"Mutual dependence and reproductive cohesion in incipient endosymbioses"Athreya, GauravEndosymbiosis is the prototypical example of an egalitarian evolutionary transition in individuality and is central to the origins of many complex biological systems. Why do only some symbioses undergo evolutionary transitions, and how does the host-symbiont relationship change during this process? In this work, we characterise endosymbiosis by two emergent collective-level properties: the relationship between the host and symbiont (mutual dependence) and reproduction (reproductive cohesion). Using methods from adaptive dynamics, we study the co-evolution of the traits underlying these properties. Our central result demonstrates robustly that even when investments in dependence and cohesion are uncorrelated, host-symbiont mutual dependence arises faster than reproductive cohesion. Further, we show that the collective's formation and shared fate, coupled with different population growth parameters, leads to an emergent asymmetry in how much they invest in the collective. Lastly, we account for biological realism in the modelling of population growth that previous models have ignored, and show that this itself can preclude a successful evolutionary transition towards stable endosymbiosis. Together, this work uncovers a fundamental property of endosymbioses, highlighting the immense effects of simple ecological factors and providing a clear way forward for theoretical and empirical investigations. |
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