Résumé
In order to understand the dynamics of contact zones between natural and cultivated populations of alfalfa (Medicago sativa L.) in Spain, we investigated the patterns of mitochondrial variation (characterized through restriction fragment length polymorphisms) for 173 individuals in four natural and four cultivated populations of this species. We also examined the relationship between mtDNA and phenotypic variation. This study shows that cytoplasmic variation occurs in both natural and cultivated populations of Medicago sativa. Among the five mitotypes identified in the species, two are specific from the natural populations. The distribution of shared mitotypes between wild and cultivated forms attested to the occurrence of gene flow through seed from cultivated towards natural populations. Comparisons between cytoplasmic and phenotypic patterns of variation give information on the patterns of gene exchange that occur in contact zones over successive generations.