Several studies of hybridization and hybrid speciation have focused on P. clevelandii (Straw 1955, 1956a,b; Wolfe and Elisens 1993, 1994, 1995; Wolfe et al. 1998a, b). P. clevelandii was proposed as diploid hybrid species with the progenitors P. centranthifolius and P. spectabilis based on morphological and artificial crossing evidence (Straw 1955, 1956a,b). Wolfe and Elisens (1993, 1994, 1995) used allozyme, chloroplast DNA and nuclear ribosomal DNA data to demonstrate that hybrid speciation was not supported, but that introgression of markers from P. centranthifolius to P. spectabilis, P. clevelandii, and P. grinnellii has occurred. More recently, Wolfe et al. (1998a, b) used hypervariable ISSR markers to demonstrate that P. clevelandii is, in fact, a diploid hybrid derivative species.
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Wolfe, A. D. and W. J. Elisens. 1994. Nuclear ribosomal DNA restriciton-site variation in Penstemon section Peltanthera (Scrophulariaceae): An evaluation of diploid hybrid speciation and evidence for introgression. American Journal of Botany 81: 1627-1635.
Wolfe, A. D. and W. J. Elisens. 1995. Evidence of chloroplast capture and pollen-mediated gene flow in Penstemon sect. Peltanthera (Scrophulariaceae). Systematic Botany 20: 395-412.
Wolfe, A. D., Q.-Y. Xiang, and S. R. Kephart. 1998. Assessing hybridization in natural populations of Penstemon (Scrophulariaceae) using hypervariable inter simple sequence repeat markers Molecular Ecology 7: 1107-1125.
Wolfe, A. D., Q.-Y. Xiang, and S. R. Kephart. 1998. Diploid hybrid speciation in Penstemon (Scrophulariaceae). Proceedings of the National Academy of Science, USA 95: 5112-5115.
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