Temporal changes in population genetic diversity and structure in red and white clover grown in three contrasting environments in northern Europe

Extending the cultivation of forage legume species into regions where they are close to the margin of their natural distribution requires knowledge of population responses to environmental stresses. This study was conducted at three north European sites (Iceland, Sweden and the UK) using AFLP markers to analyse changes in genetic structure over time in two population types of red and white clover (Trifolium pratense and T. repens, respectively): (1) standard commercial varieties; (2) wide genetic base (WGB) composite populations constructed from many commercial varieties plus unselected material obtained from germplasm collections.At each site populations were grown in field plots, then randomly sampled after 35 years to obtain survivor populations. AFLP markers were used to calculate genetic differentiation within and between original and survivor populations.No consistent changes in average genetic diversity were observed between original and survivor populations. In both species the original varieties were always genetically distinct from each other. Significant genetic shift was observed in the white clover oRamona' grown in Sweden. The WGB original populations were more genetically similar. However, genetic differentiation occurred between original and survivor WGB germplasm in both species, particularly in Sweden. Regression of climatic data with genetic differentiation showed that low autumn temperature was the best predictor. Within the set of cold sites the highest level of genetic shift in populations was observed in Sweden.The results suggest that changes in population structure can occur within a short time span in forage legumes, resulting in the rapid formation of distinct survivor populations in environmentally challenging sites.