Reactive Oxygen Defense Against Cellular Endoparasites and the Origin of Eukaryotes

Abstract

In this chapter, we propose a model for the early evolution of eukaryotic cells under pressure of intense endoparasitism. We define features of eukaryotes developed to defend against endoparasites (primarily bacteria), including a defensive system composed of an antioxidant sterol-enriched internal and external membrane system that could be used to entrap endoparasites and degrade them with superoxide produced on the membranes, cytoskeleton scaffolding for the membrane system, and a nuclear envelope to exclude endoparasites from reaching the genome. Mitochondria and chloroplasts evolved from the prokaryotes that developed ways to neutralize the reactive oxygen defense of the host. For mitochondria, hydrogen pumping to the exterior of the endoparasite enabled them to reduce superoxide to water, effectively defeating the host defense. Other features of eukaryotes that may have evolved from defense from endoparasitism include: autophagy, cell walls in fungi and plants, acquired immunity in animals, multicellularity, and apoptosis. We evaluate fossil data, where available, to provide additional information regarding the early evolution of eukaryotes and the prevalence of endoparasitic microbes. © 2018 Elsevier Inc. All rights reserved.