| Symbiotic relationships involving microalgae and invertebrates have been recognized and studied for over a century. In all of these associations, algal symbionts provide their invertebrate hosts with photosynthetically-fixed carbon in the form of small molecular weight metabolites. By far, the microalgal group with the highest number of symbiotic representatives are the dinoflagellates. Members of the genus Symbiodinium, commonly referred to as zooxanthellae, are the most common dinoflagellate symbionts in association with Protista, Cnidaria and Mollusca. The recent widespread application of DNA-based molecular techniques to ecological and evolutionary studies have lead to comprehensive studies of Symbiodinium from reef-building invertebrates. However, analyses of novel DNA molecules from the nuclear and organelle genomes may provide new insight into Symbiodinium biology, such as diversity, phylogenetics and population genetic structure. Many Symbiodinium cultures were found to be a subset of the original in hospite population. Despite this, cultures should continue to remain a vital part of studies exploring invertebrate-dinoflagellate symbioses since they offer a glimpse of the diversity present in wild zooxanthella populations as well as act as controllable sources of cells and zooxanthella DNA free of host materials. Dinoflagellate cultures isolated from a range of invertebrate host species and geographical locations were employed to construct the first Symbiodinium phylogeny based on chloroplast large subunit (cp23S)-rDNA domain V sequences, providing the first independent molecular evidence supporting the published divisions of Symbiodinium into clades. During sequencing of Symbiodinium cp23S-rDNA domain V for the phylogenetic analyses, the sequences were observed to possess several unique features, including molecules suffering internal deletion mutations of various sizes (6–298 bp), a nucleotide insertion in the large subunit rDNA that is novel across all kingdoms and the most size variable expansion segments of any known plastid-harboring organism. Microsatellite analysis suggested that Symbiodinium are haploid while demonstrating statistically significant population differentiation of Symbiodinium haplotypes inhabiting the Caribbean gorgonian Pseudopterogorgia elisabethae across the Bahamas. Although clonal propagation may maintain these distinct populations, tests of linkage disequilibrium suggest that the Symbiodinium populations associated with P. elisabethae also undergo recombination, providing evidence that Symbiodinium undergoes some form of recombination during its life cycle. |
