Abstract
Shorea (Dipterocarpaceae) is a large genus in which many closely related species often grow together in Southeast Asian lowland tropical rain forests. Many Shorea species share common pollinators, and earlier studies suggested occurrence of interspecific hybridization and introgression. Here, we show morphological and molecular evidence of hybridization between Shorea species. In the census of all the trees of Shorea curtisii, Shorea leprosula, and Shorea parvifolia (>30 cm dbh) within the 164-ha area of Bukit Timah Nature Reserve in Singapore, we found 21 morphologically recognizable hybrid individuals. All of the putative hybrids could be distinguished obviously from the parental species on the basis of vegetative characters. Population genetic analysis of DNA sequences of two nuclear (GapC and PgiC) and chloroplast (trnL-trnF) regions demonstrated that each of the three species had several species-specific mutations. The nuclear sequences of the putative hybrids were heterozygote at all the species-specific sites between two parental species. Hybrid between S. curtisii and S. leprosula was found most, while S. curtisii × S. parvifolia and S. leprosula × S. parvifolia hybrids were also found. Almost no shared polymorphism between populations of the parental species suggests rarity of introgression. The study indicated that natural hybridization between sympatric Shorea species should not be uncommon, but all of the hybrid individuals were F1, and the post-F1 hybrids were considerably rare.
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Acknowledgements
This study was conducted under permission from National Parks Board, Singapore (NP/RP713 and NP/RP862). Authors thank to the staff of the Singapore Botanic Gardens Herbarium and the herbarium of the Forest Research Institute of Malaysia for access to specimens; to Stuart Davies and Peter Ashton for invaluable help during the development of this paper; and to anonymous reviewers for helpful comments. This research was supported by the Academic Research Fund, National Institute of Education, Nanyang Technological University, Singapore (RP19/00 and RP24/96), and by the Global Environment Research Fund from the Ministry of the Environment, Japan (E-091 and RF-1010).
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Kamiya, K., Gan, Y.Y., Lum, S.K.Y. et al. Morphological and molecular evidence of natural hybridization in Shorea (Dipterocarpaceae). Tree Genetics & Genomes 7, 297–306 (2011). https://doi.org/10.1007/s11295-010-0332-8
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DOI: https://doi.org/10.1007/s11295-010-0332-8