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David Barondess


Primate adaptations such as enlarged brains have long been of interest to physical anthropologists. Most studies of primate adaptations have focused on morphological and anatomical aspects of adaptations. The advent of molecular biological techniques such as PCR allows the rapid sequencing of primate genes, and facilitates the documentation of positive Darwinian selection in primate lineages. This dissertation examines some of the adaptive molecular events that occurred in the evolution of aerobic energy metabolism in primates, and which may be linked to expansion of the anthropoid neocortex. Cytochrome c oxidase (COX), complex IV of the electron transport chain (ETC) located in mitochondria, is an enzyme important in aerobic energy metabolism, as it catalyzes the transfer of electrons from cytochrome c to oxygen. In mammals COX contains thirteen subunits, three of which are encoded by mitochondrial DNA, and the remainder by nuclear DNA. The mitochondrial-encoded subunits carry out the catalytic functions of the enzyme, and the nuclear-encoded subunits are thought to be involved in regulation and assembly of the holoenzyme. In mammals four of the nuclear-encoded subunits exist as isoforms, having tissue-specific and developmental patterns of expression. COX subunit VIII consists of a ubiquitously expressed isoform encoded by COX8L (L = liver isoform), and a muscle-specific isoform encoded by COX8H (H = heart isoform). Previous studies showed that the gene duplication that gave rise to the COX8 isoforms preceded the mammalian radiation, but that COX8H has been inactivated in humans and macaques. Here I show that COX8H is expressed in most primate clades, but coding region deletions rendered it a pseudogene in the stem of the catarrhines, between 25 and 40 million years ago. Furthermore, the rate of COX8L nonsynonymous substitutions accelerated in anthropoid evolution, indicative of positive selection for adaptive change. Inactivation of COX8H and accelerated rates of nonsynonymous substitutions in COX8L are concurrent with, and part of the positively selected co-adaptive changes that occurred in the ETC during anthropoid evolution. I propose that neocortical expansion in anthropoids has been a powerful selective force in the evolution of many components of the ETC, including COX subunit VIII-L.

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