Molecular phylogeny and comparative ecology of Mexican Stevia

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Mexican Stevia includes about 100 species of annual, perennial, and shrubs among which shurubby species are considered to be ancestral. Floral morphology is diversified among species. Autogamy and apomixis are found in many species. Chromosome numbers vary from n=11 to n=12.The goal of this project is to examine correlation in generation time, reproductive system, floral morphology and chromosome numbers. To obtain research materials, we carried out field trips in Mexico.In 1995, we had field trips in Oaxaca, Veracruz, Jalisco and some other states. We collected about 50 species of Stevia including Stevia phlebophyla and S.karwinskyana which had never been collected during the past hundred years and had been considered to be extinct. We also found several new species. In 1996, we had field trips to Chiapas, Chihuahua, Durango and some other northern states. As a result of field trips during the two years, we obtaiened materials of approximately 80 species of the 100 total. Chromosome counts made on these materials revealed that shrubby species have n=12, perennial species have n=12 or 11, and annual species have n=11 or 4 (in S.ephemera). This fact supports the idea that chromosome number reduction is associated with reduction in generation time. All materials of shrubs and annuals examined were sexual diploids while perennial plants are often triploids, tetraploids or pentaploids and are regarded as seed apomicts.As a result of a phylogenetic study using sequences of matK,Carphocaete (n=12) was shown to be a sister genus of Stevia. Another study using sequences of ITS revealed that shurubby species (n=12) are monophyletic and basal in the genus ; perrenial species belong to two monophyletic groups, one of which includes S.deltoidea (n=12) and S.decummbens (n=12), and the other includes some species with n=11 and S.origanoides (n=11,12).