Genetic conflict, genomic imprinting and establishment of the epigenotype in relation to growth

Genomic imprinting is the process that differentially modifies the parental alleles at certain genetic loci in the parental germlines. Such modificati ons of DNA and chromatin are somatically heritable and cause unequal expres sion of the parental alleles during subsequent development. In mammals, imp rinted genes encode a relatively small number of functionally heterogeneous proteins. Nevertheless, imprinted genes exert important effects, primarily on fetal development, and their deregulation is implicated in a variety of pathologies including sporadic, inherited and induced growth disorders. Im printed loci show several unusual structural and functional characteristics that may be related to mechanistic aspects of mono-allelic expression or t o modes of evolution of imprinted genetic loci. Typically, imprinted genes are clustered in certain genomic regions and have relatively reduced intron ic DNA content relative to non-imprinted genes. In addition, their regulato ry regions frequently contain a combination of features including tandem re peats associated with differentially methylated CpG islands and overlapping transcription of coding or non-coding RNAs. The evolution of imprinting ca n be understood as the stable outcome of sexual selection acting differentl y on the parental alleles of genes that influence parental investment in of fspring. Consistent with this explanation, imprinted genes are expressed pr edominantly during embryonic and postnatal development in mammals and in th e developing endosperm of plants, and maternal or paternal expression at im printed loci is associated with reduced or increased parental investment, r espectively. Such selective forces have implications for understanding mech anistic aspects of genome reprogramming in the early mammalian embryo.