Evolution of morphology, ecology and chromosome in the genus Stevia : Molecular phylogenetic approaches

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The purpose of this study is to estimate phylogenetic relationships among Stevia and its relatives using molecular data and examine evolutionary correlations of morphological, ecological and chromosomal traits based on the phylogenetic tree.The phylogenetic tree estimated from matK sequence data suggested that Carphocaete is a sister group of Stevia and n=12 is the ancestral chromosome number in the genus Stevia. Ageratum (n=10), which has been regarded as a relative of Stevia, was clustered not with Stevia but with genera with n=10. Morphological similarity between Stevia and Ageratum is considered to be a result of convergent evolution. Mcvaugiella (n=13), which has also been regarded as a relative of Stevia, was clustered not with Stevia but with Pachytamnus with n=17. This evidence suggests than n=12 in Stevia and n=13 in Mcvaugiella are independently derived from n=17.Phylogenetic relationships within the genus Stevia were examined using sequence data of ITS.An obtaiend tree showed that (1) an annual species S.aschenborniana (n=12) is basal to all other species examined, (2) a clade of shurubby species (n=12) is branched next, and then (3) perennial species diverged to two clades, one having n=12 and the other having n=11 and 12.Phylogenetic evidence showed that evolutionary changes in chromosome number and generation time are correlated, but those in chromosome number and genome size are not dorrelated.Evolutionary changes to annuality are correlated with those to self-pollination, but reduction in flower size was not correlated with changes to annuality or self-pollination.