ORIGINAL PAPER Molecular identification of natural mangrove hybrids of Rhizophora in Peninsular Malaysia Wei Lun Ng & Hung Tuck Chan & Alfred E. Szmidt Received: 5 November 2012 / Revised: 16 January 2013 / Accepted: 26 February 2013 # Springer-Verlag Berlin Heidelberg 2013 Abstract Natural hybridization is common in plants. Very often, the identity of a putative hybrid is inferred based on the observation of morphological features intermediate between two possible parental species occurring in a specific location. However, due to plasticity of morphological features and the co-occurrence of more than two possible parental species, molecular markers would be most useful to establish the origin of a putative hybrid. In mangroves, three Rhizophora species (Rhizophora apiculata, Rhizophora mucronata, and Rhizophora stylosa) and two putative hybrids (Rhizophora × lamarckii and Rhizophora × annamalayana) are distributed in the Indo-West Pacific region. Leaf samples of Rhizophora were obtained from two locations in Peninsular Malaysia, namely, Bagan Lalang and Pulau Burung, where all three species grow in sympatry. We analyzed sequences of one chloroplast and six nuclear DNA regions. Our results con- firmed earlier claims that the morphologically identified pu- tative hybrids growing in Pulau Burung are R. × lamarckii,a cross between R. apiculata and R. stylosa. Our data also pointed to the possible discovery of a new Rhizophora hy- brid—a cross between R. mucronata and R. stylosa—the identification of which would have been difficult based on morphological features alone. The directions and the stages of hybridization are also discussed. Keywords Peninsular Malaysia . Mangrove . Rhizophora . Natural hybridization . Sympatric species Introduction Natural hybridization is common in flowering plants. Mallet (2005) reported that at least 25 % of the plant species in the UK are involved in interspecific hybridization or introgres- sion. Hybridization plays an important role in the evolution of the earth’ s biodiversity, introducing new genetic variation within species and driving the emergence of new species (Arnold et al. 1999; Mallet 2007; Rieseberg 1997; Soltis and Soltis 2009; Wissemann 2007). Well-known examples of plant hybrid speciation can be found in sunflower Helianthus (Rieseberg et al. 1991) and pines Pinus (Ma et al. 2006; Ren et al. 2012) Mangrove species are no exception in terms of hybridiza- tion. There are approximately 70 known mangrove taxa dis- tributed worldwide, including naturally occurring putative hybrids (Duke et al. 1998; Spalding et al. 2010). Putative hybrids have been reported within the major genera of Rhizophora, Sonneratia, and Lumnitzera (Tomlinson 1986) and more recently in Bruguiera (Duke and Ge 2011). Three mangrove species of Rhizophora, including Rhizophora apiculata, Rhizophora mucronata, and Rhizophora stylosa, and two putative hybrids, Rhizophora × lamarckii (= R. apiculata × R. stylosa) and R. × annamalayana (= R. apiculata × R. mucronata), occur in the Indo-West Pacific (IWP) region. The morphologically recognized putative hy- brids are said to occur throughout the region (Duke et al. 2002 and references therein) and have so far been thought to be sterile and/or limited to the F 1 stage (Chan 1996; Lo 2010; Ng and Chan 2012a; Tyagi 2002). Despite the huge overlap in the geographical distribution of R. apiculata, R. mucronata, and R. stylosa (Duke 2006), their putative hybrids have been reported only in specific locations around the world. For Communicated by G. G. Vendramin Electronic supplementary material The online version of this article (doi:10.1007/s11295-013-0619-7) contains supplementary material, which is available to authorized users. W. L. Ng : A. E. Szmidt (*) Graduate School of Systems Life Sciences, Kyushu University, Fukuoka, Japan e-mail: aszmiscb@kyushu-u.org H. T. Chan International Society for Mangrove Ecosystems (ISME), Okinawa, Japan Tree Genetics & Genomes DOI 10.1007/s11295-013-0619-7