Genetic composition of zooxanthellae between and within colonies of the octocoral Plexaura kuna, based on small subunit rDNA and multilocus DNA fingerprinting

In intracellular symbiosis, the host is both the symbiotic partner and the habitat for the symbiont. Physical conditions may vary within a host individual, leading to niche partitioning within a host according to symbiont genotype. In this study, the algal symbionts (zooxanthellae) inhabiting the octocoral Plexaura kuna (Lasker, Kim, Coffroth) were characterized. Zooxan-thellae are currently divided into clades based on sequence variation of the small and large subunit ribosomal gene and internal transcribed spacer regions. Zooxanthellae isolated from the top and bottom of a P. kuna colony, from colonies sampled at various depths and geographic locations (Bahamas, Florida Keys, Panama and St. Croix, USVI), from 1988 to 1998, all belonged to clade B. Since symbiont variability within a host is an individual-level question, in the second part of the study, multilocus DNA fingerprinting was used. Each genetically distinct P. kuna colony (i.e. non-clonemate) hosted a unique prominent zooxanthella genotype. This zooxanthella genotype was found at both the top and the bottom of the host colony, although physical conditions differed between these areas. The existence of one prominent zooxanthella genotype throughout a colony may indicate a fixed symbiont-acquisition period or the active maintenance of a hostsymbiont genotypic association.