How we come to express the genes of one parent over the other is now better understood through studying the platypus and marsupial wallaby – and it doesn’t seem to have originated in association with sex chromosomes. New research published in the online open access journal, BMC Evolutionary Biology, has shed light on the evolution of genomic imprinting, in which specific genes on chromosomes that have been inherited from one parent are expressed in an organism, while the same genes on the chromosome inherited from the other parent are repressed.
Imprinting arises from some kind of ‘epigenetic memory’ – modifications to the DNA from one parent, such as the way the chromosomal material is packaged, that do not allow particular genes to be expressed. The reasons why imprinting evolved are not understood. It is known, however, that different patterns of imprinting occur in different classes of mammals, with some classes of mammals exhibiting the phenomenon and others not. Because the evolutionary relationship between mammals is well documented, patterns of imprinting in the different genomes can provide important clues about the evolution of imprinting.
One theory is that imprinted genes arose from sex chromosomes, which can be epigenetically ‘shut down’ to control the dosage of genes. Another idea is that imprinting arose from an ancestral chromosome that was itself imprinted.
A group led by father and daughter, Malcolm and Anne Ferguson-Smith, of the University of Cambridge tested these ideas by mapping known sequences of imprinted genes in two mammals, the monotreme platypus and the marsupial wallaby, which occupy distinct positions in mammalian evolution.
The results of the distribution studies suggest that imprinted genes were not located on an ancestrally imprinted chromosome, nor were they associated with sex chromosomes. Rather it appears that imprinting evolved in a stepwise, adaptive way, with each gene or cluster becoming imprinted as the need arose.
The study is also important because despite its evolutionary importance, the platypus remains cytogenetically under-characterised. By linking specific sequences to particular chromosomes, the researchers have pinpointed important markers on the platypus genome.
BioMed Central. September 2007.