IGF2R AND IGF2 GENE-EXPRESSION IN ANDROGENETIC, GYNOGENETIC, AND PARTHENOGENETIC PREIMPLANTATION MOUSE EMBRYOS - ABSENCE OF REGULATION BY GENOMIC IMPRINTING

Genomic imprinting in mammals is believed to result from modifications to chromosomes during gametogenesis that inactivate the paternal or m aternal allele. The genes encoding the insulin-like growth factor type 2 (Igf2) and its receptor (Igf2r) are reciprocally imprinted and expr essed from the paternal and maternal genomes, respectively, in the fet al and adult mouse. We find that both genes are expressed in androgene tic, gynogenetic, and parthenogenetic preimplantation mouse embryos. T hese results indicate that inactivation of imprinted genes occurs post fertilization (most likely postimplantation) and that genomic imprinti ng and gene inactivation are separate processes. We propose that impri nting marks the chromosome so that regulatory factors expressed in cel ls at later times can recognize the imprint and selectively inactivate the maternal or paternal allele. For these genes, this finding invali dates models of genomic imprinting that require them to be inactive fr om the time of fertilization.