Abstract
BackgroundIn eutherian mammals, genomic imprinting is critical for normal placentation and embryo survival. Insulin-like growth factor 2 (IGF2) is imprinted in the placenta of both eutherians and marsupials, but its function, or that of any imprinted gene, has not been investigated in any marsupial. This study examines the role of IGF2 in the yolk sac placenta of the tammar wallaby, Macropus eugenii.ResultsIGF2 mRNA and protein were produced in the marsupial placenta. Both IGF2 receptors were present in the placenta, and presumably mediate IGF2 mitogenic actions. IGF2 mRNA levels were highest in the vascular region of the yolk sac placenta. IGF2 increased vascular endothelial growth factor expression in placental explant cultures, suggesting that IGF2 promotes vascularisation of the yolk sac.ConclusionThis is the first demonstration of a physiological role for any imprinted gene in marsupial placentation. The conserved imprinting of IGF2 in this marsupial and in all eutherian species so far investigated, but not in monotremes, suggests that imprinting of this gene may have originated in the placenta of the therian ancestor.
Highlights
In eutherian mammals, genomic imprinting is critical for normal placentation and embryo survival
Insulin-like growth factor 2 (IGF2) protein was detected in the trophoblast and yolk sac endoderm of bilaminar and trilaminar regions, but rarely in the mesenchymal or endothelial cells of vitelline vessels (Fig. 1)
IGF2R protein co-localised with IGF2 in the yolk sac, but was detected in the cell membrane in addition to the cytoplasm (Fig. 2)
Summary
Genomic imprinting is critical for normal placentation and embryo survival. Insulin-like growth factor 2 (IGF2) is imprinted in the placenta of both eutherians and marsupials, but its function, or that of any imprinted gene, has not been investigated in any marsupial. Eutherians and marsupials (therian mammals) diverged between 125 and 145 million years ago [1,2] and both develop a placenta to support embryonic growth and development. The majority of marsupials, rely exclusively on a chorio-vitelline or yolk sac placenta which consists of two structurally distinct regions. An epigenetic phenomenon in which a single allele of a gene is active from only one parental chromosome has, amongst mammals, so far only been found in therians [11,12,13,14,15].
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