Abstract
Ploidy has been well studied and used extensively in the genus Opuntia to determine species boundaries, detect evidence of hybridization, and infer evolutionary patterns. We carried out chromosome counts for all members of the Humifusa clade to ascertain whether geographic patterns are associated with differences in ploidy. We then related chromosomal data to observed morphological variability, polyploid formation, and consequently the evolutionary history of the clade. We counted chromosomes of 277 individuals from throughout the ranges of taxa included within the Humifusa clade, with emphasis placed on the widely distributed species, Opuntia humifusa (Raf.) Raf., 1820 s.l. and Opuntia macrorhiza Engelm., 1850 s.l. We also compiled previous counts made for species in the clade along with our new counts to plot geographic distributions of the polyploid and diploid taxa. A phylogeny using nuclear ribosomal ITS sequence data was reconstructed to determine whether ploidal variation is consistent with cladogenesis. We discovered that diploids of the Humifusa clade are restricted to the southeastern United States (U.S.), eastern Texas, and southeastern New Mexico. Polyploid members of the clade, however, are much more widely distributed, occurring as far north as the upper midwestern U.S. (e.g., Michigan, Minnesota, Wisconsin). Morphological differentiation, although sometimes cryptic, is commonly observed among diploid and polyploid cytotypes, and such morphological distinctions may be useful in diagnosing possible cryptic species. Certain polyploid populations of Opuntia humifusa s.l. and Opuntia macrorhiza s.l., however, exhibit introgressive morphological characters, complicating species delineations. Phylogenetically, the Humifusa clade forms two subclades that are distributed, respectively, in the southeastern U.S. (including all southeastern U.S. diploids, polyploid Opuntia abjecta Small, 1923, and polyploid Opuntia pusilla (Haw.) Haw., 1812) and the southwestern U.S. (including all southwestern U.S. diploids and polyploids). In addition, tetraploid Opuntia humifusa s.l., which occurs primarily in the eastern U.S., is resolved in the southwestern diploid clade instead of with the southeastern diploid clade that includes diploid Opuntia humifusa s.l. Our results not only provide evidence for the polyphyletic nature of Opuntia humifusa and Opuntia macrorhiza, suggesting that each of these represents more than one species, but also demonstrate the high frequency of polyploidy in the Humifusa clade and the major role that genome duplication has played in the diversification of this lineage of Opuntia s.s. Our data also suggest that the southeastern and southwestern U.S. may represent glacial refugia for diploid members of this clade and that the clade as a whole should be considered a mature polyploid species complex. Widespread polyploids are likely derivatives of secondary contact among southeastern and southwestern diploid taxa as a result of the expansion and contraction of suitable habitat during the Pleistocene following glacial and interglacial events.
Highlights
Ploidy has a long tradition of utility for illuminating species boundaries, hybrid zones, and interspecific relationships among plants (e.g., Stace 2000)
Tetraploid members of O. humifusa s.l. and O. macrorhiza s.l. are much more widely distributed throughout the U.S than are their diploid relatives (Fig. 2)
Tetraploids of O. humifusa s.l. are found from Massachusetts south to the southeastern U.S where they abut the distribution of diploid taxa and throughout the eastern and midwestern U.S Tetraploid O. macrorhiza s.l. is distributed throughout parts of the Great Plains through the midwestern U.S, most of the southwestern U.S, parts of the Rocky Mountains, and the upper Sierra Madre Occidental in Sonora, Mexico (Fig. 2)
Summary
Ploidy has a long tradition of utility for illuminating species boundaries, hybrid zones, and interspecific relationships among plants (e.g., Stace 2000). Researchers have frequently used cytological data to help understand species evolution and delimitations in the nopales or prickly pear cacti, i.e., the genus Opuntia (Pinkava and McLeod 1971, Pinkava et al 1973, 1977, 1985, Weedin and Powell 1978, Pinkava and Parfitt 1982, Weedin et al 1989, Pinkava et al 1992, Powell and Weedin 2001, 2004). Opuntioideae (Opuntia s.l., as previously recognized; Benson 1982) is known to have the highest number of polyploids in Cactaceae (Cota and Philbrick 1994, Pinkava 2002), and Opuntia s.s. is well known for interspecific hybridization (e.g., Grant and Grant 1982, Griffith 2003) and subsequent genome duplication (Pinkava 2002, L.C. Majure (LCM), R.
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