Monotreme IGF2 expression and ancestral origin of genomic imprinting

IGF2 (insulin-like growth factor 2) and M6P/IGF2R (mannose 6-phosphate/insu lin-like growth factor 2 receptor) are imprinted in marsupials and eutheria ns but not in birds. These results along with the absence of M6P/IGF2R impr inting in the egg-laying monotremes indicate that the parental imprinting o f fetal growth-regulatory genes may be unique to viviparous mammals. In thi s investigation, we have cloned IGF2 from two monotreme mammals, the platyp us and echidna, to further investigate the origin of imprinting. We report herein that like M6P/IGF2R, IGF2 is not imprinted in monotremes. Thus, alth ough IGF2 encodes for a highly conserved growth factor in chordates, it is only imprinted in therian mammals. These findings support a concurrent orig in of IGF2 and M6P/IGF2R imprinting in the late Jurassic/early Cretaceous p eriod. The absence of imprinting in monotremes, despite apparent interparen tal conflicts over maternal-offspring exchange, argues that a fortuitous co ngruency of genetic and epigenetic events may have limited the phylogenetic breadth of genomic imprinting to therian mammals. (C) 2001 Wiley-Liss, Inc .