Abstract
While almost all mammalian genes express their maternally and paternally inherited copies equally, a small subset of genes (~100 in humans) are subject to parent-specific expression. This peculiar form of gene expression, known as genomic imprinting, is controlled by an epigenetic mechanism involving differential DNA methylation and histone modification that is dependent on the sex of the parent from which it is inherited. These genes play their biggest role in development where a parent-offspring conflict arises over parental investment to a growing fetus. Here, paternally expressed genes act to maximise embryonic and fetal growth, even if this restricts maternal reproductive fitness and offspring sired by other fathers. In contrast, maternally expressed genes act to limit fetal growth and share maternal resources equally, irrespective of who the father is.
In marsupials, an ancient mammalian lineage known for their unique and precocious development, genomic imprinting has been observed, yet on a far smaller scale than their eutherian mammal cousins. It has been hypothesised that this difference is due to the short-lived and less invasive marsupial placenta, meaning there is less opportunity for paternally-derived genes to influence maternal resources for the fetus. Previous investigations into genomic imprinting in marsupials have focused on the Australian tammar wallaby and the South American gray short-tailed opossum; however, New Zealand’s large and hybridised population of common brushtail possums, whose genome has recently been sequenced, provides a great opportunity to study the expression of single nucleotide polymorphisms (SNPs) within these imprinted genes in marsupials.
A search for SNPs within known mammalian imprinted genes was conducted in the brushtail possum, as well as a genome-wide search for mono-allelically expressed SNPs within candidate marsupial-specific imprinted genes. DNA amplicons were created to genotype these SNPs in multiple individuals and by creating and using pre-existing RNA sequencing datasets, the expressed allele of these SNPs was assessed to determine monoallelic expression in heterozygous individuals.
For two known mammalian imprinted genes, IGF2R and H19, monoallelic expression was discovered in the brushtail possum; however, the absence of any pairs of homozygous mothers and heterozygous pouch young meant that parent-specific imprinting of these genes could not be confirmed. Preliminary results showed that IGF2, another known imprinted gene, was expressed monoallelically in pouch young but biallelically in possum 'back-rider' juveniles and adults. Efforts to find novel, marsupial-specific imprinted genes initially looked promising, but were later found to be pseudogenes showing false monoallelic expression. Future directions should aim to confirm the monoallelic expression of IGF2R and H19 as genomic imprinting by examining more mother-offspring pairs as well as confirming the switch from monoallelic to biallelic expression of IGF2 as the possum leaves its pouch.