The genus Centaurium (Gentianaceae) consists of about 50 species, mostly annuals or biennials, distributed in two main regions of diversification. In the Mediterranean area, diploid to hexaploid species occur whereas in the western part of North America and Mexico only polyploids species are found. We have investigated the phylogenetic relationships within this group for almost all the species using sequences of nrDNA (ITS) and cpDNA (trnL-F region). Moreover, we have stated a large survey of the chromosome numbers of different population of the New World species. Phylogenetic analysis strongly supports the existence of at least five geographic groups and suggest a pattern of colonization from Eurasia to California followed by a migration in Texas and Mexico. The caryological survey of the American species comfort the evolutive scenario inferred from molecular markers. New chromosome numbers for several species were also found. In addition, our study provides some evidences of regular hybridization process among different Centaurium species followed by polyploidization. These results suggest that amphiploidy is involved in the diversification of the genus as a speciation process. Finally, this work will result in a systematic revision of the known classification of the whole genus.

Key words: biogeography, Centaurium, Gentianaceae, ITS, phylogeny, trnL-F