Mexican species of the genus Stevia (Eupatorieae, Asteraceae): Chromosome numbers and geographical distribution

Chromosome numbers from 244 accessions representing 73 species, five varieties and two putative species hybrids in Mexican Stevia are reported with comments on agamospermy, cytogeography and chromosomal evolution. The chromosome numbers of 47 species and two varieties were counted for the first time. Counts of 12 species and two varieties were new cytotypes, or differed from all previously reported numbers for the same species. B chromosomes were found in 16 species and two varieties. All species of shrubs or shrublets were sexual diploids (2n=24). Perennial herbs were diploids or polyploids based on x=11 or 12 and the widespread prevalence of agamospermous polyploids was revealed. Chromosome numbers of annual herbs varied from 2n=24 to 2n=8. The cytogeographical survey revealed five distributional patterns of agamic complexes based on the relative abundance of polyploids compared to their diploid ancestors. The wider distributional range (extending to northern and inland areas) of some agamospermous polyploids have been achieved by recent colonization accompanying climatic warming after the last glaciation. This has occurred even in lower latitudes such as Mexico, due to more successful founder population reproduction and the acceleration of seed dispersal, as well as the creation of new and open habitats by human activities.