Résumé
Weedy rices have long been known, particularly in tropical areas, as the result of introgressions between wild and cultivated forms. More recently, new weedy forms in temperate regions where wild forms were never reported have invaded rice fields. These plants show many traits similar to true wild O. rufipogon (shattering, dormancy, awning, organ pigmentation). Their occurrence concerns many parts of the world practising intensive rice growing with direct seeding. Samples of weeds from France have been analysed with 15 microsatellite markers in comparison to a representative collection of 145 Asian rice varieties. Diversity of weedy rice is in agreement with the genetic structure of O. sativa separated in two main indica and japonica groups of varieties, Weeds from France showed a greater diversity than local Mediterranean varieties (temperate japonica) as well as original alleles or alleles more typical of indica varieties. Genetic structure of weed stands appeared more probably the consequence of a predominantly independent population dynamics allowed by seed conservation and dissemination and not the result of gene flow with varieties from which they are isolated by a much greater earliness. Results suggested that distant crosses between indica and japonica varieties and their possible effects on genome expression could be also a relevant explanation to weed origin. (Résumé d'auteur)