Successful hybridization between distinct species de facto results in the creation of a new genome, providing a fascinating experimental model for evolutionary genomics. Merging two divergent genomes necessitates extensive chromatin reconfigurations and is often accompanied by genome duplications and chromosomal rearrangements. These dramatic genomic modifications trigger cascades of novel expression patterns and regulatory interactions.
One of the most intriguing consequences of hybridization as a ‘genome shock’ is the mobilization and transposition of mobile (transposable) elements (TEs), a phenomenon first predicted by the discoverer of transposition herself, Barbara McClintock, and then observed in a number of case studies (reviewed by Michalak, 2009). TEs can be classified as DNA transposons and retrotransposons. The former move within genomes as DNA fragments through a ‘cut-and-paste’ mechanism, whereas the latter duplicate through reverse-transcribed RNA intermediates (a ‘copy-and-paste’ mechanism). Retrotransposons can be divided into long terminal repeat (LTR)- and non-LTR retrotransposons based on the presence or absence of LTRs, a structural feature that they share with retroviruses. Given that mobile elements account for a large fraction of eukaryotic genomes (not uncommonly exceeding 50% of their content), their dynamics in hybrid genomes is by no means of trivial significance to our understanding of genome evolution.