From one to many: The population genetics of cnidarian- symbiodinium symbioses

Abstract

The success of the coral reef ecosystem is due in large part to endosymbioses between cnidarians such as scleractinian corals, octocorals and anemones and single-celled dinoflagellates in the genus Symbiodinium. While the rise of molecular genetic analyses have offered valuable insight into Symbiodinium biodiversity, host and geographic distributions and evolutionary relationships, less well-studied are patterns and processes at the population level. Since populations represent the fundamental unit by which evolution occurs, furthering understanding in this area is paramount towards addressing questions ranging from the basic biology of Symbiodinium and their hosts to how anthropogenic-driven global climate change may impact these symbioses in the future. Here, a synopsis of population-level characteristics for Symbiodinium and various cnidarian host species are distilled from the current literature. Mutational patterns in the most commonly utilized genetic markers for population-level studies of Symbiodinium, nuclear microsatellite loci, are also explored. Substitutions, nucleotide insertions and deletions (indels), alterations to the repeat array structure and non-stepwise changes in repeat number are drivers of both allelic variation and size homoplasy, with the latter of concern due to the potential to bias estimates of genetic structure. Such mutations, however, are also a rich source of information that complement and extend the population-level data inherent to microsatellites and provide additional insight into various facets of these symbioses. Lastly, advents in DNA sequencing technology and genomics are discussed since they offer exciting opportunities to rapidly explore pertinent questions in cnidarian hosts and their Symbiodinium populations in a rapidly changing world.