New Data on Chromosome Numbers in Aster Section Heterophylli (Asteraceae) and Their Phylogenetic Implications

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Abstract
Reports suggesting that the basic chromosome number for Aster [A. sagittifolius, A. urophyllus, A. shortii, A. lowrieanus, A. azureus, A. cordifolius, A. drummondii, A. laevis, A. maccallae and A. ciliolatus] section Heterophylli is x = 9 were cited to support the hypothesis that x = 9 is the ancestral basic number for the whole genus. Chromosome counts for 35 populations in 10 spp., 9 of which are traditionally placed in section Heterophylli, were given. The chromosome numbers in 33 populations present a polyploid series of n = 8, 16, 24 and 32. Reinterpretation of photomicrographs by Avers lends additional support to the finding that the basic number for the section is x = 8 rather than x = 9. If counts for the 3 spp. not yet re-examined agree with these results, the balance between species of the x = 8 and x = 9 groups will be significantly shifted, and phylogenetic considerations based on the number of species belonging in each of the 3 modes (x = 5, 8 and 9) in the genus must be reassessed. Inasmuch as all Old World species of Aster exhibit x = 9, this group remains the largest on a worldwide basis, but with the inclusion of section Heterophylli, the x = 8 group comprises the largest number of species in North America, the generally acknowledged center of origin of the genus. Evolution apparently proceeded from the low basic chromosome number of x = 4 or 5 rather than the high one of x = 9.