Cytogeography of Gentianaceae–Exaceae in Africa, with a special focus on <i>Sebaea</i>: the possible role of dysploidy and polyploidy in the evolution of the tribe

Unlike other tribes of Gentianaceae, Exaceae have so far received little attention regarding their karyological evolution. Indeed, only 35 chromosome number counts (19 species) have been referenced to date, representing only a negligible fraction of the tribal diversity. In this paper, we performed an intensive chromosome count on material collected in the field (South and central Africa, plus Madagascar), encompassing 155 populations and c. 60 species from four genera of <i>Exaceae</i>, including <i>Exacum</i>, <i>Ornichia</i>, <i>Sebaea</i> and <i>Tachiadenus</i>. Fifty nine species (14 <i>Exacum</i>, one <i>Ornichia</i>, 42 <i>Sebaea</i> and two <i>Tachiadenus</i>) were examined for the first time, revealing a broad set of chromosome numbers (2n = 18, 28, 32, 36, 42, 56) and the occurrence of polyploid systems within <i>Exacum</i> and <i>Sebaea</i>. These results allow us to postulate x = 7, 8 or 9 as possible base chromosome numbers for Exaceae and emphasize the importance of both dysploidy and polyploidy processes in the evolution of the tribe. Finally, chromosome numbers appear to be associated to some morphological or geographical traits, suggesting new systematic combinations and likely active speciation patterns in the group.