Hydrobiologia 413: 11–16, 1999. R.S. Dodd (ed.), Diversity and Function in Mangrove Ecosystems. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. 11 Genetic differentiation between Bruguiera gymnorhiza and B. sexangula in Sri Lanka Pushpa Damayanthi Abeysinghe 1,2 , Ludwig Triest 1,∗ , Bart De Greef 1 , Nico Koedam 1 & Sanath Hettiarachi 2 1 Vrije Universiteit Brussel, Research Group General Botany and Nature Management, Pleinlaan 2, B-1050, Brussel, Belgium. E-mail: ltriest@vub.ac.be 2 Dept. of Botany, University of Ruhuna, Matara, Sri Lanka ( ∗ Author for correspondence) Key words: DNA, RAPD, genetic diversity, Bruguiera, Sri Lanka, mangroves Abstract The identification of populations of Bruguiera sexangula, Bruguiera gymnorhiza and their putative hybrids in the field is difficult using only morphological and phenological characters. Using a PCR based technique, RAPD (Ran- dom Amplified Polymorphic DNA), the genetic variation of Bruguiera populations was studied from contrasting climatic and geographic regions along the southwest coastal region of Sri Lanka. Out of 45 primers screened, 20 primers allowed us to observe polymorphism, not only between species (interspecific) but also within the species (intraspecific). Analysis of RAPD data appears to be helpful in determining the genetic relationship among populations of B. gymnorhiza and B. sexangula. RAPD markers revealed that the two species are well separated without any hybrid position between the two taxa though they occur in mixed stands. Although sampling sizes of populations of this study were small, genetic variation among B. gymnorhiza and B. sexangula populations could be observed. For B. sexangula, it was possible to differentiate each of the three populations, even when using a small number of primers. Introduction Mangroves or mangals are tropical and subtropical forests developed in the intertidal zone (Arulchelvam, 1968; Lindén & Jernelöv, 1980; De Silva & Balas- ubramaniam, 1985; Tomlinson, 1986; Amarasinghe & Balasubramaniam, 1992; McKee, 1995; Pema- dasa, 1996). Though mangroves occur in tropical and subtropical areas of the world, the most luxuriant man- grove forests are found in Southeast Asia (Lindén & Jernelöv, 1980). The present extent of mangroves in Sri Lanka has been estimated at 4000 ha (Pemadasa, 1996). Sri Lankan mangroves include approximately 23 mangrove species (Jayawardene, 1985) from nine genera (Rhizophora, Bruguiera, Ceriops, Avicennia, Xylocarpus, Excoecaria, Lumnitzera, Aegiceras and Nypa: definition according to Tomlinson, 1986). Mangroves are ecologically important components of the coastal ecosystems because they have developed on sheltered shores of lagoons and estuaries, and because they prevent sea erosion and act as a land building system. Physiologically and morphologically, they have various features for extremely harsh and hostile dry atmospheric conditions and wet, anoxic and saline root environments. Economically, the mangroves are providing suit- able habitats for crustaceans and fish, and they deliver plant products such as tannins and dyes, timber, re- sources for alcoholic beverages and food (Lindén & Jernelöv, 1980; De Silva & Balasubramaniam, 1985; Jayawardene, 1985; Tomlinson, 1986; Pemadasa, 1996; Hellier, 1988). As a result of human activities, extensive mangrove areas are being lost at an alarming rate (Lindén & Jernelöv, 1980; Jayawardene, 1985; Hellier, 1988; Pemadasa, 1996). Despite the great interest in their conservation, in management of mangrove species (Jayawardene, 1985; Dodd et al., 1995; Farnsworth & Ellison, 1997)