Opuntia fragilis, O. humifusa s.l., and O. macrorhiza s.l. are widely distributed prickly pear taxa that have been studied cytologically mostly in western North America, but upper Midwestern United States populations, which form most of the northeastern-most extent of their distributions, have not been analyzed previously. The wide-ranging and most northern of all cacti, O. fragilis, is relatively abundant, at least historically, in the upper Midwestern United States but does not occur further southeast than Jo Daviess County, Illinois, while O. humifusa s.l. is found throughout most of the eastern United States. This difference in distribution may indicate that environmental variables impede the establishment of O. fragilis in most of the eastern United States. We present the first chromosome counts of O. fragilis, O. humifusa s.l., and O. macrorhiza s.l. for 40 populations over part of their Midwestern range and employ habitat niche modeling using 19 environmental variables to extrapolate potential reasons why O. fragilis may not be found in the eastern United States.All twelve populations of O. fragilis analyzed were hexaploid, a finding consistent with previous reports, and adding further evidence for its vegetative dispersal from the southwestern United States, where the species likely originated. Populations of O. humifusa s.l. and O. macrorhiza s.l. were all tetraploid, indicating that the northeastern-most range of those taxa is occupied by polyploid derivatives of their southern diploid relatives. Results from niche modeling suggest suitable habitat for O. fragilis in the upper Midwest is strongly predicted for areas with seasonal fluctuations in temperature but annual precipitation homogeneity, variables that may present significant environmental barriers to the existence of the species in the eastern United States.
Vol. 2012 • No. 17
Vol. 2012 • No. 17