Dissertation Research: The Origin and Molecular Evolution of the Genes Encoding the Primate Hormone Chorionic Gonadotropin

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Chorionic gonadotropin (CG) is a hormone which has a critical signaling role in establishing pregnancy. The hormone has been found exclusively in primates, and its genetic basis has been characterized only in humans and in two monkey species. No other mammals studied to date possess this hormone. This study will focus on elucidating the currently unknown origin and molecular evolutionary history of the genes encoding the two CG subunits. The researchers will test numerous primate and related mammalian species for the presence of genes encoding the beta subunit of CG (CG-beta). How is it that humans have come to possess six copies of this gene, and baboons five copies, while other mammals have none? In which species did the gene first arise? The well-established techniques of Southern blotting and hybridization will be used to test for the presence of CG-beta genes and to map the number of gene copies present in each species' genome. The origin of the CG-beta genes will be traced by mapping the genes' presence or absence onto a tree showing the known relationships of the species tested. Also, for those species possessing the gene, CG-beta genes will be sequenced using the polymerase chain reaction (PCR). DNA substitution rates and patterns will be analyzed to reveal whether and when selection has adapted CG to its current role.Chorionic gonadotropin presents a rare opportunity to study a gene from its origin to subsequent proliferation. Therefore it promises to make a unique contribution to the study of molecular evolution. This study will also provide valuable information about the evolution of the reproductive process in Primates. We know that the structure of the placenta has changed dramatically in primate evolution, from a placenta which is non-invasive in the lower primates to one which highly invades the mother's tissues and establishes an intimate connection between mother and fetus in humans. Some of this morphological change must be correlated with changes in the genes controlling reproductive function. CG is one of probably several genes responsible for this major remodeling of the primate reproductive process over time. Working out the evolution of the genes for the CG hormone, which is crucial for human reproduction yet does not exist outside the primates, is an important step in understanding how human reproduction has evolved.