The fluted giant clam (Tridacna squamosa) increases the protein abundance of the host's copper-zinc superoxide dismutase in the colorful outer mantle, but not the whitish inner mantle, during light exposure

Abstract

The colorful outer mantle of giant clams contains abundance of symbiotic dinoflagellates (zooxanthellae) and iridocytes, and has direct exposure to light. In light, photosynthesizing dinoflagellates produce O₂, and the host cells in the outer mantle would be confronted with hyperoxia-related oxidative stress. In comparison, the whitish inner mantle contains few symbiotic dinoflagellates and no iridocytes. It is involved in shell formation, and is shaded from light. CuZnSOD is a cytosolic enzyme that scavenges intracellular O₂"^-. We had obtained from the outer mantle of the fluted giant clam, <i>Tridacna squamosa</i>, the complete cDNA coding sequence of a host-derived <i>copper zinc superoxide dismutase (CuZnSOD)</i>, which comprised 462 bp and encoded for 154 amino acids with a calculated MW of 15.6 kDa. <i>CuZnSOD</i> was expressed strongly in the outer mantle, ctenidium, hepatopancreas and kidney. The transcript level of <i>CuZnSOD</i> remained unchanged in the outer mantle during light exposure, but the protein abundance of <i>CuZnSOD</i> increased ~3-fold after exposure to light for 6 or 12 h. By contrast, 12 h of light exposure had no significant effects on the gene and protein expression levels of <i>CuZnSOD</i>/CuZnSOD in the inner mantle. Hence, the increased expression of CuZnSOD in the outer mantle of <i>T. squamosa</i> was probably a host's response to ameliorate oxidative stress related to photosynthesis in the symbionts, and not simply due to increased metabolic rate in the host cells. Evidently, the host clam must possess light- or O₂-responsive anti-oxidative defenses in order to align with the light-dependent photosynthetic activity of its symbionts.