Hybridization


Penstemon centranthifoliusPenstemon spectabilisPenstemon clevelandiiPenstemon grinnellii
P. centranthifolius
P.spectabilis
P.clevelandii
P. grinnellii


Introduction

Hybridization in Penstemon is a well-known phenomenon. Many Penstemon garden ornamentals have been developed through hybridization. Hybridization occurs at both the diploid and polyploid levels in Penstemon. Our lab is interested in hybridization from the evolutionary biology perspective -- i.e., what is the long term consequence of hybridization in Penstemon? Is it a homogenization of the gene pool, or could there be some novel adaptations resulting from the combination of genes from different species?


Hypotheses of diploid hybrid speciation in Penstemon

The role of hybridization in the formation of species, particularly at the diploid level, is a central issue of plant evolutionary biology. Diploid hybrid speciation can occur when stabilization of hybrid derivatives takes place by apomixis, formation of chromosomal sterility barriers, ecological isolation as a result of colonization of habitats different from those of the parental species, or formation of external reproductive barriers such as adaptation to pollinators different than those of parental species. In plants, clear examples of the mechanisms required to prevent backcrossing and produce genetically isolated derivatives are seldom found.

We recently completed a project that specifically addressed Straw's (1955, 1956a, b) classic hypotheses of hybrid speciation for southern California species of section Peltanthera (Wolfe et al. 1998a, b). Another study is being conducted by Shannon Datwyler. Shannon is examining the purported hybrid origin of two subspecific taxa in subg. Dasanthera (P. davidsonnii var. praeteritus and P. fruticosus var. serratus).


Section Peltanthera hypotheses

Straw's hybrid complex.

(Note: the following paragraphs are excerpts from Wolfe and Elisens (1993, 1994, 1995) and Wolfe et al. (1998a, b).

Penstemon section Peltanthera has two species (P. spectabilis and P. clevelandii) that putatively arose via diploid interspecific hybridization followed by reproductive isolation based on geographical, mechanical, and/or ethological barriers (Keck 1937; Straw, 1955, 1956a, b). Straw (1955, 1956b) hypothesized that P. spectabilis is a stable hybrid derivative species between the progenitors P. centranthifolius (hummingbird-pollinated) and P. grinnellii (carpenter bee-pollinated). Because the wasp-pollination syndrome of the derivative is unlike either progenitor, he proposed that ethological isolation resulted in its stabilization as a species. Straw based his hypothesis on analyses of artificial crosses, morphological variance, and geographic distribution.

Straw (1955) also proposed that P. clevelandii originated from hybridization between P. centranthifolius and P. spectabilis. Penstemon clevelandii is found in habitats different from either putative progenitor and is pollinated by both hummingbirds and solitary bees. Thus, if Straw's hypothesis is correct, this hybrid became isolated through ecological and ethological barriers. The primary evidence for recognition of P. clevelandii as a hybrid species is that it resembles F1's produced from artificial crosses between P. centranthifolius and P. spectabilis, and it is similar in appearance to P. X parishii, a natural hybrid appearing where P. centranthifolius and P. spectabilis occur sympatrically (Keck 1937; Straw 1955). Penstemon clevelandii and P. X parishii have similar corolla shape and color, leaf margins, and leaf vestiture.

Using allozyme and restriction-site data (chloroplast DNA and nuclear ribosomal DNA), Wolfe and Elisens (1993, 1994, 1995) assessed Straw's (1955, 1956a, b) hypotheses of diploid hybrid speciation. The distribution of nuclear and organellar DNA markers among populations of P. spectabilis and P. clevelandii were evaluated to determine whether the observed patterns result from hybrid speciation, or from introgression. Nuclear markers of P. centranthifolius were found in several populations of P. grinnellii, P. spectabilis, and P. clevelandii; whereas chloroplast haplotypes of P. centranthifolius were found in only two populations of P. grinnellii. Wolfe and Elisens (1995) suggested that the patterns of introgression observed for sect. Peltanthera result predominantly from pollen-mediated gene flow, with hummingbirds the most likely vector. A comparison of the phylogenetic trees based on cpDNA vs. nuclear restriction data reveals that at least one chloroplast-capture event has occurred in Penstemon sect. Peltanthera.


Phylogenetic reconstruction of section Peltanthera

Figure caption (Wolfe and Elisens 1995): Comparison of phylogenetic reconstructions based on cpDNA and rDNA. Bootstrap values are provided above nodes. Inconsistencies between the trees are indicated by lines between taxon abbreviations. A) Majority rule consensus tree (of 33 shortest trees, each of 65 steps with CI = 0.892) resulting from analysis of cpDNA restriction-site analysis. Abbreviations refer to cpDNA haplotypes. B) Single most-parsimonious tree derived from analysis of rDNA restriction-site variation (from Wolfe and Elisens 1994); 12 steps with CI = 0.762. Two-letter abbreviations refer to rDNA type.


Using hypervariable ISSR markers, Wolfe et al. (1998a, b) were able to more fully assess Straw's hypotheses of diploid hybrid speciation in Penstemon. Wolfe et al. (1998a, b) reported numerous species-specific markers for all species in the hybrid complex. The pattern of marker distribution among taxa revealed: (1) Penstemon spectabilis is not a hybrid derivative species; (2) P. clevelandii is a diploid hybrid species derived from P. centranthifolius and P. spectabilis; (3) Wolfe and Elisens (1995) hypothesis of pollen-mediated gene flow is supported with these data; and (4) patterns of introgression among the species of the hybrid complex can be correlated with hummingbird migration patterns.


Bibliography and suggested readings

Arnold, M. L., J. L. Hamrick, and B. D. Bennett. 1990. Allozyme variation in Louisian irises: a test for introgression and hybrid speciation. Heredity 65: 297-306.

Bennett, R. W., K. Lodewick, and R. Lodewick. 1987. Penstemon nomenclature. Eugene, OR: American Penstemon Society.

Gallez, G. P. and L. D. Gottlieb. 1982. Genetic evidence for the hybrid origin of the diploid plant Stepahnomeria diegensis. Evolution 36: 1158-1167.

Grant, K. A. and V. Grant. 1968. Hummingbirds and their flowers. New York: Columbia University Press.

Grant, V. 1981. Plant speciation, 2d ed. Columbia University Press, New York.

Grant, V. 1994. Modes and origins of mechanical and ethological isolation in angiosperms. Proceedings of the National Academy of Sciences, U.S.A. 91: 3-10.

Keck, D. D. 1937. Studies in Penstemon V. The section Peltanthera. American Midland Naturalist 18: 790-829.

Lewis, H. and C. Epling. 1959. Delphinium gypsophilum, a diploid species of hybrid origin. Evolution 13: 511-525.

Mitchell, R. J. 1988. Is Penstemon centranthifolius truly hummingbird pollinated? Crossosoma 15: 1-9.

Rice, W. R. and G. W. Salt. 1990. The evolution of reproductive isolation as a correlated character under sympatric conditions: experimental evidence. Evolution 44: 1140-1152.

Rieseberg, L. H. 1991. Homoploid reticulate evolution in Helianthus (Asteraceae): evidence from ribosomal genes. American Journal of Botany 78: 1218-1237.

Rieseberg, L. H. 1995. The role of hybridization in evolution: old wine in new skins. American Journal of Botany 82: 944-953.

Rieseberg, L. H. and N. C. Ellstrand. 1993. What can molecular and morphological markers tell us about hybridization? Critical Reviews in Plant Sciences 12: 213-241.

Rieseberg, L. H., C. R. Linder and G. J. Seiler. 1995. Chromosomal and genic barriers to introgression in Helianthus. Genetics 141: 1163-1171.

Rieseberg, L. H., B. Sinervo, C. R. Linder, M. C. Ungerer and D. M. Arias. 1996. Role of gene interactions in hybrid speciation: evidence from ancient and experimental hybrids. Science 272: 741-745.

Rieseberg, L. H., C. Van Fossen and A. M. Desrochers. 1995. Hybrid speciation accompanied by genomic reorganization in wild sunflowers. Nature 375: 313-316.

Rieseberg, L. H. and J. F. Wendel. 1993. Introgression and its consequences in plants. Hybrid zones and the evolutionary process. Ed. R. Harrison. Oxford, Oxford University Press. 70-109.

Stebbins, G. L. 1959. The role of hybridization in evolution. Proceedings of the American Philosophical Society 103: 231-251.

Stebbins, G. L. 1969. The significance of hybridization for plant taxonomy and evolution. Taxon 18: 26-35.

Straw, R. M. 1955. Hybridization, homogamy, and sympatric speciation. Evolution 9: 441-444.

Straw, R. M. 1956a. Adaptive morphology of the Penstemon flower. Phytomorphology 6: 112-118.

Straw, R. M. 1956b. Floral isolation in Penstemon. American Midland Naturalist 90: 47-53.

Wolfe, A. D. and W. J. Elisens. 1993. Diploid hybrid speciation in Penstemon revisited. American Journal of Botany 80: 1082-1094.

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. and A. Liston. 1998. Contributions of the polymerase chain reaction to plant systematics. In: Soltis, D. E., P. S. Soltis, and J. J. Doyle, Molecular systematics of plants II. Kluwer pp. 43-86..

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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Last updated September 25, 1998.