A Systematic Study of Machaeranthera (Asteraceae) and Related Groups Using Restriction Site Analysis of Chloroplast DNA

Abstract

Chloroplast DNA restriction site analysis was carried out on 55 taxa of tribe Astereae, including 21 species of Machaeranthera and many additional putatively related taxa. Phylogenetic analysis delineates a well supported, monophyletic Machaeranthera alliance within Astereae that includes several segregate genera of Haplopappus and all taxa historically associated with Machaeranthera except sect. Psilactis, which is allied with Aster. The Machaeranthera alliance includes a large proportion of the taxa of tribe Astereae with x = 4, 5, and 6, and contains no taxa with base chromosome numbers greater than x = 6. Chloroplast DNA data indicate five well supported monophyletic groups within the Machaeranthera alliance. One of these groups includes Xanthocephalum, Isocoma, Prionopsis, and Grindelia (all with x = 6), but does not include the x = 6 genera Xylorhiza and Pyrrocoma. A second group includes M. sect. Sideranthus and the monotypic M. sect. Stenoloba. A third group consists of M. sect. Blepharodon and two species not normally associated with the section, M. viscida and M. gypsitherma. Comparison of cpDNA evidence with one rDNA site mutation suggests that chloroplast and nuclear genomes in M. viscida, M. gypsitherma, and M. heterophylla have different origins. A fourth group includes Pyrrocoma and M. sect. Arida. In the fifth group, cpDNA data suggest a close alliance between Oonopsis and M. sects. Machaeranthera and Hesperastrum. Molecular evidence, when considered with other data, is consistent with an hypothesis of aneuploid reduction in base chromosome number from x = 6, or ascending and descending aneuploidy from x = 5 in the Machaeranthera alliance. Machaeranthera Nees consists of mostly taprooted annual or perennial herbs that are distributed in western North America. As treated by Hartman (1976, 1990; table 1), and as here defined, the genus consists of 36 species. Except for three discoid species, all have both ray and disc florets. Ray color is variable within the genus, with both xanthic (yellow) and cyanic (white, blue, pink, purple) rays present. The composition of Machaeranthera, relationships among its sections, and its affinities to other genera have been the subject of much debate (i.e., Cronquist and Keck 1957; Hartman 1976, 1990; Shinners 1950; Turner and Horne 1964; Watson 1977). Large morphological and cytological data bases exist, but synthesis of these data with evidence from hybridization, flavonoid, and micromorphological studies has not resulted in a consensus concerning the systematics of Machaeranthera. Circumscription of Machaeranthera has been extremely difficult because the closest relatives of many species that have been placed in the genus may actually be in several different genera (i.e., Haplopappus Cass., Xylorhiza Nutt., Psilactis A. Gray, and Aster L.). The task of resolving relationships within Machaeranthera is further compounded when one considers that such taxa as Haplopappus and Aster may be even more poorly understood than Machaeranthera. Hall (1928) treated Haplopappus as a large genus of 21 sections, but most or all of the North American sections may eventually be treated as segregate genera or included in other genera. Haplopappus sect. Blepharodon is the group most commonly allied with Machaeranthera. The section has included cyanic-rayed, xanthic-rayed, and eradiate species, all of which Shinners (1950) combined with Machaeranthera. Cronquist and Keck (1957), on the other hand, included only the cyanic-rayed and eradiate species of the section in Machaeranthera (series Originales, table 2). Hartman (1976, 1990) placed the eradiate and cyanic-rayed species in Machaeranthera sect. Blepharodon and the xanthic-rayed species in Machaeranthera sects. Sideranthus, Stenoloba and Havardii (table 1). Xylorhiza has been variously treated as a section of Aster, a section of Machaeranthera, or as a distinct genus. Cronquist and Keck (1957) in-