GENETIC-MARKERS AND QUANTITATIVE GENETIC-VARIATION IN MEDICAGO-TRUNCATULA (LEGUMINOSAE) - A COMPARATIVE-ANALYSIS OF POPULATION-STRUCTURE

Two populations of the selfing annual Medicago truncatula Gaertn. (Leg uminoseae), each subdivided into three subpopulations, were studied fo r both metric traits (quantitative characters) and genetic markers (ra ndom amplified polymorphic DNA and one morphological, single-locus mar ker). Hierarchical analyses of variance components show that (1) popul ations are more differentiated for quantitative characters than for ma rker loci, (2) the contribution of both within and among subpopulation s components of variance to overall genetic variance of these characte rs is reduced as compared to markers, and (3) at the population level, within population structure is slightly but not significantly larger for markers than for quantitative traits. Under the hypothesis that mo st markers are neutral, such comparisons may be used to make hypothese s about the strength and heterogeneity of natural selection in the fac e of genetic drift and gene flow. We thus suggest that in these popula tions, quantitative characters are under strong divergent selection am ong populations, and that gene flow is restricted among populations an d subpopulations.