164. THE IMPRINT STATUS AND EXPRESSION OF INS IN THE TAMMAR WALLABY, MACROPUS EUGENII

Abstract

Genomic imprinting is an epigenetic mechanism that differentially regulates the expression of certain genes, resulting in expression from only one parental allele. It is presumed to have first evolved after the divergence of therian mammals from the monotremes. One imprinted gene, INS is maternally imprinted (paternally expressed) in the eutherian and marsupial yolk sac^1,2. INS encodes the precursor to the hormone insulin, which regulates carbohydrate metabolism and has a role in cell growth and, by regulating amino acid and fatty acid transporters, protein synthesis. In rats, mice and several other mammals insulin, in addition to cortisol and prolactin, is an absolute requirement for the onset of lactation and the synthesis of milk³. As imprinting plays an important role in regulating nutrition and growth the role of imprinted genes in the placenta has been the focus for imprinting research. Since the mammary gland provides a critical source of nutrition for the neonate in all mammals it is possible that genomic imprinting may have developed and been maintained in this organ. Given that marsupials deliver tiny, altricial young, it is in the relatively long and complex lactation phase where the mother has most control of the young's growth. Therefore, there may be greater selection for genomic imprinting in the marsupial mammary gland than in the eutherian mammary gland. This study examined the expression and the imprint status of INS in the mammary gland and neonatal tissues of the tammar wallaby, Macropus eugenii. INS expression was detected using PCR and direct sequencing provides evidence of INS imprinting in the mammary gland. This is the first study to identify imprinting in the mammary gland of a marsupial and the first to identify INS imprinting outside of the yolk sac.