PROGRESS IN CHARACTERIZING THE ENDOSYMBIOTIC DINOFLAGELLATES OF SORITID FORAMINIFERA AND RELATED STUDIES ON SOME STAGES IN THE LIFE-CYCLE OF MARGINOPORA-VERTEBRALIS

Foraminifera have developed symbiotic relationships with a wide range of different algae. In all the cases studied, the requirement for a sy mbiotic relationship with a particular type of alga (not necessarily a particular species of alga) is obligate for the foraminiferal host. M embers of one family of larger foraminifera, the Soritidae, are the ho sts for endosymbiotic dinoflagellates. Preliminary evidence obtained f rom ssrRNA sequences, obtained from single samples of endosymbionts fr om three different hosts, have shown that the endosymbionts are phylog enetically more closely related to coelenterate zooxanthellae than the y are to each other. One interpretation is that, over time, the hosts and symbionts have maintained widely flexible relationships. This infe rence is also supported by data obtained from diatom-bearing and chlor ophyte-bearing larger foraminifera, which seem to have exceptionally f lexible relationships with their endosymbionts. We speculate that the soritids got their zooxanthellae from environmental pools contributed to by coelentrate host taxa, rather than by co-evolution with their di noflagellates. These results run contrary to a paradigm based on solid data derived from studies of termites and their parabasalian endosymb ionts that shows that hosts and symbionts co-evolved. We suggest that larger foraminifera have taken an alternative symbiotic evolutionary p athway by developing a system of host/symbiont fit that is not finical , but which could have advantages for survival and adaptation in chang ing habitats.