Rjw. Oneill et al., UNDERMETHYLATION ASSOCIATED WITH RETROELEMENT ACTIVATION AND CHROMOSOME REMODELING IN AN INTERSPECIFIC MAMMALIAN HYBRID, Nature, 393(6680), 1998, pp. 68-72
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.