Allelic expression of IGF2 in marsupials and birds

Genomic imprinting, the parent-of-origin-specific expression of genes, has been observed in a variety of eutherian mammals. One gene that has been sho wn to be imprinted in all eutherians examined is the IGF2 gene. This gene e ncodes a potent fetal-specific growth factor that is expressed almost exclu sively from the paternal chromosome. Several other imprinted genes in the I GF2 pathway are imprinted as well, suggesting that IGF2 is a focal point fo r the selective pressure leading to imprinted gene expression. This observa tion is in keeping with a proposal that imprinting arose as the result of a genetic conflict between parents over the allocation of maternal resources to the embryo. One prediction of this model is that imprinting exists in s pecies in which there is at least some contribution of maternal resources t o the embryo, and in which polyandry is observed. To test this prediction t he allelic expression of the IGF2 gene was examined in two noneutherian spe cies. The IGF2 gene was shown to be expressed in a paternal-specific manner identical to that in eutherians in Monodelphis domestica, a placental Sout h American opossum. In contrast, the IGF2 gene is biallelic in expression i n chickens, which are oviparous, and make no postfertilization contribution of maternal resources to the offspring.