taylor & Francis

Data from: Plant dispersal in the sub-Antarctic inferred from anisotropic genetic structure

Climatic conditions and landscape features often strongly affect species’ local distribution patterns, dispersal, reproduction and survival, and may therefore have considerable impacts on species' fine-scale spatial genetic structure (SGS). In this paper we demonstrate the efficacy of combining fine-scale SGS analyses with isotropic and anisotropic spatial autocorrelation techniques to infer the impact of wind patterns on plant dispersal processes. We genotyped 1304 Azorella selago (Apiaceae) specimens, a wind-pollinated and wind-dispersed plant, from four populations distributed across sub-Antarctic Marion Island. SGS was variable with Sp values ranging from 0.001 to 0.014, suggesting notable variability in dispersal distance and wind velocities between sites. Nonetheless, the data supported previous hypotheses of a strong NW – SE gradient in wind strength across the island. Anisotropic autocorrelation analyses further suggested that dispersal is strongly directional, but varying between sites depending on the local prevailing winds. Despite the high frequency of gale-force winds on Marion Island, gene dispersal distance estimates (σ) were surprisingly low (< 10 m), most likely because of a low pollen dispersal efficiency. An SGS approach in association with isotropic and anisotropic analyses provides a powerful means to assess the relative influence of abiotic factors on dispersal, and allow inferences that would not be possible without this combined approach