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C4 photosynthesis is a complex trait that sustains fast growth and high productivity in tropical and subtropical conditions and evolved repeatedly in flowering plants. One of the major C4 lineages is Andropogoneae, a group of ∼ 1,200 grass species that includes some of the world's most important crops and species dominating tropical and some temperate grasslands. Previous efforts to understand C4 evolution in the group have compared a few model C4 plants to distantly related C3 species, so that changes directly responsible for the transition to C4 could not be distinguished from those that preceded or followed it. In this study, we analyse the genomes of 66 grass species, capturing the earliest diversification within Andropogoneae as well as their C3 relatives. Phylogenomics combined with molecular dating and analyses of protein evolution show that many changes linked to the evolution of C4 photosynthesis in Andropogoneae happened in the Early Miocene, between 21 and 18 Ma, after the split from its C3 sister lineage, and before the diversification of the group. This initial burst of changes was followed by an extended period of modifications to leaf anatomy and biochemistry during the diversification of Andropogoneae, so that a single C4 origin gave birth to a diversity of C4 phenotypes during 18 million years of speciation events and migration across geographic and ecological spaces. Our comprehensive approach and broad sampling of the diversity in the group reveals that one key transition can lead to a plethora of phenotypes following sustained adaptation of the ancestral state.

International audience; Understanding the nature of competitive interactions among trees is critical to understand forest community structure and dynamics, and to predict their responses to global change. However, predicting the outcome of competitive interaction between numerous pairs of tree species and across large gradients of abiotic conditions remains challenging. A classical hypothesis -the competition-relatedness hypothesis- assumes that because of niche and trait conservatism, closely related species tend to be ecologically more similar and thus compete more severely than distantly related species. However, this hypothesis has remained largely untested with direct measurements of competition. Rather, it was only evaluated from simple patterns of species co-occurrence in the past. Recent publications have challenged this hypothesis on the ground that coexistence requires more complex explanations than simply niche differentiation. Coexistence is also driven by differences in species' competitive ability, and this could have effects opposed to the competition- relatedness hypothesis. For instance, pairs of species sharing similar values of trait responsible for a high competitive ability should compete less strongly than species with more distinct trait values. Using data on individual tree radial growth from more than 17000 forest plots in the French Alps, we explore the relative role of these two contrasting views of competitive interactions for 130 pairs of species. We developed a Bayesian neighbourhood growth competition model to estimate the intensity of competition between pairs of species. We then tested the relation between strength of competition between pairs of species and i)their phylogenetic distance or ii)their functional similarity for several key functional traits.

International audience; Aim Assessing whether environmental and human factors influenced the spatial distribution and the dynamics of regionally rare plant species since the late nineteenth century, and whether these spatial and temporal patterns of rare species occurrences differ according to their chorology (level of endemism and biogeographic affinity). Location An area extending over 6250 km2 in the French Mediterranean Region. Methods We used two botanical surveys achieved in 1886 and in 2001, and considered species rare if occurring in only one or two sites in the study area. Each rare species was assigned to a group of endemism level (restricted endemic, non-endemic), and of biogeographic affinity (Mediterranean, South/Central European, Mountain, Eurosiberian). A 1 × 1 km grid was applied to the study zone. Generalized linear models were developed to study the spatial distribution and the fate of rare species occurrences (local extinction vs. local persistence between 1886 and 2001), as a function of environmental and human variables. Multivariate analyses were used to test whether the spatial distribution and the fate of rare species occurrences differed according to their chorology. Results In 2001, rare species as a whole tended to occur at higher altitude, in zones dominated by semi-natural open habitats, and where cultivated area had decreased in the last 30 years. Between 1886 and 2001, rare species were the most prone to local extinction in zones where human population density, cultivated area and livestock density had increased the most. Between 1886 and 2001, rare species had a higher probability of local persistence in zones of high altitude and steep slope, on basic bedrocks and with low cultivated area. Rare species with Mountain and Eurosiberian affinities occurred in marginal habitats in the study region, i.e. on gneiss-micaschist bedrocks and at high altitudes, whereas Mediterranean and South/Central European rare species occupied more varied environmental conditions. Between 1886 and 2001, Eurosiberian rare species showed high rates of local extinction whereas Mediterranean rare species had a significantly higher probability of local persistence. Restricted endemic species mostly occurred in zones of high slope, low human population density, and where cultivated area had decreased in the last 30 years. Occurrences of restricted endemics remained significantly stable between 1886 and 2001. Main conclusions Environmental and land-use changes that occurred over the twentieth century in the Mediterranean Basin had significant impacts on the spatial distribution and on the long-term dynamics of rare species occurrences. Urbanization and recent agriculture intensification, occurring mainly in coastal plains and littoral zones, caused most local extinctions of rare species from 1886 to 2001. Local populations of Eurosiberian species, which reach their range limits in marginal zones of the Mediterranean, also appear to be highly vulnerable. Conversely, most restricted endemic species occur in habitats with harsh topography and low human disturbance and have a higher potential of local persistence.

Darwin proposed two seemingly contradictory hypotheses for a better understanding of biological invasions. Strong relatedness of invaders to native communities as an indication of niche overlap could promote naturalization because of appropriate niche adaptation, but could also hamper naturalization because of negative interactions with native species ( ‘ Darwin ’ s naturalization hypothesis ’ ). Although these hypotheses provide clear and opposing predictions for expected patterns of species relatedness in invaded communities, so far no study has been able to clearly disentangle the underlying mechanisms. We hypothesize that confl icting past results are mainly due to the neglected role of spatial resolution of the community sampling. In this study, we corroborate both of Darwin ’ s expectations by using phylogenetic relatedness as a measure of niche overlap and by testing the eff ects of sampling resolution in highly invaded coastal plant communities. At spatial resolutions fi ne enough to detect signatures of biotic interactions, we fi nd that most invaders are less related to their nearest relative in invaded plant communities than expected by chance (phylogenetic overdispersion). Yet at coarser spatial resolutions, native assemblages become more invasible for closely-related species as a consequence of habitat fi ltering (phylogenetic clustering). Recognition of the importance of the spatial resolution at which communities are studied allows apparently contrasting theoretical and empirical results to be reconciled. Our study opens new perspectives on how to better detect, diff erentiate and understand the impact of negative biotic interactions and habitat fi ltering on the ability of invaders to establish in native communities.