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reports: Zachariah Gompert, James A. Fordyce, Matthew L. Forister, Arthur M. Shapiro, and Chris C. Nice Homoploid Hybrid Speciation in an Extreme Habitat Science 2006; 314: 1923-1925 [Abstract] [Full text] [PDF]

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Response to Lukhtanov

Zachariah Gompert, James A. Fordyce, Matthew L. Forister, Arthur M. Shapiro, Chris C. Nice   (13 March 2007)

Homoploid Hybrid Speciation

Vladimir Lukhtanov   (13 March 2007)

Response to Lukhtanov 13 March 2007
Zachariah Gompert Department of Biology, Population and Conservation Biology Program, Texas State University, James A. Fordyce, Matthew L. Forister, Arthur M. Shapiro, Chris C. Nice Respond to this E-Letter: Re: Response to Lukhtanov	Lukhtanov correctly states that we do not provide evidence demonstrating a causal link between hybridization and the characteristics that contribute to prezygotic reproductive isolation between the hybrid Lycaeides species and its parental species, L. idas and L. melissa. However, the demonstration of hybrid speciation does not require such a link. Hybrid speciation is defined as hybridization between two species that gives rise to a new lineage that is fertile and true breeding, but reproductively isolated from its parental species (1, 2). Reproductive isolation can be nearly instantaneous, as is generally the case when it is a consequence of chromosomal rearrangements (2). In some instances, ecological isolation can arise very rapidly as well (3–5). However, reproductive isolation can also evolve as a consequence of natural selection. Indeed, Stebbins (6) and Anderson (7) envisioned a clear role for natural selection acting on recombinant genotypes during the evolution of reproductive isolation. Thus, Lukhtanov’s assertion that hybrid speciation specifically describes a scenario in which reproductive isolation is a direct consequence of hybridization is erroneous. Hybrid speciation simply requires that the incipient hybrid species becomes reproductively isolated from the parental species, which has been demonstrated for the alpine Lycaeides in our Report. Lukhtanov provides an alternative scenario to explain the molecular patterns documented in our Report. He proposes that a single ancestral species that consisted of genetically differentiated populations, one corresponding to the ancestor of L. idas and the other corresponding to the ancestor of L. melissa, produced hybrids in the Sierra Nevada; following this hybridization event, all three of these lineages became reproductively isolated species. However, genetically differentiated populations that occur in parapatry or sympatry (a requirement for hybridization), yet maintain their distinctiveness and persist to the present, would be considered species by most definitions (2, 8–10). Furthermore, an implicit assumption of this scenario is that L. idas and L. melissa are sister species. However, the relationship between these two species is unclear. In fact, there is some evidence to the contrary. Lycaeides melissa may be more closely related to the old world species L. argyrognomon than to L. idas (11). We agree with Lukhtanov that an understanding of the traits under selection following the hybridization event is desirable. However, such evidence is not required to conclude that the alpine Lycaeides represents a stable species of hybrid origin with a genomes that is a mosaic of alleles from the parent species and, consistent with theoretical predictions, occupies a different, and relatively extreme, habitat compared with either of the parental species. Zachariah Gompert,1 James A. Fordyce,2 Matthew L. Forister,3 Arthur M. Shapiro,4 Chris C. Nice1 1Department of Biology, Population and Conservation Biology Program, Texas State University, San Marcos, TX 78666, USA. 2Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, TN 37996, USA. 3Department of Natural Resources and Environmental Science, University of Nevada, Reno, NV 89512, USA. 4Section of Evolution and Ecology, University of California, Davis, CA 95616, USA. References 1. V. Grant, Plant Speciation (Columbia Univ. Press, New York, 1981). 2. J. A. Coyne, H. A. Orr, Speciation (Sinauer Associates, Sunderland, MA, 2004). 3. J. Mavarez et al., Nature 441, 868 (2006). 4. L. H. Rieseberg, Annu. Rev. Ecol. Syst. 28, 359 (1997). 5. B. L. Gross, L. H. Rieseberg, J. Hered. 96, 241 (2005). 6. G. L. Stebbins, Proc. Am. Philos. Soc. 103, 231 (1959). 7. E. Anderson, Introgressive Hybridization (Wiley, New York, 1949). 8. J. Hey, Trends Ecol. Evol. 16, 326 (2001). 9. R. G. Harrison, in Endless Forms: Species and Speciation, D. J. Howard, S. H. Berlocher, Eds. (Oxford Univ. Press, Oxford, UK, 1998), pp. 19–31. 10. K. de Queiroz, in Endless Forms: Species and Speciation, D. J. Howard, S. H. Berlocher, Eds. (Oxford Univ. Press, Oxford, UK, 1998), pp. 57–78. 11. C. C. Nice, N. Anthony, G. Gelembiuk, D. Raterman, R. ffrench- Constant, Mol. Ecol. 14, 1741 (2005).
Homoploid Hybrid Speciation 13 March 2007
Vladimir Lukhtanov Department of Entomology, St. Petersburg State University Respond to this E-Letter: Re: Homoploid Hybrid Speciation	In their Report “Homoploid hybrid speciation in an extreme habitat” (22 Dec. 2006, p. 1923), Z. Gompert and colleagues present strong evidence for ancient hybridization in the genus Lycaeides and analyze an interesting case of ecological divergence in an alpine-adapted lineage of these butterflies. The authors argue that this alpine species is the product of hybrid homoploid speciation. However, this interpretation is disputable. Hybrid speciation assumes an essential contribution of hybridization to the establishment of reproductive isolation between the new emerged lineage and both parental species. This isolation may be (i) postzygotic (“classic” mode of homoploid hybrid speciation) or (ii) prezygotic (“unorthodox” mode of homoploid hybrid speciation) (1). The plausibility of both modes is supported by empirical data: Hybridization in itself triggered the formation of postzygotic isolation between sunflowers (2) and prezygotic isolation between Heliconius butterflies (3). In the case of Lycaeides, there is no clear evidence for postzygotic or prezygotic isolation promoted by hybridization. The actual postzygotic isolation between L. idas and the “hybrid” alpine species, if any, seems to be weak, because these taxa produce viable offspring in greenhouse conditions (4). At the time of ancient hybridization, the hybrid offsprings could be expected to have even greater fitness, i.e., to be completely viable and fertile. The prezygotic isolation is present in the form of differences in oviposition behavior and food plants. However, no data exist to support the idea that evolution of these characteristics was related to the hybridization event. An alternative scenario explaining the genetic patterns found in Lycaeides can be suggested. Half a million years ago, the progenitors of L. melissa and L. idas represented two genetically diverged but conspecific lineages and produced mixed populations in Sierra Nevada. Only later, the mixed populations and both parental lineages developed reproductive isolation either by classic allopatric speciation (1) or by sympatric speciation through the food plant shift (5), but without any contribution of hybridization. The data presented do not conflict with the model of homoploid hybrid speciation (6), but additional evidence for the creative role of hybridization in evolution of reproductive isolation would be very desirable. Vladimir Lukhtanov Department of Entomology, St. Petersburg State University, Universitetskaya nab. 7/9, St. Petersburg 199034, Russia. References 1. J. A. Coyne, H. A. Orr, Speciation (Sinauer Associates, Sunderland, MA, 2004). 2. L. H. Rieseberg, C. Van Fossen, A. M. Desrochers, Nature 375, 313 (1995). 3. J. Mavarez et al., Nature 441, 868 (2006). 4. See Gompert et al.’s Supporting Online Material (www.sciencemag.org/cgi/content/full/1135875/DC1). 5. I. Emelianov, F. Marec, J. Mallet, Proc. R. Soc. London B 271, 97 (2003). 6. C. A. Buerkle, R. J. Morris, M. A. Asmussen, L. H. Rieseberg, Heredity 84, 441 (2000).