Systematics of the Opuntia humifusa complex.

摘要

The systematics of the Opuntia humifusa complex was analyzed using a combination of traditional morphology, SEM characteristics, guard cells, pollen grains, chromosome counts, chlorophyll levels, and flavonoid patterns. In general, the conclusions are consistent with the taxonomy of Benson (1982). Opuntia stricta is the most distinctive species in the complex, differing from other members in vegetative morphology, flavonoid profiles, seed, pollen, and guard cell characteristics, although the two varieties of O. stricta (vars. stricta and dillenii) are only weakly delimited. Opuntia humifusa separates into three varieties: vars. humifusa, ammophila, and austrina. Opuntia pusilla is morphologically defined by its prostrate growth, small terete pads, and acicular barbed spines, although many populations are intermediate between O. pusilla and O. humifusa. However, Opuntia pusilla is less distinctive in terms of its flavonoids and micromorphological characters. In several respects, many populations of O. pusilla are more similar to geographically neighboring O. humifusa populations than to more distant populations of O. pusilla. A partial distinction between these two species is found in chromosome number. O. pusilla counts were diploid, whereas counts for O. humifusa var. humifusa were tetraploid. However, guard cell and pollen grain data imply O. humifusa vars. austrina and ammophila may be diploid, and northern populations of O. pusilla may be tetraploid. Northern populations of O. humifusa (from the Virginia mountains westward) have high similarity to O. macrorhiza, particularly in midwestern populations. Further work is necessary to determine whether these northern populations should be included in O. macrorhiza.; The patterns of variation found in the Opuntia humifusa complex relate to the reproductive and dispersal characteristics of the species. All fields of evidence exhibited clinal variation along geographical gradients. These patterns may also indicate postglacial migration routes of the eastern Opuntia. There appears to have been two separate migration routes: one from the south, moving north along the coast; the second from the west, moving eastward, perhaps during the hypsithermal.