Genomic Imprinting in Viviparous Fishes

University Of Connecticut, Storrs CT

Investigators

Abstract

Higher organisms contain two copies, or alleles, of every gene, one inherited maternally and the other inherited paternally, and for most genes both copies are expressed. Having two copies of every gene, known as diploidy, is thought to protect organisms from the deleterious effects of recessive mutations. For imprinted genes, however, only one copy is expressed. Which copy of an imprinted gene pair is to be expressed is determined by which parent it was inherited from. In other words, some genes are expressed only when inherited maternally and some only when inherited paternally. About forty genes have been identified in mice, humans and other mammals that show imprinted patterns of expression, and there is evidence to suggest that there may be 200 or more imprinted genes in the mammalian genome. Because silencing of one allele of a gene pair abolishes the protection against recessive mutation, it is difficult to explain why such a system of gene expression exists and how it evolved. Of several evolutionary theories proposed to explain genomic imprinting, only one, the conflict model, has been broadly supported by molecular genetic, population genetic and embryological evidence. The conflict model posits that genomic imprinting is the outcome of a conflict between parents over their relative contributions to developing offspring. In mammals, the mother is the sole source of nutrients to gestating young. In polygamous breeding animals, the genes transmitted by fathers will evolve mechanisms, or imprints, to take advantage of the mothers exclusive resource input to his genetic offspring. This puts the mothers genes at a disadvantage since focussing resources on the offspring of one father compromises her output to the offspring of her other mates and may reduce her reproductive output altogether. Maternally transmitted genes have, therefore, evolved mechanisms to counteract the imprinted genes transmitted paternally. The conflict model predicts the general rule that growth-enhancing genes are paternally active and maternally silent, and that growth-suppressing genes are maternally active and paternally silent. This type of imprinted gene expression is peculiar to mammals, which are all viviparous, and in all of which developing embryos are supported by a placental connection with the mother. An essential test of the conflict model is to search for imprinted genes in animals that have evolved viviparity and placentation independent of mammals. This project investigates whether a genus of viviparous fish, Poeciliopsis, has evolved genomic imprinting in the context of parental conflict. This work promises not only to test the evolutionary model, but also to provide a greater insight into the molecular mechanisms underlying imprinted gene expression and the function of imprinting in embryonic development.