Dna Content of Seven Species of Astereae and Its Significance to Theories of Chromosome Evolution in the Tribe

Relative amounts of nuclear DNA were determined in root tip cells of seven species of Astereae: Aster hydrophilus Greene, A. oblongifolius Nutt., A. riparius H.B.K., Machaeranthera boltoniae (Greene) Turner and Home, M. brevilingulata (Sch‐Bip.) Turner and Home, M. parviflora Gray, and M. tenuis (S. Wats.) Turner and Home. The results show that A. hydrophilus and M. brevilingulata , with a chromosome number of n = 9, have less nuclear DNA than other closely related species which are either n = 4 or n = 5. Cytological analyses of meiosis in the intergeneric hybrid M. parviflora X A. hydrophilus showed cells with two or more small chromosomes of the latter species pairing with single large chromosomes of the former. Pachytene cells of the hybrids M. parviflora X A. hydrophilus, M. parviflora X A. riparius , and M. boltoniae X M. tenuis showed some unpaired chromosome segments. The significance of these results to chromosome evolution in the tribe Astereae is discussed.