Effects of Isolation of the Endosymbiont Symbiodinium microadriaticum (Dinophyceae) from Its Host Aiptasia pallida (Anthozoa) on Cell Wall Ultrastructure and Mitotic Rate

Abstract

The dinoflagellate Symbiodinium microadriaticum inhabits the gastroderm of the anemone Aiptasia pallida. The symbionts were isolated from their hosts and processed for transmission electron microscopy either immediately or after culturing for one day. Symbionts inside anemone tissues were compared with isolated symbionts. Cell wall structure of symbionts within anemone tissues was found to be similar to that of symbionts fixed immediately following isolation. In symbionts cultured for one day, the wall became much thicker and more homogeneous. This was thought to be related to a change in mitotic rate from 1.2% to 13.2% during the first day after isolation. Because of increased mitotic rates, entire new walls were synthesized in much of the population. The increased mitotic rate was maintained in culture. The younger walls were identified by their thicker and more homogeneous fine granular structure. A. pallida developed multiple membranes surrounding the symbionts, the abundance of which seemed to be associated with the length of time the symbiont remained undivided in the host. The dinoflagellate Symbiodinium (=Gymnodinium) microadriaticum (Freudenthal, 1962) inhabits the gastrodermal cells of the anemone Aiptasia pallida (Verrill) (Fig. 1). The nature of symbiont membranes and wall have been described by Taylor (1968), Kevin et al. (1969), and Smith (1979). In situ studies on the relationship between the symbiont periplast and anemone cells by Taylor (1968) included the observation of a tendency for the surrounding cnidarian cells to form thread-like extensions of cytoplasm encircling the symbiont that appeared as additional Smith (1979) observed symbionts that appeared to be bounded by a complex of membranes. Duclaux (1977, cited by Smith, 1979) observed most membranes to be of host origin and suggested that they increased in abundance, the longer the symbiont remained within the host. Schoenberg & Trench (1980) described variation in the cell wall and associated membranes of S. microadriaticum from Tridacna gigas. In this study, we observed the anemone-symbiont interface in order to compare the wall of symbionts living inside anemones with walls of symbionts isolated from anemones. Because S. microadriaticum is able to survive both inside and outside anemones, structural differences that appeared following isolation of symbionts from anemones might be regarded as modifications necessary to support life outside the host cells. We observed the dinoflagellates in vitro over a 24-h period to determine whether any additional changes occurred as they adjusted to life in a new environment. Muscatine & Pool (1979) observed that the mitotic rates of symbiont and host must be closely balanced in order to maintain optimum levels of both cell populations. Colley & Trench (1983) noted that the mitotic rate of symbionts TRANS. AM. MICROSC. SOC., 107(1): 53-66. 1988. ? Copyright, 1988, by the American Microscopical Society, Inc. This content downloaded from 157.55.39.45 on Thu, 01 Sep 2016 04:47:08 UTC All use subject to http://about.jstor.org/terms TRANS. AM. MICROSC. SOC.