Karyotype and cytogeography of the genusHeracleum(Apiaceae) in the Hengduan Mountains

Abstract

Abstract In the present study, the karyotypes of 34 populations belonging to 11 species and one variety ofHeracleumfrom the Hengduan Mountains in China were examined. Chromosome numbers and the karyotypes of three species (H. souliei,H. kingdoni, andH. wenchuanense) are reported for the first time, as are the karyotypes ofH. moellendorffiiandH. henryi(tetraploid). Populations ofH. candicans,H. franchetii, andH. kingdoniin the Hengduan Mountains were found to consist of a mixture of diploid and tetraploid plants. Except for four species ofHeracleum, namelyH. candicans,H. franchetii,H. henryi, andH. kingdoni, which have both diploid and tetraploid karyotypes, all other species ofHeracleumare were found to be diploid. All karyotypes were found to belong to the 2A type of Stebbins, with the exception ofH. candicansvar.obtusifolium, which belongs to 2B, andH. hemsleyanumandH. franchetii(Mt. Dujuan, Daocheng, Sichuan, China), which belong to 1A. There was only a slight difference in the karyotype asymmetry index, which suggests a close kinship for species ofHeracleumand that the entire phylogenetic development ofHeracleumis relatively primitive. Species that exhibited advanced morphological features were also more advanced in karyotype structure, with the order of karyotype evolution being 1A→2A→2B. This phenomenon indicates that the species distributed in the Hengduan Mountains have not diverged completely and that the Hengduan Mountains are a relatively young and active area for the evolution ofHeracleum. Polyploidization inHeracleummay be an important evolutionary mechanisms for some species, generating diversity. The biological attributes, distribution range, and the geological history of the genus have all played a part in accelerating the evolution through polyploidization or aneuploidization. It is known that as the distribution latitude ofHeracleumdecreases from north to south, the chromosome number, ploidy level, and asymmetry structure appear to increase. In the Hengduan Mountains, these tendencies are also evident. Finally, based on all the available cytogeographic data, we speculate that the more advanced tetraplont or aneuploid species ofHeracleumin India may be derived from early diplont species that were distributed in the Caucasus region and Hengduan Mountains. The dispersal ofHeracleumwas from Eurasia to India, because this correlates with the emergence of the Himalayan Mountains through tectonic movement. Thus, the Hengduan Mountains are not only a center of diversity forHeracleum, but also a center of active speciation in modern times.