MBE Advance Access published online on September 28, 2007
Molecular Biology and Evolution, doi:10.1093/molbev/msm209
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Research Article |
Population Genetics of Speciation in Non-Model Organisms: I. Ancestral Polymorphism in Mangroves
1 State Key Laboratory of Biocontrol and Key Laboratory of Gene Engineering of the Ministry of Education, Sun Yat-Sen University, Guangzhou 510275, China
2 Department of Ecology and Evolution, University of Chicago, Chicago, Illinois 60637, USA
3 Department of Biology, University College London, Gower Street, London WC1E 6BT, UK
These authors contributed equally to this study
* Corresponding authors: Suhua Shi, School of Life Sciences, Sun Yat-Sen University, Guangzhou 510275, Guangdong, People's Republic of China, Phone: 86-20-84113677; Fax: 86-20-34022356, E-mail: lssssh{at}mail.sysu.edu.cn, Chung-I Wu, Department of Ecology and Evolution, University of Chicago, Chicago, Illinois 60637, USA, Phone: 773-702-2565; Fax: 773-702-9740, E-mail: ciwu{at}uchicago.edu
Received for publication May 23, 2007. Revision received August 6, 2007. Revision received September 6, 2007. Accepted for publication September 24, 2007.
The level of DNA polymorphism in the ancestral species at the time of speciation can be estimated using DNA sequences from many loci sampled from two or more extant species. The comparison between ancestral and extant polymorphism can be informative about the population genetics of speciation. In this study, we collected and analyzed DNA sequences of 
60 genes from four species of Sonneratia, a common genus of mangroves on the Indo-Pacific coasts. We found that the 3 ancestral species were comparable to each other in terms of level of polymorphism. However, the ancestral species at the time of speciation were substantially more polymorphic than the extant geographical populations. This ancestral polymorphism is in fact larger than, or at least equal to, the level of polymorphism of the entire species across extant geographical populations. The observations are not fully compatible with speciation by strict allopatry. We suggest that, at the time of speciation, the ancestral species consisted of interconnected but strongly divided geographical populations. This population structure would give rise to high level of polymorphism across species range. This approach of studying the speciation history by genomic means should be applicable to non-model organisms.
Key Words: ancestral polymorphism • extant polymorphism • population subdivision • Sonneratia • mode of speciation
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