Farming the mitochondrial ancestor as a model of endosymbiotic establishment by natural selection

Zachar, István and Szilágyi, András and Számadó, Szabolcs and Szathmáry, Eörs (2018) Farming the mitochondrial ancestor as a model of endosymbiotic establishment by natural selection. PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA. pp. 1-7. ISSN 0027-8424

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Abstract

The origin of mitochondria is a challenging and intensely debated issue. Mitochondria are ancestrally present in eukaryotes, and their endosymbiotic inclusion was an extremely important step during the transition from prokaryotes to eukaryotes. However, because of the unknown order of eukaryotic inventions (e.g., cytoskeleton, phagocytosis, and endomembranes), it is unknown whether they led to or followed the acquisition of mitochondria. According to the farming hypothesis, the mitochondrial ancestor was captured by a phagocytotic host, but the advantage was not direct metabolic help provided by the symbiont; rather, it was provisioning captured prey to farmers in poor times, like humans farm pigs. Our analytical and computational models prove that farming could lead to stable endosymbiosis without any further benefit assumed between partners.The origin of mitochondria was a major evolutionary transition leading to eukaryotes, and is a hotly debated issue. It is unknown whether mitochondria were acquired early or late, and whether it was captured via phagocytosis or syntrophic integration. We present dynamical models to directly simulate the emergence of mitochondria in an ecoevolutionary context. Our results show that regulated farming of prey bacteria and delayed digestion can facilitate the establishment of stable endosymbiosis if prey-rich and prey-poor periods alternate. Stable endosymbiosis emerges without assuming any initial metabolic benefit provided by the engulfed partner, in a wide range of parameters, despite that during good periods farming is costly. Our approach lends support to the appearance of mitochondria before any metabolic coupling has emerged, but after the evolution of primitive phagocytosis by the urkaryote.

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