Chromosomal evolution in Balsaminaceae, with cytological observations on 45 species from Southeast Asia

Abstract Balsaminaceae consists of two genera, Hydrocera with only one species H. triflora and Impatiens with over 900 species. The chromosome number of H. triflora was consistently reported as 2n=16 or n=8. The somatic chromosome numbers vary greatly from 2n=6 to 2n=66 in Impatiens. In order to provide more complete information to understand the chromosomal evolution and cytogeography of Balsaminaceae, we counted chromosome numbers for 45 species of Impatiens from southwest China and the adjacent areas. Chromosome numbers were confirmed for 11 species, and numbers different from previous reports were found for two species. 32 species were examined for the first time, and the numbers 2n=12, 14, 16, 18, 20, 40, 54 (or the relevant gametic numbers) were found. The number n=27 found in I. pseudokingii is new for the family. The number 2n=18, mostly involving a bimodal karyotype with one pair of chromosomes conspicuously longer than others, is predominant among the species studied. Considering all the available chromosomal data, x=7, 8, 9, 10 are the most frequent basic numbers of the family. Previous authors have suggested x=7 or x=8 to be ancestral. Based on the present data, we suppose that x=8, x=9, or x=10 are all possible candidates of the ancestral basic numbers in Impatiens. Geographic distributions of the most frequent basic numbers show interesting patterns: x=7 and 8 occur in Africa, x=7, 8, 10 in Southern India and Sri Lanka, x=7, 9, 10 in the Himalayas, x=7, 8, 9, 10 in Southeast Asia, and x=10 in northern Asia, Europe and North America.