The Ecology and Evolution of Coral-Associated Apicomplexans

Abstract

Apicomplexans are protists within the eukaryotic supergroup S.A.R. that infect a wide- range of animal species and can cause disease. Although apicomplexans are an important parasitic group, little is published regarding those associated with many invertebrates, such as reef-building scleractinian corals. There is a single described species of coral-associated apicomplexans, Gemmocystis cylindrus, which is hypothesized to diverge early within the coccidian lineage. This group contains many opportunistic human and livestock pathogens. To resolve the potential effect of apicomplexans on coral health, it is first necessary to further describe this enigmatic group of putative parasites and determine their prevalence among host species. This study utilized previously collected seasonal samples from four Caribbean scleractinian coral species (Montastraea annularis, Montastraea faveolata, Siderastrea siderea, and Porites astreoides) over a nine-year period (May 2000-2008) from two reefs in the Florida Keys as well as over five-and-a-half years (May 2001-Nov 2005) for two Bahamian reefs. Using PCR-based screening, these colonies exhibited consistent infection for the sampling duration. There was a significant effect of season and species, with apicomplexans more likely to be associated during the winter and less likely associated with S. siderea. High prevalence may be partially explained by life-history traits as apicomplexans were found associated with planulae larvae of P. astreoides, indicating vertical transmission in species that brood (i.e. undergo fertilization internally). Conversely, apicomplexans are not associated with larvae of broadcasting (i.e. undergo external fertilization) species, implying horizontal transmission in these species. To determine the evolutionary history of these as well as coral-associated apicomplexans from an additional 16 coral hosts, small subunit (18S) ribosomal DNA (rDNA) was utilized to generate phylogenetic trees. This group of apicomplexans forms a monophyletic clade with strong bootstrap support near the coccidians. Altogether, these data provide insights into the symbiotic association between coral hosts and apicomplexans.