Endosymbiosis: metabolic trade-offs drive reproductive synchronization

Endosymbiotic relationships have fueled the evolution of complex life. The persistence of these relationships relies on coordinating the reproduction of hosts and their guests. Yet, such coordination may come at a cost, as one or both partners may need to slow their growth to synchronize. Here, we examine the fitness consequences of synchronization by using a combination of mathematical and bacterial metabolic models. Analyzing millions of putative host-guest pairings, we find that synchronization is typically costly for hosts but beneficial for guests. We show that synchronization can occur when hosts relinquish metabolic resources to guests. Given the costs to hosts, we investigate whether hosts can benefit from growing faster, at the risk of losing their endosymbionts. Our mathematical model suggests this outcome is theoretically possible, but metabolic modeling consistently indicates that synchronization increases overall fitness-- provided the formation of new endosymbioses is rare. When new relationships can readily form, the optimal strategy shifts: hosts maximize fitness by growing as fast as possible, leading to regular cycles of loss and reacquisition of their guests. Overall, our findings indicate that while synchronization is costly for hosts, it offers long-term fitness benefits in contexts where endosymbionts are not easily replaced.