Abstract
The coral symbiosis is the linchpin of the reef ecosystem, yet the mechanisms that promote and maintain cooperation between hosts and symbionts have not been fully resolved. We used a phylogenetically controlled design to investigate the role of vertical symbiont transmission, an evolutionary mechanism in which symbionts are inherited directly from parents, predicted to enhance cooperation and holobiont fitness. Six species of coral, three vertical transmitters and their closest horizontally transmitting relatives, which exhibit environmental acquisition of symbionts, were fragmented and subjected to a 2-week thermal stress experiment. Symbiont cell density, photosynthetic function and translocation of photosynthetically fixed carbon between symbionts and hosts were quantified to assess changes in physiological performance and cooperation. All species exhibited similar decreases in symbiont cell density and net photosynthesis in response to elevated temperature, consistent with the onset of bleaching. Yet baseline cooperation, or translocation of photosynthate, in ambient conditions and the reduction in cooperation in response to elevated temperature differed among species. Although Porites lobata and Galaxea acrhelia did exhibit the highest levels of baseline cooperation, we did not observe universally higher levels of cooperation in vertically transmitting species. Post hoc sequencing of the Symbiodinium ITS-2 locus was used to investigate the potential role of differences in symbiont community composition. Interestingly, reductions in cooperation at the onset of bleaching tended to be associated with increased symbiont community diversity among coral species. The theoretical benefits of evolving vertical transmission are based on the underlying assumption that the host-symbiont relationship becomes genetically uniform, thereby reducing competition among symbionts. Taken together, our results suggest that it may not be vertical transmission per se that influences host-symbiont cooperation, but genetic uniformity of the symbiont community, although additional work is needed to test this hypothesis.
Highlights
Cooperation between species has played a fundamental role in the evolution and diversification of life
A total of corals of each species, Galaxea acrhelia and Galaxea astreata were collected from Davies Reef (1849.816′, 14737.888′, April 2015), 10 corals each of A. millepora and M. aequituberculata were collected from Pelorus Island (1833.358′, 14630.276′, 18 April 2015) and 10 corals each of Goniopora columna and P. lobata were collected from Pandora Reef (1848.778′, 14625.593′, 21 April 2015) from depths of
The fixed difference among species was driven by the low symbiont density on average in P. lobata, which differed significantly from densities in A. millepora, M. aequituberculata, Galaxea astreata and Galaxea acrhelia (Tukey’s HSD < 0.05, Fig. 1)
Summary
Cooperation between species has played a fundamental role in the evolution and diversification of life Exploring mechanisms that affect coral cooperation: symbiont transmission mode, cell density and community composition. In the case of reef-building corals, the intracellular symbiosis between dinoflagellates in the genus Symbiodinium and a calcifying Cnidarian host forms the basis of one of the most biodiverse and productive ecosystems on the planet (Hatcher, 1988; Knowlton et al, 2010). The process of host calcification, which builds the three dimensional structure of the reef, is largely powered by symbiont primary productivity (Roth, 2014). Climate change and other anthropogenic processes threaten reefs because of the sensitivity of the coraldinoflagellate symbiosis to environmental stress (Hoegh-Guldberg et al, 2007; Hughes et al, 2003), indicating that host-symbiont cooperation is not stable over ecological timescales
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
