Distribution and variation of sexual and agamospermous populations of Stevia (Asteraceae: Eupatorieae) in the lower latitudes, Mexico

Cytogeographical studies have revealed the distribution of sexual diploids and agamospermous polyploids of Stevia ovata and four related species: Stevia nepetifolia, Stevia oligophylla, Stevia origanoides and Stevia triflora in Mexico. These species are distributed mainly in the temperate and subhumid montane region. Agamospermous triploids are the forms most abundantly and widely distributed geographically in each of the five species. Diploids occur in low frequencies except in S. origanoides. For the five species, diploids are restricted mainly to the temperate montane ranges of the south-western Pacific side of Mexico, but they also occur sporadically in the relatively isolated areas from Oaxaca to Durango, Mexico D. F. and Nuevo Leon. Although the distribution of the temperate forests must have moved southward and northward following climatic fluctuations, even in the lower latitudes of Mexico, the largely stable temperate vegetation could have been maintained along the Y-shaped montane ranges (Sierra Madre del Oriental, Sierra Madre del Occidental, Sierra Madre del Sur and the Trans-Mexico Volcanic Belt) and have provided some refuge areas. The present distribution ranges of sexual diploids of some species of Stevia in Mexico indicate that diploids were not restricted to a single refuge during the last ice age.