Origins, distribution, and local co-occurrence of polyploid cytotypes in Solidago altissima (Asteraceae)

There is growing realization that intraspecific polyploidy in plants has important implications for the evolution of plant diversity and for plant and plant-herbivore community dynamics. Nonetheless, the rate at which polyploid cytotypes arise and their fate in local mixed-cytotype populations are not well understood. Although within- and especially among-population cytotype variation has been documented in many plants, particularly at the largest (continental) spatial scales, local and regional distributions of cytotypes have been well characterized only for a handful of species. Furthermore, few if any studies have addressed both local and regional patterns in the same plant species with sufficient power to detect cytotype variation on both scales. We assessed the cooccurrence and frequency distributions of diploid, tetraploid, and hexaploid cytotypes of Solidago altissima (Asteraceae) at local and regional spatial scales, sampling across a zone of presumed ploidy-form overlap in the midwestern United States. Determining ploidy by flow cytometry, we found many local populations containing two or three cytotypes, with the relative frequencies of cytotypes varying dramatically even among neighboring populations. AFLP marker data suggest that polyploid cytotypes of S. altissima likely have multiple origins from different diploid lineages. Our results are consistent with recent perspectives on polyploidization as an evolutionarily dynamic phenomenon with frequent multiple origins of higher-ploidy cytotypes.