The importance of spatio-temporal heterogeneity in natural populations subject to exposure to geneflow from agriculture

Tom Harwood, Michael Shaw, Mark Beaumont, Joel Allainguillaume, Luisa Elliott, Mike Wilkinson; The University of Reading, UK.

email (Tom Harwood): t.d.harwood@reading.ac.uk

Concern over the potential impact of transgenes on ecosystems has led to studies of the mechanisms of movement of genes between cultivated and wild plants and within wild populations. This study concentrated on a single system, that of hybridisation in populations of Wild Turnip (Brassica rapa) as a consequence of exposure to pollen of cultivated Rape (Brassica napus) as a model system. In the UK, Brassic rapa grows largely as a riverside plant, apparently genetically separate from the weedy populations on arable land.

A spatial population model of gene flow by pollen and seed dispersal has been developed. Although not yet fully parameterised, the model can be used to illustrate the principles of the study system. Due to the asymmetric flow of genes downstream, the probability of fixation of a single hybridisation event is dependent upon the position of the population in the linear system. Genes introduced nearer to the source of the river are more likely to spread.

Recent fieldwork has revealed that the riverside Wild Turnip populations are transient, with more than half the total recorded populations being present in only one of the three survey years. It is postulated that there are large areas where seed is present in the soil, and that the above ground breeding populations are a sample by disturbance from this pool. The genetically relevant population is therefore that of the seedbank, violating some of the assumptions of the initial spatial model. Importantly this indicates that the timing of any hybridisation event would also be significant.