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First published online July 6, 2009; doi:10.3732/ajb.0800357 American Journal of Botany 96: 1519-1531 (2009) © 2009 Botanical Society of America, Inc.

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Tangled trios?: Characterizing a hybrid zone in Castilleja (Orobanchaceae)¹

² Department of Biology, Northern Arizona University, S. Beaver St. Box 5640 Flagstaff, Arizona 86011 USA ³ U.S. Forest Service, Pacific NW Research Station, 3200 SW Jefferson Way, Corvallis, Oregon 97331 USA

ABSTRACT

Hybridization and polyploidization are exceedingly important processes because both influence the ecological envelope and evolutionary trajectory of land plants. These processes are frequently invoked for Castilleja (Indian paintbrushes) as contributors to morphological and genetic novelty and as complicating factors in species delimitations. Here, we provide a detailed analysis of morphological and genetic evidence for hybridization in a well-characterized hybrid swarm involving three broadly sympatric species (C. miniata, C. rhexiifolia, C. sulphurea) in western Colorado. Field-classified hybrids are present at high frequencies at these sites and show morphological intermediacy to and segregate for chloroplast DNA haplotypes with C. rhexiifolia and C. sulphurea. Contrarily, DNA content and AFLP variation show that field-classified hybrids are not recent hybrids but a distinctive fourth taxon. Actual hybrids (plants showing admixture 10% for two genotypic groups) comprised 13% of our sample, with most admixture involving C. rhexiifolia, C. sulphurea, and the unknown taxon. The identity of the field-classified "hybrids" remains unknown; they either represent a stabilized hybrid species or a species with uncharacteristically high diversity for color alleles. This study highlights the importance of examining concordance and discordance between morphology, cytology, and genetic criteria to understand the complex evolutionary history of diverse groups such as Castilleja.

Key Words: admixture • Castilleja • hybridization • introgression • Orobanchaceae • polyploidy • population structure

Received for publication 21 October 2008. Accepted for publication 16 March 2009.

FOOTNOTES

¹ The authors thank B. Husband and P. Kron at the University of Guelph for help with flow cytometry work, B. Husband for providing laboratory space, and B. Roy for great discussions of this work along the way. They also thank J. Jernstedt, D. Tank, and an anonymous reviewer for helpful comments on an earlier draft of the manuscript. This research was supported by a National Institutes of Health graduate training grant fellowship (NIH GM007413-30), an IGERT graduate training fellowship (NSF DGE-0504627), a National Science Foundation Dissertation Improvement Grant (to B. Roy, NSF DGE-25821), and a National Science Foundation predoctoral fellowship awarded to E.I.H-G.

⁴ Author for correspondence (e-mail: erikahersch{at}gmail.com)

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