MBE Advance Access published online on April 15, 2008
Molecular Biology and Evolution, doi:10.1093/molbev/msn092
Research Article |
Polyploid speciation did not confer instant reproductive isolation in Capsella (Brassicaceae)
* Evolutionary Functional Genomics, Department of Evolution, Genomics and Systematics, Uppsala University, Norbyvägen 18 D, SE-752 36 Uppsala, Sweden
South China Botanical Garden, Chinese Academy of Sciences, Leyiju, Guangzhou, Guangdong 510650, China
Department of Cell and Organism Biology (Genetics), Lund University, Sölvegatan 29, SE-223 62 Lund, Sweden
Corresponding author: Tanja Slotte E-mail: tanjas{at}gmail.com Phone: +1-416-736-2100-44034
Received for publication December 11, 2007. Revision received March 12, 2008. Accepted for publication April 7, 2008.
Polyploid formation is a major mode of sympatric speciation in flowering plants. Unlike other speciation processes, polyploidization is often assumed to confer instant reproductive isolation. Shared polymorphism across ploidy levels has therefore often been attributed to multiple polyploid origins, while the alternative hypothesis of introgressive hybridization has rarely been rigorously tested. Here, we sequence 12 nuclear loci representing six genes duplicated by polyploidy in 92 accessions of the tetraploid C. bursa-pastoris, together with the corresponding loci in 21 accessions of its close diploid relative C. rubella. In C. bursa-pastoris accessions from western Eurasia, where the two species occur in partial sympatry, we find higher levels of nucleotide diversity than in accessions from eastern Eurasia, where C. rubella does not grow. Furthermore, haplotypes are shared across ploidy levels at four loci in western, but not in eastern Eurasia. We test whether haplotype sharing is due to retention of ancestral polymorphism or to hybridization and introgression using a coalescent-based isolation-with-migration model. In western, but not in eastern Eurasia, there is evidence for uni-directional gene flow from C. rubella to C. bursa-pastoris. An independent estimate of the timing of dispersal of C. bursa-pastoris to eastern Eurasia indicates that it probably pre-dated introgression. Our results show that polyploid speciation need not result in immediate and complete reproductive isolation, that post-polyploidization hybridization and introgression can contribute significantly to genetic variation in a newly formed polyploid, and that divergence population genetic analysis constitutes a powerful way of testing hypotheses on polyploid speciation.
Key Words: Capsella • introgression • polyploidy • Isolation-Migration model • speciation
Current address: Department of Biology, York University, 4700 Keele St, Toronto ON, Canada, M3J 1P3.