The particular case of Medicago sativa in Spain

Eric Jenczewski, Joëlle Ronfort and Jean-Marie Prosperi

INRA SGAP Montpellier Domaine de Melgueil. 34130 Mauguio. France

Wild and cultivated plants of Medicago sativa from Spain are autotetraploid, outcrossing and cross compatible ^(3,4). They grow in parapatry in many locations, overlap in flowering periods and share the same pollinators. But, they differ for many morphological traits ^(1,2). In this study, we investigated the mechanisms that allow wild populations to maintain their morphological originality. We have examined two hypothesis : Restricted crop-to-wild gene flow or Strong selection pressures exerted against cultivated traits in the wild.

While the level of differentiation at neutral loci only depends on a balance between gene flow and genetic drift, the pattern of variation for quantitative traits may also reflect the effects of selection. Comparing the population structure for neutral and quantitative traits give insight on the relative importance of natural selection and gene flow to explain the morphological differentiation. In 1985 and 1986, one hundred and three natural populations were collected and evaluated in a dense swards experiment. A great variability for both morphological and agronomic traits was detected among populations ⁽¹⁾. For this study, a subset of 15 natural populations and 5 Spanish landraces was chosen to represent the whole range of morphological variability observed. Forty plants per population were individually scored for 13 quantitative traits and 5 allozymic loci.

Combining patterns of population structure suggests ⁽²⁾ :

(1) Evidence for crop to wild gene flow. Natural populations 64 and 147 contained a continuous range of plants with intermediate morphological features between typical wild and cultivated plants. They were neither significantly differentiated from the typical wild populations (Fst=0.009) nor from the landraces (Fst<0.001). These populations are likely to be of hybrid origin. This result strongly suggests that gene flow occurred from crop to wild populations.

(2) Strong divergent selection. Natural populations 32 and 66 contained mostly typical wild plants but they were not significantly differentiated from the landrace Aragon for allozymes (Fst<0.064). These contrasting patterns still provide evidence for gene flow from crop to wild populations, but also suggests that a strong divergent selection eliminated the cultivated traits from natural populations.

(3) Isolated wild populations exist. The remaining natural populations were significantly different from all the landraces with respect to both allozymes and quantitative traits. They are likely to be spatially or temporally isolated from the cultivated populations. The variation of the importance of the gene flow (crop-to-wild) may be due to a combination of factors (agricultural rotations, open man-made habitats).

References.

(1). Jenczewski E., Angevain M., Charrier A., Génier G., Ronfort J. and J.M. Prosperi 1998a. Contrasting patterns of genetic diversity in neutral markers and agro-morphological traits in wild and cultivated populations of Medicago sativa from Spain. Genetics Selection and Evolution. In press.

(2). Jenczewski E., Prosperi J.M. and J. Ronfort 1998b. Evidence for gene flow between wild and cultivated Medicago sativa (Leguminosae) based on allozyme markers and quantitative traits. American Journal of Botany. In press.

(3). Prosperi J. M., Delgado Enguita I. and M. Angevain 1989. Prospection du genre Medicago en Espagne et au Portugal. Plant Genetic Resources Newsletter 78/79: 27-29.

(4). Prosperi J.M., Angevain M., Bonnin I., Chaulet E., Génier G., Jenczewski E., Olivieri I. and J. Ronfort 1996. Genetic diversity, preservation and use of genetic resources of the Mediterranean legumes: alfalfa and medics. In 'The genus Medicago in the Mediterraneon region: current status and prospects in research' Génier and Prosperi [eds.] Cahiers Options méditerranéennes vol.18, 71-89, CIHEAM.