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. THE SAGEBRUSHEs-section Tridentatae Rydb. of Artemisia L. (Anthemideae, Compositae)are perhaps the most important shrubs in midlatitude (3O? -50) western North Arnerica. This shrub group dominates tens of millions of hectares, is present on millions more hectares, provides browse and habitat for many kinds of 1 Received for publication 14 April 1980; revision accepted 10 December 1980. This work was aided by Federal funds for wildlife restoration through Pittman-Robertson W-82-R Project (cooperators: Intermountain Forest and Range Experiment Station and Utah State Division of Wildlife Resources). The Snow Field Station is cooperatively maintained by the above agencies, Utah State University, and Snow College. We thank A. Perry Plummer, Paul J. Edgerton, and Elizabeth McClintock for providing plant material. Alma H. Winward gave us precise locations for his and Tisdale's chromosome counts (Fig. 1). The use of trade, firm, or corporation names in this publication is for the information and convenience of the reader. Such does not constitute an official endorsement or approval by the U.S. Department of Agriculture of any product or service to the exclusion of others which may be suitable. 2 Located at the Intermountain Station's Shrub Scienc,s Laboratory, 735 North 500 East, Provo, Utah 84601; and adjunct associate professor of botany, Brigham Young University. 3Present address is Rice Researchers Inc., Glenn, California 95943. 4Present address is Department of Biology, Wayne State University, Detroit, Michigan 48202. wildlife and livestock, and gives much of the countryside its characteristic appearance. Members of the group grow in many soil types on arid planes, in canyons and valleys, and on foothills and mountain slopes from 100 to 3,400 m in elevation (Fig. 1, 2). Tridentatae grow with many grasses, forbs, other shrubs, and trees. The plants have been an important vegetative component of the land they occupy for several thousand years (Tidwell, Rushforth, and Simper, 1972; Van Devender, 1977). Even so, they are relative newcomers and have become important in the flora only since the late Tertiary or early Quaternary periods (Axelrod, 1950). Alternating moist and dry climates during the Pleistocene Epoch favored repeated evolutionary cycles that provided evolutionary opportunity for the production of present-day Tridentatae (Stebbins, 1974; Cronquist, 1978; McArthur and Plummer, 1978). Since the settlement of western North America by Caucasians, the sagebrushes have increased in numbers in local areas (Young, Eckert, and Evans, 1979). How great the increase has been is subject to continuous debate (Cottam, 1961; Vale, 1975; Young et al., 1979). Management of sagebrush lands has often entailed attempts to eradicate completelv or