This study experimentally examines the symbiosis between the upside-down jellyfish and its algal symbiont. This mutualism, with infectious transmission of symbionts, represents a class of symbioses for which symbiont cooperation remains unexplained. A selection experiment addresses two opposing hypotheses for the maintenance of symbiont cooperation: Partner fidelity, in which repeated interactions between partners favors cooperators over cheaters and partner choice, in which hosts can differentially reward cooperating symbionts and/or punish cheaters. Symbiont transmission between hosts is experimentally manipulated; one treatment enforces infectious transmission- between unrelated hosts (selection for cheating ) and the other vertical transmission- between parent and offspring (selection for cooperation). Fitness assays on experimentally evolved hosts and symbionts will resolve whether partner choice or partner fidelity stabilizes cooperation in this symbiosis and whether cheater symbionts exist in nature.
Mutualisms with infectious transmission of symbionts are an unsolved problem in biology. Theory states that while vertical transmission of symbionts favors cooperation, infectious transmission favors parasitism. Nonetheless, nature abounds with mutualisms in which the symbionts are horizontally acquired. Furthermore, bleaching in jellyfish and related corals represents an ecologically important, yet poorly understood breakdown of this mutualism. This study promises to help unravel the evolutionary maintenance of these poorly understood symbioses.
