Paris is a genus of medicinal plants in the family Melanthiaceae. For the use in traditional medicine
and for trading in local markets, rhizomes of Paris plants are collected from their native habitats. Due to its slow
growth and excessive harvesting, the species is currently facing the risk of extinction. The aim of the present study
was to cytogenetically characterize Paris plants from the mountainous regions of northern Thailand in order to obtain
characteristics that may be species-specific for use in taxonomical and diversity studies. Chromosomes were isolated
from rhizome-grown root tips from the total of 20 Paris accessions. Of these, 12 were diploid (2n = 2x = 10), 2 were
triploid (2n = 3x = 15), one was tetraploid (2n = 4x = 20) and 5 were mixoploid accessions. The diploid accessions
from Chiang Rai and Nan provinces had the most common basic karyotype 6m+4t whereas those from Chiang Mai
province showed 6m+4st and 4m+2sm+4st, indicating differentiation at the species level. The analysis of karyotypes
and ribosomal gene mapping by FISH (fluorescence in situ hybridization) indicated autotriploidy arising spontaneously
within a diploid population and autotetraploidy via endoreduplication in the mitotic cell division. The ribosomal FISH
also revealed a novel map of 5S and 45S rDNA, including the ancestral 5S–45S linked sites. The findings of our research
support the ongoing species diversification among Thai Paris taxa.