Phylogenetic patterns and polyploid evolution within the Mediterranean genus Centaurium (Gentianaceae-Chironieae)

The genus Centaurium (centauries) comprises ca. 27 annual species and subspecies primarily distributed in the Mediterranean basin. Phylogenetic relationships, inferred from sequences of both nrDNA (internal transcribed spacer, ITS) and cpDNA (trnL intron and trnL-F spacer) regions, confirm the monophyly of Centaurium and reveal the importance of polyploidy within the genus. In each ITS clade, diploid to hexaploid species of centauries occur. Furthermore, polyploidization is often associated with hybridization events, as suggested by the importance of additive polymorphic sites detected in the ITS region. Hence, reticulation appears to be the main cause of phylogenetic incongruence-and resulting systematic controversies-detected in the genus. Based on molecular and cytological evidence, the origin of several polyploid systems is discussed. Allopolyploidy, associated or not with introgression, appears to be the main evolutionary trend in Centaurium, and only a few autopolyploid taxa have been detected so far.