MATERNAL AND PATERNAL GENOMES FUNCTION INDEPENDENTLY IN MOUSE OVA IN ESTABLISHING EXPRESSION OF THE IMPRINTED GENES SNRPN AND IGF2R - NO EVIDENCE FOR ALLELIC TRANS-SENSING AND COUNTING MECHANISMS

It has often been suggested that the parental-specific expression of m ammalian imprinted genes might be dependent on maternal-paternal inter genomic or interallelic interactions, Using quantitative allele-specif ic RT-PCR single nucleotide primer extension assays developed for two imprinted genes, Snrpn and Igf2r, we demonstrate: (i) No role for mate rnal-paternal allelic interactions: the modes of parental-specific exp ression of Snrpn and Igf2r in normal ova were unchanged in gynogenetic and androgenetic ova; the latter contain two maternal and two paterna l genomes respectively, and cannot undergo maternal-paternal interacti ons, (ii) No role for allelic counting or exclusion mechanisms: in ind ividual blastomeres of androgenetic ova, both paternal Snrpn alleles w ere active (Snrpn was not expressed in gynogenetic ova), and in indivi dual gynogenetic and androgenetic blastomeres, both maternal and pater nal Igf2r alleles, respectively, were active, (iii) No role for ploidy : the mode of parental-specific expression of Snrpn and Igf2r in norma l diploid ova was unchanged in individual blastomeres of triploid and tetraploid ova, Thus, the maternal and paternal genomes function indep endently in establishing the pre-implantation mode of parental-specifi c expression of Snrpn and Igf2r, with no role for trans-allelic/genomi c interaction phenomena. In addition, the results show that inactive a nd biallelic modes of expression of imprinted genes are potential mech anisms for the death of gynogenones and androgenones at the periimplan tation stage.