Abstract: In this paper, karyotypes from twenty-one populations in eight species of Allium section Rhiziridium from China were investigated. The karyotypes of two species, A. eduardii and A. flavovirens, and the hexaploidy karyotype of A. strictum are reported for the first time. By studying karyotypes of this section in Allium, we hope to understand evolutionary mechanisms and relationships among the taxa in this section. We find that five species, A. eduardii, A. flavovirens, A. lineare, A. mongolicum and A. oreoprasum are diploid, and that their karyotypes are the 2A type of Stebbins. The two species, A. tuberosum and A. ramosum, are tetraploid and their karyotypes are also the 2A type. Four populations of the species A. strictum are hexaploid and their karyotypes are the 2B type. From the karyotype analysis we obtain the following results: (1) In this section, there are many polyploids and polyploid series. The variation in the chromosome number in this section is closely related to the evolution history of the species. Therefore polyploidization might be one of the important evolutionary mechanisms that has shaped species diversity in sect. Rhiziridium. (2) The satellite chromosomes are either subterminal or terminal chromosomes and satellites are located in the short arm. (3) It is suggested that A. strictum might represent an incipient species and has exploited a new niche through ployploidization. (4) The cultivated species A. tuberosum might be derived from the artificial naturalization of the wild diploid and tetraploid ancestors. The cultivated triploid species is suggested to have originated from the hybridization between the wild diploid and tetraploid ancestors, and its survival and widespread cultivation might have been achieved through asexual reproduction.

Key words: Allium, section Rhiziridium, karyotype, polyploid, intraspecific differentiation, evolution