Acquisition and proliferation of algal symbionts in bleached polyps of the upside-down jellyfish, Cassiopea xamachana

摘要

The mutualistic relationship between dinoflagellates in the genus Symbiodinium microadriaticum and animals in the phylum Cnidaria, specifically corals, make up one of the most well-known symbioses in the marine environment. The nature of the relationship is such that these symbiotic algae provide products of photosynthesis to their cnidarian hosts, while the algae are afforded refuge within the endodermal cells of the host. Use of the model system Cassiopea xamachana (a.k.a. the 'upside down jellyfish'), a scyphozoan that can be reared throughout its entire life cycle in a laboratory setting, allowed for the study of the cnidarian-algal symbiosis without many of the constraints involved when working with corals and other cnidarians. The aposymbiotic polyp stage of C. xamachana was used in this study to visualize the acquisition of algal cells by endodermal cells of host animals, a requisite phenomenon for the transition to a free living medusae (strobilation). Polyps harboring algae from environmental sources prior to experimentation were subjected to high temperatures to initiate the expulsion of algae and to obtain polyps that were completely free of symbionts (aposymbiotic); bleaching was confirmed with high resolution confocal microscopy. Aposymbiotic polyps were subsequently introduced to low numbers of Symbiodinium, and the intrinsic growth rate of algae calculated based on count data generated at 7-day intervals. Symbiodinium used for reintroduction in experimental trials were obtained from three sources: (1) commercially cultured algal cells (type A194), (2) algae that had been expelled from their host tissues following heat-induced bleaching in the laboratory, and (3) algae freshly isolated from an adult medusa. These three sources of algal cells were used to represent the possible reservoirs from which bleached animals might replenish their symbiont population following a bleaching event. Symbiodinium cells were acquired by C. xamachana polyps in all three treatment groups, although cultured Symbiodinium A194 exhibited the slowest mean intrinsic growth rate (0.054 d(-1)) once they were housed within endodermal cells of their host. Mean time to strobilation was accordingly slow ( 120 days) for those polyps harboring type A194 symbionts. In fact, polyps harboring other sourced Symbiodinium strobilated in 70 days on average. Though it is known that strobilation does not occur without the acquisition of symbionts during the polyp stage, the results provided in this study demonstrate clearly that the number of symbionts at the time of strobilation can vary greatly. This variation in symbiont number just prior to strobilation, substantiates previous documented evidence that strobilation is not only stimulated by acquisition of Symbiodinium, but by other triggers as well. Our methods allow quantitative methods to be used to discover additional components involved in the establishment of successful symbiosis between these species.