Abstract: Fifteen populations of four species of Maianthemum from southwestern China were analyzed in terms of base number , polyploidy, dysploidy , chromosome size , chromosome morphology , satellited chromosomes , structural heteromorphism, and karyotype asymmetry and karyotype evolution . The results show that 1) chromosomal number of all the species examined is 2 n = 36 , with M. tatsienensis (Population Lijiang , Yunnan) as an exceptional case of which the chromosomal number is 2 n = 72 ; 2) karyotypes show difference among populations, especially in the numbers of metacentric , submetacentric, and acrocentric chromosomes as well as satellites. The karyotypes of M. nanchuanense and M. szechuanicum are documented the first time . B- chromosome is discovered in this genus the first time, too . It is inferred that the evolutionary mechanisms in the genus Maianthemum include the frequent somatic chromosome aberration and the polyploidy at various
levels , and that the southwestern China is one of the differentiation centers of this genus .

Key words: Chromosome