Stages of the symbiotic zooxanthellae–host cell division and the dynamic role of coral nucleus in the partitioning process: a novel observation elucidated by electron microscopy

Abstract

The relationship of the symbiotic algae or commonly referred as zooxanthellae (zo) and its host invertebrate species is uniquely observed among cnidarians where these tiny single-celled endosymbionts are used to reside densely within the gastrodermal tissue. However, at the ultrastructural level, little has been known on how zo are maintained through proliferation within its host cells. In the present study, the stages of division of the zo that occurred in the coral cells were investigated by electron microscopy. Scleractinian Pocillopora damicornis was fixed by freeze-substitution methods using a high-pressure freezer and embedded in resin. Ultrathin sections were observed using transmission electron microscope. Symbiotic cell at interphase showed the cortical fine structures of the non-dividing zo enclosed by the outermost plasma membrane (zpm). Overlying them, two distinct layers of intermediate membranes (inm) and a symbiosome membrane (sm) were found. When cytokinesis occurred, the zpm at the opposite division regions formed ‘U-shaped’ furrows which led amphiesmal vesicle (av) to protrude inwardly. As they invaginates, new avs and series of cortical microtubules (mi) are used to develop concomitant with the symbiont’ nuclear division. After the cytokinesis, the overlying membranes successively invaginate; thus, the daughter zooxanthellae were transformed into coccoid cells. These cells, however, remain contiguous to each other until when the coral nucleus (cnu) migrated toward one of the cleavage sites to intervene and dissociate them. Upon occupying the interzooxanthellar space, host nuclear division occurred prior to cytokinesis. These events involving the role of cnu in the partitioning process are first reported in this study, in which its dynamic intervening behavior had been elucidated as a novel finding from the cnidarian species.