Polyamory in Corals - An adaptive mechanism to environmental changes

Obligate mutualistic symbiosis occurs when the host must possess symbionts in order to survive. The most studied symbiosis of this type is the one between reef building corals and dinoflagellates, but how much do we actually know? What happens in the symbiosis between dinoflagellate and coral throughout the hosts ontogenesis is rarely studied yet important for a better understanding of the host-symbiont relationship.

There are two ways in how symbionts are transmitted from one generation to the next, both having positive and negative aspects for the host. In vertical transmission, the symbiont is directly transferred from the host parent to the offspring. Due to physiological differences between the symbionts clades (light and temperature sensitivity), which symbiont the offspring inherit may affect the hosts survivorship in different habitats. The diversity of symbiont inheritance may vary between and even within a single parent. Vertical transmission secures the offspring having symbionts, however, these may not be advantageous if the offspring lives in an environment that differs from that of their parent. In opposite, in horizontal symbiont transmission the offspring must acquire the symbiont form the environment which might be advantageous in a new environment but could also mean that beneficial symbionts gained in the last generation would have been lost.

In presence of global warming, neither vertical nor horizontal symbiont alone provides mechanism for the adaptation of an obligate symbiosis to the changing environment. This may be the reason for loss of whole ecosystems. Only combining both acquisition strategies may provide mechanisms for adaptation, Byler et al. (2013) therefore investigated whether or not both symbiont transmission modes could occur in an eukaryote invertebrate obligate symbiosis using the coral Stylophora pistillata and its symbiotic dinoflagellate Symbiodinium. For the purpose of this study, samples were collected from two depths, shallow and deep. The Symbiodinium genetic identity in S. pistillata adults, their released planulae and juvenile colonies were examined using technics such as DNA Extraction, amplification, denaturing-gradient gel electrophoresis and a real-time PCR to detect both abundant and possible low-level symbiont populations. 

As S. pistillata planulae contained only one Symbiodinium type while some of the juvenile colonies at shallow and deep depths harboured mixed symbioses, this study provides evidence for the possibility of both symbiont acquisition modes, horizontal and vertical. This is a great start point but further investigations are needed as there is no evidence that juveniles that acquired symbionts from the environment actually survive to adulthood. Recording the whole coral life-cycle would be a challenge, considering corals being such a slow growing organism. However, if it was possible that coral juveniles that show both vertical and horizontal symbiont transmission maintain advantageous symbionts into adulthood and later on inherited them to their offspring, novel Symbiodinium could be maintained over generations. Such adaptation to the environment as an evolutionary mechanism can potentially establish novel host symbiont combinations that may be advantageous during changing environmental conditions. Even though only 35% of coral species vertically transmit Symbiodinium, these species belong to several widely distributed dominant coral genera. The future of coral ecosystems may not be as depressing as we thought but in the end we are still talking about corals, sparsely sexual reproducing and painfully slow growing in a fast-paced world.

Byler K. A., Carmi-Veal M., Fine M. & Goulet T. L. (2013) Multiple Symbiont Acquisition Strategies as an Adaptive Mechanism in the Coral Stylophora pistillata. PLoS ONE 8(39): e59596. doi:10.1371/journal.pone.0059596