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(American Journal of Botany. 2009;96:853-861.) doi: 10.3732/ajb.0800153 © 2009 Botanical Society of America, Inc.

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Investigating species boundaries in the Giliopsis group of Ipomopsis (Polemoniaceae): Strong discordance among molecular and morphological markers¹

² Institute for Evolution and Biodiversity, Plant Evolutionary Ecology, University of Münster, Hüfferstrasse 1, Münster 48149, Germany ³ Department of Biology, Indiana University, 1001 E. Third Street, Bloomington, Indiana 47405 USA

ABSTRACT

As a first step in elucidating mechanisms of speciation in the Giliopsis group of Ipomopsis (Polemoniaceae), we examined patterns of morphological and genetic differentiation and crossability. This group comprises three species that diverged very recently: two perennials, I. guttata and I. tenuifolia, and one annual, I. effusa. Analysis of phenotypic variation established that the three species are distinct for floral characters, and this differentiation is maintained in a locality containing both perennial species. Next, we assessed the genealogical relationships with AFLPs. All sampled individuals of I. effusa clustered together, a result in accord with its genetic isolation. The perennials, which retain interfertility, were not resolved as sister taxa. Rather, individuals sampled from the single I. guttata population that is sympatric with I. tenuifolia were genetically more similar to I. tenuifolia samples than they were to conspecifics. This pattern may be due to substantial introgression of I. tenuifolia genomic regions that do not contribute to floral phenotype in I. guttata. Our result adds to mounting evidence that plant species, as defined by morphological characters, are often not genomically cohesive. Taken together, our data warrant caution in delimiting species with genetic markers alone, and, importantly, suggest that selection on species-diagnostic morphological characters can be sufficiently strong to counteract extensive gene flow.

Key Words: AFLP • Giliopsis group • introgression • Ipomopsis • Polemoniaceae • selection • species • taxonomy

Received for publication 30 April 2008. Accepted for publication 9 December 2008.

FOOTNOTES

¹ The authors thank J. M. Porter for suggesting the Giliopsis group as an interesting study system and for providing DNA samples, D. Bradfute and D. Vickers for their ability to see red (i.e., finding I. tenuifolia in the field), and G. Gastony, L. Rieseberg, and two anonymous reviewers for critiques of earlier versions of this manuscript. This work was funded in part by an NSF-IGERT fellowship, the Indiana Academy of Sciences, and a Sigma XI Grant-in-Aid of Research to T.E.W.

⁴ Author for correspondence (e-mail: trowood{at}indiana.edu)

⁵ Current address: Department of Biology, The University of Memphis, 3700 Walker Avenue, Memphis, TN 38152 USA

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