Evidence for multiple, autoploid origins of agamospermous populations in Eupatorium sessilifolium (Asteraceae)
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Comparative analyses were made of agamospermous populations of Eupatorium sessilifolium, which have previously been documented to be polyploid, to determine whether they are alloploid or autoploid in origin and to assess the possibility that they have arisen more than once. There was no variability in ITS sequences among seven agamospermous and eight sexual diploid populations of E. sessilifolium, which is consistent with morphological observations in suggesting that the agamospermous populations were autoploids. The ITS sequence characteristic of E. sessilifolium differs from all other North American species by a minimum of 15 changes, and heterogeneity or polymorphism would be expected if the agamospermous populations were alloploids. Analysis of the chloroplast-based trnC-psbM spacer region showed variability among both sexual diploid and agamospermous populations of E. sessilifolium, which suggested that the agamospermous populations stem from multiple origins. Analysis of ISSR data revealed considerable intraspecific variability within E. sessilifolium, and the distribution of variability, with agamospermous populations showing variability from one another, added further evidence for multiple origins of agamospermous populations. The results in conjunction with distributional evidence that the sexual diploid populations of E. sessilifolium are geographically restricted and uncommon suggest that monitoring of populations might be warranted to evaluate whether measures are needed to enhance their continued survival.