Gene flow of ceriops tagal (rhizophoraceae) across the kra isthmus in the thai malay peninsula

The Malay Peninsula (formerly ancient Sundaland) is regarded as a barrier that isolates organisms of the South China Sea from those of the Bay of Bengal. During the interglacial period, approximately 5 Mya, sea levels rose and organisms migrated across the narrowest part of this peninsula, the Kra Isthmus. In the present study, we examine the chloroplast genomes of Ceriops tagal along the coasts of both sides of the Kra Isthmus to retrace divergence events and to evaluate the probability of previous long distance dispersal. The haplotype distributions support the hypothesis that the Kra Isthmus was an effective geographic barrier that caused genetically differentiated populations. Based on comparison of the chloroplast genomes, the estimated time of divergence between the two populations is consistent with the emergence time of the Kra Isthmus. However, ancient and recent gene flow obscures the phylogenetic relationships between eastern and western haplotypes. We used nested clade analysis (based on user-defined-distances corresponding to the distances across the peninsula and the sea route around it) and provide evidence of pre-isthmus range expansion and restriction of gene flow that resulted from geographic isolation. Trans-isthmus long distance dispersal probably occurred at the pre-isthmus region ̃5 Mya via the southern Malay Peninsula. Our results indicate that the Malay Peninsula has had separate populations on opposite sides of the Kra Isthmus since its formation, but that interglacial migration at the Strait of Malacca may have provided a corridor for gene flow. This is an instance of arrested allopatric speciation due to genetic homogenization via rare long distance dispersal.