Diversity of dinoflagellate symbionts in Red Sea soft corals: mode of symbiont acquisition matters

Symbiotic associations are ubiquitous in terrestrial and marine environments and are of great ecological importance. The onset of a symbiotic relationship differs among associations. Symbionts can be vertically transmitted from host parent to offspring or they can be acquired horizontally from the surrounding environment with each new host generation. Cnidarian-algal symbioses, the subject of our study, exhibit both strategies. We investigated the clade identity of symbionts in soft coral hosts (Eilat, Red Sea) in relation to their hosts' mode of symbiont acquisition. We found for the first time that all hosts using horizontal transmission harbored symbionts belonging to Clade C while those with vertical transmission uniquely harbored symbionts from Clade A. The latter, capable of coping with a wide array of environmental conditions, evolved to be optimal vertically transmitted symbionts. The limitation of Clade A symbionts to hosts with vertical transmission suggests a coevolution of the hosts and symbionts. Clade C symbionts, characterized by large sub-clade variability, are found in corals with horizontal transmission and, most probably, each of its genotypes exhibits a more specialized set of physiological capabilities.