Vertebrate Genome Size and the Impact of Transposable Elements in Genome Evolution

In eukaryotes, the haploid DNA content (C-value) varies widely across lineages without an apparent correlation with the complexity of organisms. This incongruity has been called the C-value paradox and has been solved by demonstrating that not all DNA is constituted by genes but, on the contrary, most of it is made up of repetitive DNA. In vertebrates, the increasing number of sequenced genomes has shown that differences in genome size between lineages are ascribable to a variation in transposon content. These mobile elements, previously perceived as junk DNA or selfish DNA, are now recognized as the major players in shaping genomes. During vertebrate evolution, transposable elements have been repeatedly co-opted and exapted to generate regulatory sequences, coding exons, or entirely new genes that lead to evolutionary advantages for the host. Moreover, transposable elements are also responsible for substantial rearrangements such as insertions, deletions, inversions, and duplications potentially associated with, or following, speciation events.