Symbiont Transmission onto the Cell Surface of Early Oocytes in the Deep-Sea Clam Phreagena okutanii

Abstract

Deep-sea hydrothermal vent and seep sites often harbor high densities of a variety of invertebrates, such as annelids, arthropods, and mollusks. These invertebrates survive in the deep sea, where little or no sunlight reaches, by establishing symbiotic relationships with chemoautotrophic bacteria. These symbiotic bacteria use the chemical energy contained in reduced compounds such as sulfide, methane, or hydrogen for chemo-biosynthetic processes (Dubilier et al., 2008; Petersen et al., 2011). Deep-sea vesicomyid clams, including those of the genus Phreagena (formerly Calyptogena), are endemic and dominant members of deep-sea chemosynthesis-based communities (Fisher, 1990). They harbor sulfur-oxidizing symbiotic bacteria (Gram-negative Gammaproteobacteria) in their gill epithelial cells (Kuwahara et al., 2007). The clams depend on these symbionts for nutrition as their digestive tracts are not functional (Le Pennec et al., 1995). Furthermore, the genome of Phreagena symbionts lacks several genes essential for life in free-living Gram-negative bacteria (Kuwahara et al., 2007). The maintenance of such obligate host-symbiont associations is contingent on the transmission of the symbionts across host generations. In general, symbiotic microbes can be transmitted horizontally (between contemporary hosts or through reinfection by free-living forms of symbionts), vertically (directly from parent to offspring, often via gametes), or through a combination of these two transmission mechanisms (Bright and Bulgheresi, 2010). Recently, Ikuta et al. (2016) reported that the endosymbiotic bacteria of Phreagena okutanii are associated with the host's eggs and transmitted vertically, but they are attached extracellularly on the outer surface of the plasma membrane of spawned eggs, exclusively at the vegetal pole, forming an oval-shaped cluster. Phreagena okutanii is a gonochoristic species with a male or female gonad in separate adult individuals, and no evidence for hermaphroditism has been found (Ikuta et al., 2016). The symbiont resides in the gills of both males and females, but it is not detected in the male testis. The female ovary consists of a cluster of acini, also called follicles, where the symbionts are localized in a small part of the periphery of the oocyte and in the acinar wall cells (Ikuta et al., 2016). These observations suggest that behavior of the symbiont during the host development may be dynamic, exhibiting transmission of symbiotic bacteria among host cells. However, the processes and mechanisms of how and when the symbionts enter gill epithelial cells, enter gonadal acinar wall cells in females but not in males, and attach to the next generation of eggs, are totally unknown.