in the long run. However, migration rate in our experiment was density independent and migration was confined to the two nearest neighbors, whereas it is known that the dynamics of a metapopulation can vary depending on the exact form of density dependence (31) and scheme of migration (11). Moreover, growth rates of Drosophila (and most insects, microbes, and fishes) are higher than those of mammals and birds, which are generally of greater concern for conservation. The intrinsic growth rates of subpopulations are also known to interact strongly with migration rate in producing observed metapopulation dynamics (12). Therefore, due caution should be exercised when extrapolating our results to natural populations.

DNA methylation is an epigenetic mark that silences transposable elements (TEs) and repeats. Whereas the establishment and maintenance of DNA methylation are relatively well understood, little is known about their dynamics and biological relevance in plant and animal innate immunity. Here, we show that some TEs are demethylated and transcriptionally reactivated during antibacterial defense in Arabidopsis. This effect is correlated with the down-regulation of key transcriptional gene silencing factors and is partly dependent on an active demethylation process. DNA demethylation restricts multiplication and vascular propagation of the bacterial pathogen Pseudomonas syringae in leaves and, accordingly, some immune-response genes, containing repeats in their promoter regions, are negatively regulated by DNA methylation. This study provides evidence that DNA demethylation is part of a plant-induced immune response, potentially acting to prime transcriptional activation of some defense genes linked to TEs/repeats. epigenetics | RNA silencing | MAMP