How are different gentians related?

The most basal (oldest) group of gentians is tribe Saccifolieae.  Next up in the family tree the tribe Exaceae branched off, followed by tribe Chironieae.  The three tribes Gentianeae, Helieae, and Potalieae are of approximately the same age, and the specific relationship among these three tribes are still unclear. 

Detailed view of relationships within the tribe based on matK and trnL intron DNA sequence data.