Genetic variability of the symbiotic dinoflagellates from the wide rangingcoral species Seriatopora hystrix and Acropora longicyathus in the Indo-West Pacific

The scleractinian coral species, Seriatopora hystrix and Acropora longicyat hus, are widely distributed throughout the latitudinal range of the tropica l west Pacific. These 2 coral species live in a mutually beneficial relatio n with symbiotic dinoflagellates (zooxanthellae), which are passed to their progeny by vertical transmission (zooxanthellate eggs or larvae) and horiz ontal transmission (eggs or larvae that acquire symbionts from the environm ent), respectively. For S. hystrix, vertical transmission might create biog eographically isolated and genetically differentiated symbiont populations because the extent of its larval migration is known to be limited. On the o ther hand, horizontal transmission in corals such as A. longicyathus may re sult in genetically connected symbiont populations, especially if its zooxa nthellae taxa are widely distributed. To examine these hypotheses, symbiont s were collected from colonies of S. hystrix and A. longicyathus living in the Great Barrier Reef (Australia), South China Sea (Malaysia) and East Chi na Sea (Ryukyus Archipelago, Japan), and were examined using restriction fr agment length polymorphism and sequence analysis of large and small subunit rRNA genes. Phylogenetic analysis assigned the symbionts to 1 of 3 taxonom ically distinct groups, known as clades. Symbionts from Australian and Japa nese S. hystrix were placed in Clade C, and Malaysian S. hystrix symbionts in the newly described Clade D. Seven of 11 Australian and all Japanese and Malaysian colonies of A. longicyathus had symbiotic dinoflagellates that a lso grouped with Clade C, but symbionts from the remaining Australian colon ies of A. longicyathus grouped with Clade A. Analysis of molecular variance of Clade C symbionts found significant genetic variation in 1 or more geog raphic groups (69.8%) and to a lesser extent among populations within geogr aphic regions (13.6%). All populations of Clade C symbionts from S. hystrix were genetically differentiated according to geographic region. Although C lade C symbionts of A. longicyathus from Japan resolved into a distinct geo graphic group, those from Australia and Malaysia did not and were genetical ly connected. We propose that these patterns of genetic connectivity correl ate with differences in the dispersal range of the coral or symbiont propag ules and are associated with their respective modes of symbiont transmissio n.