Genetic comparisons of freshly isolated versus cultured symbiotic dinoflagellates: Implications for extrapolating to the intact symbiosis

Zooxanthellae, algal symbionts in divergent marine invertebrate hosts, are a genetically heterogeneous group. All species descriptions and most physio logical and infectivity studies of zooxanthellae have been conducted using cultured material. However, few studies have attempted to quantify the repr esentation of cultures isolated from cnidarians to the in hospite zooxanthe lla populations of the individual host or host species from which they were established. RFLPs of small subunit (18S) rDNA, internal transcribed space r (ITS)rDNA sequence data, and microsatellite analyses were conducted to as sess the relatedness between cultured zooxanthellae and the in hospite popu lations of the individual host or host species from which they were isolate d. RFLP data demonstrated that cultures may represent either the numericall y dominant symbiont or ones present in lower number. ITS-rDNA sequences fro m zooxanthella. cultures were disconcordant with ITS-rDNA sequences identif ied from in hospite zooxanthellae of the same host species, and microsatell ites present in in hospite zooxanthella populations were absent from the co rresponding cultures. Finally, reexamination of the literature revealed exa mples of zooxanthellar cultures being nonrepresentative of in hospite popul ations. These data suggest that, in most cases, cultures are a subset of th e original in hospite population. Factors such as failing to homogenize bul k cultures before transfer, growth medium used, and the picking of single m otile cells may contribute to many zooxanthella cultures being nonrepresent ative.