Gen0mic imprinting in mammalian development: a parental tug-0f-war

T h e term genomic imprinting has been used to refer to all cases where genes have differential expression depending on the sex of the parent from which they are inherited~-3. At least two distinct phenomena have been described as imprinting and it is important to distinguish between them. In fungus gnats (Sciaridae), gall midges (Cecidomyidae) and scale insects (Coccoidea), the paternal set of chromosomes is eliminated during spermatogenesis4, 5. This can be understood as a special form of meiotic drive, under the control of the maternal genome. Paternal chromosomes are marked for elimination in the egg cytoplasm and do not contribt~te gdnes to the next generation 4~. Imprinting in mammals and flowering plants belongs in another category, because maternal and paternal genomes have different phenotypic effects during development but contribute equally to. gametes at meiosis. Our paper presents a selective process that can explain the evolution of imprinting in mammals but does not address the underlying molecular mechanisms.