# LBMRPM, CNRS-INRA, BP27, 31326 Castanet-Tolosan Cedex, France.

Medicago truncatula is an annual forage, diploid, selfing plant of low DNA content and with which transgenic plants can be routinely obtained. This Mediterranean forage legume is used as a model-plant instead of alfalfa (Medicago sativa) to combine analyses of the genetic diversity and studies of molecular biology. M. truncatula is nodulated by well-known Rhizobium meliloti strains and is of good agronomical value for Mediterranean regions where they are used to improve the soil structure, increase soil nitrogen and as a source of winter forage.

Genetic resources and diversity of natural populations. Since 1985, more than 300 natural populations have been collected from 9 countries (North-Africa and Europe) and using agronomic and electrophoresis tools (isozymes), a core-collection of 130 populations was built⁽¹⁾. A high level of polymorphism was detected both between and within natural populations for both neutral markers (RAPD) and morphological traits^(1,2,3,4). Concurrently, a selection of medics for introduction in Mediterranean marginal lands started and one cold tolerant variety was selected⁽⁶⁾.

Molecular biology. We are engaged in the construction of a genetic map based on crosses between 4 genotypes. From three of them, recombinant inbreed lines have been obtained by single seed descent. From the fourth cross between Jemalong and a natural population, we have mapped more than 250 markers (RAPD, AFLP, isozymes, known genes, micro-satellites and morphological markers). This map spans over 980 cM (an average 600kb/cM) in 6 linkage groups (2n=16). Haploid DNA content was measured by flow cytometry to be 0.58pg/1C, that is approximately four times Arabidopsis genome size. Level of segregation distortions varies from cross to cross and seem to be related to the genome size of parents⁽⁵⁾. This genetic map is a powerful tool for the positional cloning of symbiotic genes and for the breeding of Mediterranean forage legumes as well as for studying evolutionary processes within Medicago genus.

References

(1) Balfourier F, Charmet G., Prosperi J.M., Goulard M. and Monestiez P. 1998. Comparison of different spatial strategies for sampling a core-collection of natural populations of fodder crops. Genetics Selection and Evolution. In press.

(2) Bonnin I., Huguet T., Gherardi M., Prosperi J.M. and I. Olivieri 1996a. High level of polymorphism and spatial structure in a selfing plant species, Medicago truncatula (Leguminosae), using RAPD markers. American Journal of Botany. 83(7) 843-855.

(3) Bonnin I., Prosperi J.M. and I. Olivieri 1996b. Genetic markers and quantitative genetic variation in Medicago truncatula (Leguminosae): a comparative analysis of population structure. Genetics. 143:1795-1805.

(4) Bonnin I., Prosperi J.M. and Olivieri I. 1997. Comparison of quantitative genetic parameters between two natural populations of a selfing plant species, Medicago truncatula Gaertn. Theoretical and Applied Genetics 94(5) 641-651.

(5) Jenczewski E., Gherardi M., Bonnin I., Prosperi J.M., Olivieri I. and T. Huguet 1997. Insight on segregation distorsions in two intraspecific crosses between annual species of Medicago (Leguminosae). Theoretical and Applied Genetics 94(5) 682-691.

(6) 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.