Genetic structure of the annual weed Senecio vulgaris in relation to habitat type and population size

Throughout the world, the highly selfing annual common groundsel, Senecio v ulgaris (Asteraceae) is a common weed. Recently, it has also colonized ecol ogical compensation areas in agro-ecosystems. We investigated the genetic s tructure of S. vulgaris using random amplified polymorphic DNA (RAPD) profi les of 80 plants from nine populations representing three habitat types in two regions in Switzerland. RAPD variation among regions (19.8%), among pop ulations within regions (19.2%) and within populations (61.1%) was highly s ignificant (AMOVA; P< 0.001). Gene now estimated from the observed differen tiation among populations (<Phi>(ST) = 0.382) was low (assuming Wright's is land model, N(e)m = 0.404). Genetic distances between pairs of populations were significantly correlated with geographical distances (Mantel test; r = 0.37, P< 0.03). Molecular variance obtained with AMOVA was lowest in the s mall populations in compensation areas (1.13), intermediate in vineyard pop ulations (2.49), all located in northern Switzerland and highest in the lar ger vegetable field populations from western Switzerland (3.41; P< 0.05). O verall, there was a positive correlation of molecular variance and populati on size (P < 0.05), as expected under genetic drift. However, molecular var iance was negatively correlated with population size among populations in e cological compensation areas, suggesting that selection was also important. We also applied triazine herbicide to leaves of three offspring of each of the 80 plants. Plants from populations of compensation areas showed higher mean levels and reduced variation in the resistance to triazine herbicide than plants from vineyards and vegetable fields. This suggests that compens ation areas were colonized from adjacent corn fields, in which there has be en selection for herbicide resistance. We discuss the implications of our r esults for the biological control of S. vulgaris.