cation of symbiotic dino X agellates in Paci W c corals in the genus Pocillopora

This study focused on the association between corals of the genus Pocillopora, a major constituent of PaciWc reefs, and their zooxanthellae. Samples of P. meandrina, P. verrucosa, P. damicornis, P. eydouxi, P. ligulata and P. molokensis were collected from French Polynesia, Tonga, Okinawa and Hawaii. Symbiodinium diversity was explored by looking at the 28S and ITS1 regions of the ribosomal DNA. Most zooxanthellae were found to belong to clade C, sub-clade C1, with little diVerentiation between populations. Interestingly, individuals of P. damicornis harbored sub-clade C1, clade D and clade A, depending on location. The symbiotic association of P. damicornis with its zooxanthellae may be somewhat more Xexible than those of other Pocillopora species. Introduction Most tropical corals live in symbiosis with photosynthetic algae, the zooxanthellae (reviewed in Trench 1993). Most zooxanthellae belong to the genus Symbiodinium and 11 species have now been deWned based on morphological, physiological and molecular criteria (reviewed in Baker 2003). Diversity of zooxanthellae from a wide range of hosts has been explored using molecular techniques with diVerent levels of resolution. The Wrst studies used allozymes and Restriction Fragment Length Polymorphism (RFLP) to diVerentiate between zooxanthella lineages (Rowan and Powers 1991). Later, Symbiodinium molecular phylogenetics became dominated by the study of nuclear DNA coding for ribosomal RNAs (nrDNA):18S nrDNA encoding the small ribosomal subunit (SSU), 28S nrDNA encoding the large ribosomal subunit (LSU) and the internal transcribed spacers (ITS1 and ITS2) located in between (reviewed in Baker 2003). Later studies turned to chloroplast DNA (23S rDNA) and mitochondrial DNA sequences (Santos et al. 2002a, b, 2003; Takabayashi et al. 2004). Recently, microsatellite markers have also been used to explore Symbiodinium diversity, particularly at the population level (e.g., Santos and CoVroth 2003; Magalon et al. 2006). All these studies have deWned multiple clades (reviewed in CoVroth and Santos 2005) that can further be subdivided into “sub-clades” or “types” (based on ITS1; e.g., van Oppen et al. 2001, and based on ITS2 DGGE proWles; e.g., LaJeunesse 2001). Clades were Wrst thought to correspond to ecological types with diVerent photoacclimatory abilities (Rowan and Knowlton 1995). Studies on zooxanthellae depth distribution and on their responses to stress conditions have attempted to determine the characteristics of each clade. However, these categories are highly simplistic and artiWcial since diVerential sensitivity to H. Magalon (&) UMR CNRS 7625, Laboratoire Fonctionnement et Evolution des Systèmes Ecologiques, Université Pierre et Marie Curie, 7 quai saint Bernard, 75005 Paris, France e-mail: hmagalon@snv.jussieu. fr J.-F. Flot UMR UPMC-CNRS-MNHN-IRD 7138, Département Systématique et Evolution, Muséum National d’Histoire Naturelle, 43 rue Cuvier, 75005 Paris, France E. Baudry UMR CNRS 8079, Laboratoire Ecologie, Systématique et Evolution, Université Paris-Sud, Bat 362, 91405 Orsay Cedex, France thermal stress among the various species of Symbiodinium seems to be distributed across all clades (Tchernov et al. 2004), as is sensitivity to irradiance (Savage et al. 2002). Researchers became interested in examining zooxanthellae diversity over a wide range of hosts and geographic locations. For instance, van Oppen et al. (2001) examined symbiont association across a large number of coral species belonging to the genus Acropora along the Great Barrier Reef and LaJeunesse (2005) published a complete survey of clade C diversity, encompassing nearly 100 host genera across the Atlantic and the Indo-PaciWc. In this study, zooxanthellae diversity was investigated in six Pocillopora species (Veron 2000): P. meandrina, P. verrucosa, P. damicornis, P. eydouxi, P. ligulata and P. molokensis, collected from French Polynesia, Tonga, Hawaii and Japan. Pocillopora is a major constituent of PaciWc reefs where it may play an important role in coral reef ecosystems. Many studies have reported that P. damicornis, P. verrucosa and P. eydouxi from diVerent regions [Japan (Hirose et al. 2000, 2001; Harii et al. 2002), Eastern PaciWc (Glynn et al. 1991), Maldives (Sier and Olive 1994) and South Africa (Kruger and Schleyer 1998)] contained zooxanthellae in their oocytes prior to spawning, indicating maternal symbiont transmission. Even if P. meandrina, P. ligulata and P. molokensis are not mentioned speciWcally in these studies, it is probable that these latter species also transmit their zooxanthellae vertically since they are closely related to the ones cited above. Clade-speciWc 28S nrDNA primers were used and the ITS1 region was sequenced to investigate (1) whether the diVerent species of the Pocillopora genus harbor diVerent zooxanthellae, and (2) whether the association between the host and its symbiont is speciWc. Materials and methods