Chromosomal Studies of Subgenus Tridentatae of Artemisia: Evidence for Autopolyploidy

The sagebrushes (subgenus Tridentatae of Artemisia—new combination presented in the text) are western North America's most widespread and populous shrub group. Chromosome counts from 120 populations confirm the base chromosome number at x = 9 with numerous 2n = 2x = 18 diploids and 2n = 4x = 36 tetraploids. Few higher polyploids are known, and aneuploidy and supernumerary chromosomes are rare. All 11 Tridentatae species are now known cytologically. All but the narrowly endemic A. argillosa are known from at least three locations: A. arbuscula (2n = 18, 36), A. bigelovii (2n = 18, 36), A. cana (2n = 18, 36), A. longiloba (2n = 18, 36), A. nova (2n = 18, 36), A. pygmaea (2n = 18, 36), A. rigida (2n = 18, 36), A. rothrockii (2n = 18, 36, 54, ca. 72), A. tridentata (2n = 18, 36, 54), and A. tripartita (2n = 18, 36). The chromosome number of A. argillosa, reported here for the first time, is 2n = 36. Chromosome numbers of eight subspecies also have been determined. The subgenus is characterized by autopolyploidy as indicated by morphologically indistinguishable 2x and 4x plants, a few mixed ploidy populations, consistent formation of IVs in 4x PMCs, a relatively uniform 2x karyotype, and a 4x karyotype, which is approximately twice the 2x one. Karyotypic differences, if they exist at all, are on a populational level rather than a systematic taxonomic level. The Tridentatae have apparently rapidly differentiated in situ in North America under the stimulus of recurring aridic cycles since late Tertiary or early Quaternary. They likely derive from more primitive circumboreal stock originating from the Eurasian homeland of Artemisia. The differentiation of myriad forms of Tridentatae was seemingly achieved through genic rather than genomic means. Karyotypic analysis supports a position within Tridentatae of A. rigida, A. bigelovii, and A. pygmaea.