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SummaryTo gain an understanding of the genetic basis of adaptation, we conducted quantitative trait locus (QTL) mapping for flowering time variation between two winter annual populations of Arabidopsis thaliana that are locally adapted and display distinct flowering times.QTL mapping was performed with large (n = 384) F 2 populations with and without vernalization, in order to reveal both the genetic basis of a vernalization requirement and that of variation in flowering time given vernalization.In the nonvernalization treatment, none of the Sweden parents flowered, whereas all of the Italy parents and 42% of the F 2 s flowered. We identified three QTLs for flowering without vernalization, with much of the variation being attributed to a QTL co-localizing with FLOWERING LOCUS C (FLC). In the vernalization treatment, all parents and F 2 s flowered, and six QTLs of small to moderate effect were revealed, with underlying candidate genes that are members of the vernalization pathway. We found no evidence for a role of FRIGIDA in the regulation of flowering times.These results contribute to a growing body of evidence aimed at the identification of ecologically relevant genetic variation for flowering time in Arabidopsis, and set the stage for functional studies to determine the link between flowering time loci and fitness.

The development of integrated weed management programs requires a clear understanding of the factors and mechanisms conditioning weed community dynamics in agroecosystems. This study evaluated the effect of different agricultural management systems on the aboveground and seedbank weed communities in annual row crops at the Long Term Ecological Research project in agricultural ecology at the W. K. Kellogg Biological Station, Michigan, USA. Weed biomass and species composition were sampled for six years over two corn–soybean–wheat sequence cycles in four agricultural management systems: (1) conventional (high external chemical input, moldboard plowed); (2) no‐till (high external chemical input, no tillage); (3) low‐input (low external chemical input, moldboard plowed); and (4) organic (no external chemical input, moldboard plowed). A greenhouse germination study assessed variation in the abundance and composition of the weed seedbank across the studied systems in the first and sixth year of this study. Aboveground weed biomass, species density, and diversity were lowest in the conventional system, intermediate in the no‐till system, and highest in the low‐input and organic systems, but there were significant year‐by‐system interactions. Monocot and dicot species were equally common in the conventional system, whereas annual grasses, such as Digitaria sanguinalis (large crabgrass) and Panicum dichotomiflorum (fall panicum), dominated the no‐till system. Two perennial weed species (Trifolium pratense [red clover] and Elytrigia repens [quackgrass]) and one annual dicot (Chenopodium album [common lambsquarters]) dominated the low‐input and organic systems. A multivariate ordination of all four systems revealed close associations between the conventional and no‐till systems and between the low‐input and organic systems. Separate ordinations of the four management systems revealed a crop effect in the low‐input and organic systems, but no differentiation in the conventional and no‐till ones. The seedbank study revealed a significant increase in the number of weed seeds and species, mainly of annual grasses such as D. sanguinalis and P. dichotomiflorum, in the conventional and no‐till systems over the six years of study. During the same period, the number of weed seeds declined in low‐input and organic systems. Three annual dicots (Stellaria media [common chickweed], Veronica peregrina [purslane speedwell], and C. album) dominated the seedbank of the low‐input and organic systems. Weed aboveground and seedbank community composition were more constant over time in the low‐input and organic systems than in the conventional and no‐till systems over the study period. These results demonstrate that agricultural management systems can have both immediate and long‐term effects on weed species density, abundance, and diversity. The differences observed among management systems in weed biomass, species composition, diversity, and community constancy indicate challenges that exist for the development of ecologically based weed manag...

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Premise: Anthropogenic nitrogen (N) addition alters the abiotic and biotic environment, potentially leading to changes in patterns of natural selection (i.e., trait-fitness relationships) and the opportunity for selection (i.e., variance in relative fitness). Because N addition favors species with light acquisition strategies (e.g., tall species), we predicted that N would strengthen selection favoring those same traits. We also predicted that N could alter the opportunity for selection via its effects on mean fitness and/or competitive asymmetries. Methods: We quantified the strength of selection and the opportunity for selection in replicated populations of the annual grass Setaria faberi (giant foxtail) growing in a long-term N addition experiment. We also correlated these population-level parameters with community-level metrics to identify the proximate causes of N-mediated evolutionary effects. Results: N addition increased aboveground productivity, light asymmetry, and reduced species diversity. Contrary to expectations, N addition did not strengthen selection for trait values associated with higher light acquisition such as greater height and specific leaf area (SLA); rather, it strengthened selection favoring lower SLA. Light asymmetry and species diversity were associated with selection for height and SLA, suggesting a role for these factors in driving N-mediated selection. The opportunity for selection was not influenced by N addition but was negatively associated with species diversity. Conclusions: Our results indicate that anthropogenic N enrichment can affect evolutionary processes, but that evolutionary changes in plant traits within populations are unlikely to parallel the shifts in plant traits observed at the community level.