Progress in characterizing the endosymbiotic dinoflagellates of soritid Foraminifera and related studies on some stages in the life cycle of Marginopora vertebralis

Abstract

Foraminifera have developed symbiotic relationships with a wide range of different algae. In all the cases studied, the requirement for a symbiotic relationship with a particular type of alga (not necessarily a particular species of alga) is obligate for the foraminiferal host. Members of one family of larger foraminifera, the Soritidae, are the hosts for endosymbiotic dinoflagellates. Preliminary evidence obtained from ssrRNA sequences, obtained from single samples of endosymbionts from three different hosts, have shown that the endosymbionts are phylogenetically more closely related to coelenterate zooxanthellae than they are to each other. One interpretation is that, over time, the hosts and symbionts have maintained widely flexible relationships. This inference is also supported by data obtained from diatom-bearing and chlorophyte-bearing larger foraminifera, which seem to have exceptionally flexible 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 dinoflagellates. These results run contrary to a paradigm based on solid data derived from studies of termites and their parabasalian endosymbionts that shows that hosts and symbionts co-evolved. We suggest that larger foraminifera have taken an alternative symbiotic evolutionary pathway by developing a system of host/symbiont fit that is not finical, but which could have advantages for survival and adaptation in changing habitats.