Differentiation between natural and cultivated populations of Medicago sativa (Leguminosae) from Spain: analysis with random amplified polymorphic DNA (RAPD) markers and comparison to allozymes

The conservation of a crop's wild relatives as genetic resources requires a n understanding of the way genetic diversity is maintained in their populat ions, notably the effect of crop-to-wild gene flow. In this study, the amou nt of differentiation between natural and cultivated populations of Medicag o sativa was analysed using random amplified polymorphic DNA (RAPD) markers and an extension of the AMOVA procedure adapted to autotetraploid organism s. Simulations of structured populations were performed to test whether AMO VA provides estimates of population structure in autotetraploids that can b e directly compared to those obtained for allozyme data. Simulations showed that phi-statistics allow a good estimation of population differentiation when unbiased allelic frequencies are used to correct the conditional expec tations of squared genetic distances. But such unbiased estimates can not b e practically guaranteed, and population structure is notably overestimated when some populations are fixed for the presence of amplified fragments. H owever, removing fixed loci from the data set improves the statistical powe r of the test for population structure. The genetic variation of 15 natural and six cultivated populations of M. sativa was analysed at 25 RAPD loci a nd compared to estimates computed with allozymes on the same set of populat ions. Although RAPD markers revealed less within-population genetic diversi ty than allozymes, the quantitative and qualitative patterns of population structure were in full agreement with allozymes. This confirmed the conclus ions drawn from the allozymic survey: crop-to-wild gene flow occurred in ma ny locations, but some other mechanisms opposed cultivated traits to be mai ntained into natural populations.