Cyto-Taxonomic Studies in Oryzopsis

Abstract

1. The Old World species Oryzopsis miliacea, O. virescens, O. paradoxa, O. coerulescens, and O. holciformis-which constitute most of the section Piptatherum-form a coherent group marked by specialization in characters which clearly distinguish them from the genus Stipa. On these characters O. virescens is most representative of the genus Oryzopsis, while O. holciformis shows the greatest specialization or departure from the generic characters of Oryzopsis and still greater divergence from the generic characters of Stipa. O. miliacea shows evidence of reduction from a specialized condition. O. holciformis and O. miliacea were found to have 2n = 24 chromosomes. Their idiograms are similar and both differ notably from the idiograms of other species of Oryzopsis studied. Previous counts of 2n = 24 have been reported for O. miliacea and O. virescens. It is concluded that the basic number for the section Piptatherum is x = 12 (or 6). This has been previously suggested as the basic number for the genus Oryzopsis and as the primary basic number for the genus Stipa. 2. The North American species O. micrantha, O. pungens, O. exigua, O. canadensis, and O. kingii-which constitute most of the section Euoryzopsis-form a group marked by specialization in characters which merge with the genus Stipa. O. micrantha is somewhat detached from the other species by its resemblance to the section Piptatherum on some characters, while the highly specialized O. kingii departs farthest from the generic characters of Oryzopsis and intergrades completely with Stipa. Chromosome counts of 2n = 22 were obtained for O. micrantha, O. pungens, and O. kingii. Their idiograms are similar, and all have longer chromosomes than in the Old World species. It is concluded that the basic chromosome number for the section Euoryzopsis is x = 11. A secondary basic number of x = 11 has been reported previously for Stipa. O. 3. The North American species racemosa and O. asperifolia combine morphological characters from the two lines in Oryzopsis with x = 12 (or 6) and x = 11 chromosomes. In size of plants and floral parts, they are larger than the species of either line. Both were found to have 2n = 46 chromosomes. The idiograms show that in range of chromosome lengths O. racemosa includes most of the range for the x = 12 (or 6) line plus most of the range for the x = 11 line. It is concluded that they are allopolyploids between those two lines. O. racemosa is referred to the section Piptatherum. O. asperifolia is the type of the section Euoryzopsis. Another species, O. hendersoni, may represent another such allopolyploid, as judged from morphological characters. It is referred to Euoryzopsis. 4. The North American species O. hymenoides, which is the only one in the section Eriocoma, resembles the genus Stipa in some features and O. virescens of the section Piptatherum in other features. A count of 2n = 48 chromosomes has been previously reported for O. hymenoides. Its idiogram shows a range in chromosome lengths which greatly exceeds that for the x = 12 (or 6) line in Oryzopsis. It may be an allopolyploid between that line and the line in Stipa with a similar basic number or possibly a polyploid from the Stipa line alone. It hybridizes naturally with a number of species of that genus to produce sterile hybrids. 5. A variety of O. hymenoides with a contracted panicle is described. 6. O. webberi is transferred to Stipa on the basis of its close similarity to S. pinetorum. Both species have 2n = 32 chromosomes. 7. O. bloomeri and O. caduca are excluded from Oryzopsis on the basis of proof published elsewhere showing that they are sterile hybrids between O. hymenoides on the one hand and S. occidentalis and S. viridula, respectively, on the other hand. 8. It is concluded that the section Piptatherum and the genus Stipa in part represent divergent lines of specialization on a common primary basic number of x = 6 chromosomes and that these lines may have reticulated to form the polyploid section Eriocoma. The section Euoryzopsis represents mainly the diploid species of a line that has tentatively become isolated from the x = 12 (or 6) lines by a change to a secondary basic number of x = 11. The latter line in turn has reticulated with the Piptatherum line to form allopolyploid species of Oryzopsis and with the Stipa line to contribute to the modified polyploid series reported for that genus. 9. It is suggested that the secondary basic number of x = 11 probably originated through a change from 2n = 12 to 2n = 10 within the Stipa line, with subsequent allopolyploidy between 2n = 10 individuals and the original 2n = 12 lines in both Stipa and Piptatherum.