PERSPECTIVE A UNITS-OF-EVOLUTION PERSPECTIVE ON THE ENDOSYMBIONT THEORY OF THE ORIGIN OF THE MITOCHONDRION.

Discussions of mitochondria and their hosts often conceptualize this relationship in a more or less modern form, focusing on the metabolic benefits of mitochondria to the host cell or on the possibility of intragenomic conflict. A more inclusive units-of-evolution perspective recognizes that both costs and benefits must be viewed from the level of the cells that initiated this interaction, the protomitochondrion and the primitive host cell. From this perspective, ecological and physiological considerations become central to the characterization of initial and subsequent host-mitochondria associations. Foremost among these considerations is the generation of superoxide radicals by modern mitochondria and the deleterious effects of these endogenous oxidants on modern eukaryotic cells. Because of their photosynthetic and aerobic ecologies, protomitochondria likely were relatively tolerant of such oxidants; anaerobic, heterotrophic, primitive host cells, on the other hand, likely were not. In the initial association of host and symbiont, the latter may have manipulated the former's life history by means of both endogenous oxidants and a superabundance of ATP. A resolution of this units-of-evolution conflict was necessary to continue this association, and this resolution, in a ritualized form, may have shaped the evolution of many features of modern eukaryotic cells and mitochondria, for example, the messenger functions of calcium ions, the regulatory role of phosphorylation cascades in cell-division cycles, the absence from the mitochondrial genome of replication factors, transcription factors, and adenine nucleotide carrier genes. The initial host-mitochondria interaction may have further channeled the evolution of multicellular eukaryotes, particularly animals, resulting in the association of mitochondria and the germinal plasm and in the use of extracellular ATP and endogenous oxidants as developmental signals. Evolutionary explanations for "free-radical" theories of development and aging are thus suggested.