Abstract: A karyomorphological study was performed on 3 Alstonia species in Yunnan Province, China . The karyotype of A. rostrata is reported for the first time . The results show that the chromosome numbers in the 3 species are 2n = 42 , and the chromosome numbers of A. scholaris and A. yunnanensis are both unexpectedly different from the previous reports“2n= 44. The interphase nuclei of A. rostrata , A. scholaris and A. yunnanensis were observed to be the rod-shaped prochromosome type, the round prochromosome type and the complex chromocenter type respectively, and the prophase chromosome types of the 3 species were all the interstitial one . The karyotype formulas are as follow: 2n = 42 = 3M+ 21m+ 18sm( A. rostrata) , 2n = 42 = 14m+ 24sm+ 4st ( A. scholaris) and 2n = 42 = 5m+ 37sm ( A. yunnanensis ). Karyotype asymmetries are of type 2A in A. rostrata , 3A in A. scholaris and 3B in A. yunnanensis. Based on the results, combined with the evidence from comparative morphology and cytology, the systematic position and evolutionary trend of this genus were briefly discussed .

Key words: Alstonia