UNDERMETHYLATION ASSOCIATED WITH RETROELEMENT ACTIVATION AND CHROMOSOME REMODELING IN AN INTERSPECIFIC MAMMALIAN HYBRID

Genetic models(1,2) predict that genomic rearrangement in hybrids can facilitate reproductive isolation and the formation of new species by preventing gene flow between the parent species and hybrid (sunflowers are an example(3)). The mechanism underlying hybridization-induced ch romosome remodelling is as yet unknown, although mobile element activi ty has been shown to be involved in DNA rearrangement in some dysgenic Drosophila hybrids(4,5), It has been proposed that DNA methylation ev olved as a means of repressing the movement of mobile elements (the ho st defence model(6,7)). If such a protective mechanism were to fail, m obile elements could be activated, and could cause major and rapid gen ome alterations(8,9), Here we demonstrate the occurrence of genome-wid e undermethylation, retroviral element amplification and chromosome re modelling in an interspecific mammalian hybrid (Macropus eugenii X Wal labia bicolor). Atypically extended centromeres of Macropus eugenii de rived autosomes in the hybrid were composed primarily of an unmethylat ed, amplified retroviral element not detectable in either parent speci es. These results, taken with the observation of deficient methylation and de novo chromosome change in other mammalian hybrids, indicate th at the failure of DNA methylation and subsequent mobile-element activi ty in hybrids could facilitate rapid karyotypic evolution.