Allelic Expression of IGF2 in Live-bearing, Matrotrophic Fishes

The parental conflict, or kinship, theory of genomic imprinting predicts that parent-specific gene expression may evolve in species in which parental investment in developing offspring is unequal. This theory explains many aspects of parent-of-origin transcriptional silencing of embryonic growth regulatory genes in mammals, but it has not been tested in any other live-bearing, placental animals. A major embryonic growth promoting gene with conserved function in all vertebrates is insulin-like growth factor 2 (IGF2). This gene is imprinted in both eutherians and marsupials, as are several genes that modulate IGF2 activity. We have tested for parent-of-origin influences on developmental expression of IGF2 in two poeciliid fish species, Heterandria formosa and Poeciliopsis prolifica, that have evolved placentation independently. We found IGF2 to be expressed bi-allelically throughout embryonic development in both species.