The biological and evolutionary consequences of competition between DNA sequences that benefit the cell and DNA...

Mobile genetic elements (e.g. transposons and introns) are genomic parasites, DNA sequences that can proliferate within host genomes. In prokaryotes, these elements are minimized by purifying selection and host-cell defenses. In eukaryotes, however, these parasitic sequences may comprise most of the genome. This extreme imbalance in eukaryotes is maintained by a metabolically quiescent germline through which the genomic parasites are transmitted to the next generation via gametes and spread throughout the population via sexual reproduction. Some eukaryotes avoid the threat posed by genomic parasites by eliminating them from somatic cells, whereas others retain the parasites in somatic cells and attempt to suppress them by epigenetic silencing mechanisms. Here, we review the evidence for the ongoing competition between host and parasite for genome occupancy. We conclude that defenses against mobile genetic elements vary greatly among organisms, and this variation accounts for the enormous range in genome size among organisms.