Evidence for gene flow between wild and cultivated Medicago sativa (Leguminosae) based on allozyme markers and quantitative traits

Genetic differentiation between co-occurring crops and their wild relatives will be greatly modified by crop-to-weed gene flow and variation between h uman and natural selective pressures. The maintenance of original morpholog ical features in most natural populations of Medicago sativa in Spain quest ions the relative extent of these antagonistic forces. In this paper, we me asured and compared the pattern of population differentiation within and am ong the wild and cultivated gene pool with respect to both allozymes and qu antitative traits. Patterns of diversity defined three kinds of natural pop ulations. First, some populations were intermediate with respect to both al lozymes and quantitative traits. This suggests that crop-to-weed gene flow may have created hybrid populations in some locations. Second, some populat ions were different from all the cultivated landraces with respect to both allozymes and quantitative traits. This probably results from variable gene Row in space and in time, due to demographic stochasticity in either natur al or cultivated populations. Third. differentiation from cultivated landra ces was only achieved for the quantitative traits but not for allozymes in two populations. This suggests that natural selection in some locations may oppose gene flow to establish cultivated traits into the natural introgres sed populations.