Figure 5.

Summary illustration. The branched black arrow represents the evolutionally divergence between marsupials and eutherians which occurred at least 130-148 million years ago. The broken red arrow represents the evolution of eutherian-type gestation including the prolongation of inter-uterine development with a chorioallantoic placenta. The broken green arrow represents the evolution of the advanced complex lactation system as one of the remarkable and specialised features of marsupials. The acquisition of genomic imprinting in the KCNQ1 domain, accompanied with gene dosage reduction of CDKN1C and PHLDA2, occurred only in the evolution of the eutherian linage as well as the SNURF-SNRPN and DLK1-GTL2 domains [29,32]. On the other hand, imprinting of IGF2-H19 domain, IGF2R, PEG1/MEST and PEG10 occurred before the divergence of marsupials [26-28,30,31,33]. This study and others provide evidence that imprinting occurred at two critical time points during the evolution of mammals. For the third time point, whether marsupial-specific imprinting occurred or not, is currently still unknown.