Genomic imprinting is a form of epigenetic gene regulation that plays a key role in placental development. Imprinted expression is highly enriched within developmental pathways and, as a consequence, the disruption of imprinted pathways results in a range of congenital developmental disorders in humans and other mammals. Mammalian hybrids often manifest many of the same placental growth abnormalities, raising the intriguing possibility that recurrent developmental syndromes within and between species may reflect the disruption of common regulatory pathways. However, surprisingly little is known about the evolution of imprinted regulatory networks, and the contribution of disrupted genomic imprinting to the evolution of reproductive barriers between species remains unresolved. The proposed research will begin to overcome these fundamental gaps in knowledge by generating novel comparative genomic data on placental gene expression and genomic imprinting among closely related species and their reciprocal hybrids. Specific Aims 1 and 2 will use a series of hybrid rodent systems to generate an atlas of genome-wide placental transcription, DNA methylation, and chromatin structure across five species and two major lineages of rodents (house mice and dwarf hamsters). These unprecedented comparative data will allow us to resolve the basic epigenetic mechanisms controlling genomic imprinting and quantify how placental expression and imprinting has evolved over 30 million years of divergence. Specific Aim 3 will use a systems genetics approach to examine the genetic architecture of hybrid overgrowth, placental expression, and imprinting in dwarf hamsters and mice. These experiments will allow us to understand the mechanistic and genetic underpinnings of a common form of placental dysplasia and test the novel hypothesis the X chromosome plays a central role in the regulation of imprinted autosomal regulatory networks. The long-term goals of this research program are to (1) illuminate the basic epigenetic mechanisms controlling genomic imprinting, (2) provide insights into the evolutionary tempo and theoretical drivers of imprinting and other aspects of placental expression, (3) test hypotheses on the causal connection between disrupted imprinting and abnormal placental development, (4) understand the organization and evolution of imprinted expression networks, and (5) link these conceptual advances to understand the contribution of genomic imprinting to the origin of biological diversity.