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First published online August 19, 2009; doi:10.3732/ajb.0800337 American Journal of Botany 96: 1646-1655 (2009) © 2009 Botanical Society of America, Inc.

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The genomic organization of Ty3/gypsy-like retrotransposons in Helianthus (Asteraceae) homoploid hybrid species¹

² Miami University, Department of Botany, 316 Pearson Hall, Oxford, Ohio 45056 USA ³ Kansas State University, Division of Biology, 426 Ackert Hall, Manhattan, Kansas 66506 USA

ABSTRACT

The origin of new diploid, or homoploid, hybrid species is associated with rapid genomic restructuring in the hybrid neospecies. This mode of speciation has been best characterized in wild sunflower species in the genus Helianthus, where three homoploid hybrid species (H. anomalus, H. deserticola, and H. paradoxus) have independently arisen via ancient hybridization events between the same two parental species (H. annuus and H. petiolaris). Most previous work examining genomic restructuring in these sunflower hybrid species has focused on chromosomal rearrangements. However, the origin of all three homoploid hybrid sunflower species also is associated with massive proliferation events of Ty3/gypsy-like retrotransposons in the hybrid species’ genomes. We compared the genomic organization of these elements in the parent species and two of the homoploid hybrid species using fluorescence in situ hybridization (FISH). We found a significant expansion of Ty3/gypsy-like retrotransposons confined to the pericentromeric regions of two hybrid sunflower species, H. deserticola and H. paradoxus. In contrast, we detected no significant increase in the frequency or extent of dispersed retrotransposon populations in the hybrid species within the resolution limits of our assay. We discuss the potential role that transposable element proliferation and localization plays in the evolution of homoploid hybrid species.

Key Words: Asteraceae • fluorescence in situ hybridization • homoploid hybrid species • genome evolution • Helianthus • hybridization • sunflower • transposable elements

Received for publication 3 October 2008. Accepted for publication 23 April 2009.

FOOTNOTES

¹ The authors thank J. Jiang (University of Wisconsin, Madison) and P. Heslop-Harrison (University of Leicester, Leicester, UK) for valuable advice with optimizing FISH. M.H.H. Stevens (Miami University, Oxford, OH) and L. Murray (Kansas State University) advised us on our data and statistical analyses. M. Duley and R. Edelmann of the Electron Microscopy Facility (Miami University) assisted with microscopy and image analysis. Helianthus paradoxus seeds were kindly provided by B. Blackman, and collected under U.S. Fish and Wildlife Permit TE093663-1 and Science Studies Permit for Endangered Plant Species #129. They also thank two anonymous reviewers for helpful comments. This work was supported by a Miami University Department of Botany Academic Challenge grant to S.E.S. and a Miami University Committee on Faculty Research grant to R.C.M.

⁴ Present address: Department of Genetics, University of Georgia, Athens, GA

⁵ Author for correspondence (e-mail: moorerc{at}muohio.edu), phone: (513) 529-4278

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