Ecological and genetic correlates of long-term population trends in the Park Grass Experiment

The Park Grass Experiment (PGE) is the longest observed set of experimental plant communities in existence. Although the gross composition of the vegetation was at equilibrium over the 60-yr period from 1920 to 1979, annual records show that individual species exhibited a range of dynamics. We tested two hypotheses to explain why some species initially increased nd why subsequently some of these (the outbreak species) decreased gain. The study was designed around eight phylogenetically ndependent contrasts (PICs), each containing related species with ifferent dynamics. Our first hypothesis was that persistent increasers and utbreakers have higher intrinsic rates of natural increase than ontrol species (species without trends), allowing them to spread hen interspecific competition is reduced by drought. This was tested by measuring establishment and seed production of species in ield experiments, with and without interspecific competition. Seed production in outbreak species responded more strongly to release from interspecific competition than it did in either of the ther groups of species. Our second hypothesis was that outbreak species eventually declined because they lacked the genetic variation ecessary to adapt to the novel habitats to which they had initially spread. We tested this by measuring mating systems and genetic diversity in persistent and outbreak species in the PGE. In seven out of seven PICs tested, the outbreak species was more selfing than its persistent relative. There was a significant positive correlation between outcrossing rate and gene diversity. These results support roles for both ecological and genetic traits in long-term dynamics

Successful amplification of rice chloroplast microsatellites from century-old grass samples from the Park Grass Experiment

We report the successful amplification of microsatellite markers for the chloroplast genome from century-old samples of 2 grasses growing in the Park Grass Experiment (PGE): Anthoxanthum ordoratum and Festuca rubra. This opens the possibility of establishing a long-term genetic time series for the PGE, which began in 1856 and is believed to be the oldest ecological experiment in existence. Although the plant samples used were not originally prepared or stored with molecular analysis in mind, the hexadecyltrimethylammonium bromide (CTAB) method of DNA extraction was successfully used. Obtained DNA was degraded but could be amplified by means of PCR. It produced bands around the expected size for chloroplast microsatellite primers derived from rice. When sequenced, bands showed good homology with sequences from rice chloroplast genomes listed in GenBank (accession No X15901)

Universal primers for the amplification of chloroplast microsatellites in grasses (Poaceae)

Attempts to design truly universal primers to amplify chloroplast microsatellites have met with limited success due to nonconservation of repeat loci across widely divergent taxa. We have used the complete chloroplast genome sequences of rice, maize and wheat to design five pairs of primers that amplify homologous mononucleotide repeats across the Poaceae (grasses). Sequencing confirmed conservation of repeat motifs across subfamilies and a preliminary study in Anthoxanthum odoratum revealed polymorphism at two loci with a haplotype diversity value of 0.495. These primers provide a valuable tool to study cytoplasmic diversity in this extensively studied and economically important range of taxa

Contrasting patterns of pollen and seed flow influence the spatial genetic structure of sweet vernal grass (<i>Anthoxanthum odoratum</i>) populations

The spatial genetic structure of plant populations is determined by a combination of gene flow, genetic drift, and natural selection. Gene flow in most plants can result from either seed or pollen dispersal, but detailed investigations of pollen and seed flow among populations that have diverged following local adaptation are lacking. In this study, we compared pollen and seed flow among 10 populations of sweet vernal grass (Anthoxanthum odoratum) on the Park Grass Experiment. Overall, estimates of genetic differentiation that were based on chloroplast DNA (cpDNA) and, which therefore resulted primarily from seed flow, were lower (average FST = 0.058) than previously published estimates that were based on nuclear DNA (average FST = 0.095). Unlike nuclear DNA, cpDNA showed no pattern of isolation by adaptation; cpDNA differentiation was, however, inversely correlated with the number of additions (nutrients and lime) that each plot had received. We suggest that natural selection is restricting pollen flow among plots, whereas nutrient additions are increasing seed flow and genetic diversity by facilitating the successful germination and growth of immigrant seeds. This study highlights the importance of considering all potential gene flow mechanisms when investigating determinants of spatial genetic structure, and cautions against the widespread assumption that pollen flow is more important than seed flow for population connectivity in wind-pollinated species

The Park Grass Experiment 1856 - 2006: Its Contribution to Ecology

From the summary: The Park Grass Experiment, begun in 1856, is the oldest ecological experiment in existence. Its value to science has changed and grown since it was founded to answer agricultural questions. In recent times the experiment has shown inter alia how: plant species richness, biomass and pH are related; community composition responds to climatic perturbation and nutrient additions; soil is acidified and corrected by liming. It also provided one of the first demonstrations of the evolution of adaptation at a very local scale and contains a putative case of the evolution of reproductive isolation by reinforcement. The application of molecular genetic markers to archived plant material promises to reveal a whole new chapter of genetic detail about the long-term dynamics of plant populations