The role of patch processes in regulating regional dynamics in annual plant metapopulations

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Annual plants occupy a broad range of natural, semi-natural and human-disturbed habitats. They thus may constitute an important component of a region's biodiversity. Because of their short generation times, annual plants may also respond quickly to new selective pressures, but this depends on how much genetic diversity is maintained in their populations, and how this is structured. A knowledge of the fine-scale structure of annual plant populations might seem trivial, but it is in fact essential for understanding the factors that regulate the genetic diversity of annual plant populations and thus their ability to respond to natural selection. It is this potentially important for the management of weed populations, the measurement and management of road-side biodiversity, and for predicting responses of annual plants to environmental change. Previous work has focussed on two contrasting levels of spatial organisation and processes. On the one hand, studies have addressed how density affects the mating system and patterns of gene flow within populations, and how these in turn affect population genetic structure. On the other hand, studies have addressed the importance of gene flow amongst populations and potential metapopulation dynamics (i.e., extinctions and re-colonisations) in a fragmented landscape for the genetic diversity both within and between populations. The likely possibility that continuous populations might be established as a result of discrete patch-level colonisation has hardly been addressed. Under this model, populations expand locally not by density-dependent diffusion but rather by the establishment of patches over small spatial scales (due to very local dispersal), followed by the closure of gaps between patches through patch expansion by diffusion. The result is a spatially extended population whose genetic structure will be very different from its physical shape. This project will test the hypothesis that within-population patch dynamics play a key role in limiting and structuring within-population genetic diversity. In other words, we hypothesise that colonisation and extinction dynamics at scales of a few metres within a population play an important role in patterning genetic diversity. The definition of population and patch is evidently important; here, as is common, a population is a more or less continuous array of individuals distributed over space; a patch is an spatial array of individuals separated from other such arrays by a distance at least an order of magnitude of the spatial extent of an individual plant. Thus, for annual plants with a diameter of 30 cm, patch-colonisation will depend on dispersal over several metres. We will test our hypothesis by using within-patch diversity and between-patch differentiation for patches within a range of populations of the annual plant Mercurialis annua. Because the impact of patch colonisations on patterns of genetic diversity will be eroded by subsequent gene flow amongst patches, we will compare diversity and differentiation for patches that differ in whether they contain males; patches with males are expected to exchange genes at a greater rate than those without males. We will also use our study to showcase a novel technique that we have developed to genotype polyploid plants in a way that allows standard diploid population genetics. This approach involves the amplification of microsatellites from just one of the constituent genomes of the polyploid genome. In the case of M. annua, we will use microsatellites for our population genetic assays that amplify in only the genome contributed by the sister species M. huetii.