**2.2.3 Evolution of the Callipyge imprinted locus**

The Callipyge locus, so named after a muscle trait observed in sheep, contains a cluster of three paternally expressed genes (*DIO3, DLK1, RTL1*). In order to carry out a comprehensive analysis of this locus, seven platypus and 13 wallaby overlapping BAC clones were fully sequenced and assembled into a single contig for each species (Edwards et al., 2008). Comparative genome analysis revealed that the genomic landscape of this locus has undergone a number of changes during mammalian evolution. In marsupials, the locus is twice the size of the orthologous region in eutherians as a result of an accumulation of LINE1 repeats. In addition, there has been selection against SINE repeats in eutherians along with an increase in GC and CpG island content. Over 140 evolutionary conserved regions were found by phylogenetic footprinting but none of these regions corresponded to the imprint control element identified in eutherians. These findings were consistent with the absence of imprinted expression for this locus both in monotremes and marsupials. Similar to the situation described above for the Prader-Willi/Angelman locus, it appears that a retrotransposition event resulted in the formation of a novel gene in eutherians and it was suggested that this may have been the driving force behind the evolution of imprinting at this locus (Edwards et al., 2008).
