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Bidirectional history of hybridization in California wild radish, Raphanus sativus (Brassicaceae), as revealed by chloroplast DNA¹

² Department of Botany and Plant Sciences, University of California, Riverside, California 92521 USA ³ Center for Conservation Biology, University of California, Riverside, California 92521 USA

ABSTRACT

The evolutionary processes that take place in invasive plant populations are not well documented or understood. Interspecific hybridization between cultivated radish (Raphanus sativus) and R. raphanistrum is known to be responsible for the origin of the invasive California wild radish, but little is known about the nature of the hybridization events that produced the hybrid-derived lineage. We analyzed the trnL-rpl32 intergenic region of chloroplast DNA (cpDNA) obtained from 37 cultivated radish individuals from four different cultivars, 53 R. raphanistrum individuals from six European populations and 104 California wild radish individuals from 11 populations covering its entire range throughout the state. We found that cultivated radish and R. raphanistrum shared no cpDNA haplotypes but that they both shared haplotypes with California wild radish, evidence for bidirectional hybridization between the progenitor species in the creation of the California lineage. We also found evidence that multiple cultivars and multiple European source populations contributed to the diversity of cpDNA haplotypes within California. Studies like this will continue to be important for our understanding of the origin of invasive populations and the mechanisms by which they succeed.

Key Words: cpDNA • haplotype • hybridization • intergenic region trnL-rpl32 • multiple introductions • Raphanus

Received for publication 1 April 2008. Accepted for publication 2 September 2008.

FOOTNOTES

¹ The authors thank J. Heraty and O. Bacani for laboratory assistance. The research described in this paper was done by C.E.R. in partial completion of her Ph.D. requirements and has been funded by the U. S. Environmental Protection Agency (EPA) under a Science to Achieve Results (STAR) Graduate Fellowship to C.E.R. and by NRI-CSREES-USDA grant 2003-35320-13559 to N.C.E. EPA has not officially endorsed this publication, and the views expressed herein may not reflect the views of the EPA.

⁴ Author for correspondence (e-mail: ridle026{at}umn.edu); present address: Department of Ecology, Evolution and Behavior, 1987 Upper Buford Circle, University of Minnesota, St. Paul, MN 55108 USA