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 ismade 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

Measuring and interpreting transposable element expression

International audienceTransposable elements (TEs) are insertional mutagens that contribute greatly to the plasticity of eukaryotic genomes, influencing the evolution and adaptation of species as well as physiology or disease in individuals. Measuring TE expression helps to understand not only when and where TE mobilization can occur, but also how this process alters gene expression, chromatin accessibility or cellular signalling pathways. Although genome-wide gene expression assays such as RNA-sequencing include transposon-derived transcripts, the majority of computational analytical tools discard or misinterpret TE-derived reads. Emerging approaches are improving the identification of expressed TE loci and helping to discriminate TE transcripts that permit TE mobilization from gene-TE chimeric transcripts or pervasive transcription. Here, we review the main challenges associated with the detection of TE expression, including mappability, insertional and internal sequence polymorphisms, and the diversity of the TE transcriptional landscape, as well as the different experimental and computational strategies to solve them