Genetic diversity and population dynamics of weedy rice in Camargue area.

Weedy rice have long been known, particularly in tropical areas, as the result, of introgressions between wild and cultivated forms. More recently, rice fields have been frequently invaded by new weedy forms in temperate regions where wild forms were never reported. These plants show many traits similar to true wild O. rufipogon (shattering, dormancy, awning, organ pigmentation). Their occurrence concerns many parts of the world practicing intensive rice-growing with direct seeding. Samples of weeds from Camargue have been analyzed 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 mainly organized in two main groups of indica and japonica. Weeds from Camargue showed a greater diversity than Mediterranean varieties (temporate japonica) as well as the presence of original alleles or alleles more typical of indica varieties. Genetic structure of weed stands appeared more probably the consequence of a predominantly autonomous dynamics allowed by seed conservation and dissemination than the result of gene flow with varieties from which they are isolated by a much greater precocity,. These results suggested that distant crosses between indica and japonica varieties and their effect on genome structure and expression could be a relevant explanation to this "weediness" process.