Luc Descroix

Spatial and temporal factors of erosion by water of black marls in the badlands of the French southern Alps

Abstract Jurassic black marls constitute one of the most extensive outcrops in the Alpine massif. They include the most eroded terrains in the French Alps. Black marls are subject to strong weathering and their erosion rate can reach values close to 1 cm year−1 (i.e. 100 m3 ha−1 year−1). Many studies have focused on the processes and forms of erosion of these black marls and on the development of an original metrology to improve our understanding of the seasonal processes in these badlands areas. This study shows that alternating freeze-thaw and wet-dry cycles are the main causes of disaggregation in the marls. The exposure of hillslopes and the dip-slope angle are the main site factors explaining the variability in the action of these processes and thus, in the soil thickness lost as a result of erosion. The kinetic energy of rainfall is similar everywhere, but the volumes of regolith transported by runoff depend on the effects of previous weathering.

Spatial and Temporal Variation in Selection of Genes Associated with Pearl Millet Varietal Quantitative Traits In situ

Ongoing global climate changes imply new challenges for agriculture. Whether plants and crops can adapt to such rapid changes is still a widely debated question. We previously showed adaptation in the form of earlier flowering in pearl millet at the scale of a whole country over three decades. However, this analysis did not deal with variability of year to year selection. To understand and possibly manage plant and crop adaptation, we need more knowledge of how selection acts in situ. Is selection gradual, abrupt, and does it vary in space and over time? In the present study, we tracked the evolution of allele frequency in two genes associated with pearl millet phenotypic variation in situ. We sampled 17 populations of cultivated pearl millet over a period of 2 years. We tracked changes in allele frequencies in these populations by genotyping more than seven thousand individuals. We demonstrate that several allele frequencies changes are compatible with selection, by correcting allele frequency changes associated with genetic drift. We found marked variation in allele frequencies from year to year, suggesting a variable selection effect in space and over time. We estimated the strength of selection associated with variations in allele frequency. Our results suggest that the polymorphism maintained at the genes we studied is partially explained by the spatial and temporal variability of selection. In response to environmental changes, traditional pearl millet varieties could rapidly adapt thanks to this available functional variability.