Jumping genes and epigenetics

Speciation is the evolutionary process by which new species emerge. This can happen through several mechanisms, for example, a population can become geographically split and no longer interbreed. The two subgroups will then slowly form new species just by random drift alone. In general any change that forces a split in interbreeding will have a similar effect, as for example a strong selective sweep, or chromosomal rearrangements.

Each chromosome is made of a molecule of double-stranded DNA. If the double strand breaks, mechanisms within the cell will attempt to repair the molecule. However, the mechanism is not perfect and it could end up joining together different broken ends, in which case a new chromosomal rearrangement takes place. They can arise through deletions, duplications, inversions, and translocations. If the change is such to prevent a subgroup of the population to interbreed with the rest of the population, speciation may indeed occur.

In [1], Rebollo et al. discuss how transposable elements can induce speciation by causing chromosomal rearrangements. A quick refresher: a transposable element (TE) is a DNA sequence that can move from one place to another within the same genome. It can make copies of itself that are then inserted at different DNA locations. We "inherited" some of them when an ancient virus got "stuck" in our germ line and became part of our genome (see here and here).

Rebollo et al. notice how epigenetic responses to environmental stimuli can trigger a burst of TE transposition, which in turn can induce chromosomal rearrangements. Therefore they suggest that TE bursts of transposition might induce new speciation in response to environmental changes.

TEs are intriguing because on the one hand they are highly mutagenic (they induce mutations), and in many cases have been associated with cancer and disease. However, if they were truly deleterious to organisms, you would expect them to slowly disappear from genomes. Instead, they are not only quite abundant in mammalian genomes, they seem to increase with genomic complexity, indicating that they may impart an evolutionary advantage.

"TE abundance, TE-derived genomic features and chromosomal rearrangements involving TE sequences are frequently lineage specific and, therefore, suggest that TEs have contributed to the process of speciation, either as a cause, or an effect."

"Significant TE activity is observed in several species, often during periods of radiation, suggesting that massive speciation and massive TE activity may be associated. The genetic distance between two organisms is calculated as a function of their genetic divergence, so every episode that creates divergence, such as lineage-specific transposition events, could contribute to the reproductive isolation of those organisms. TE patterns that differ between individuals of the same species, whether as a cause or a consequence of genetic differentiation, may not only provide genetic markers for researchers, but also constitute evidence of a speciation process occurring within the species concerned."

Transposons are strictly regulated by epigenetic mechanisms that follow different pathways, from noncoding RNAs, to chromatin remodeling and DNA methylation. Interestingly, despite being very rigorous, this epigenetic regulation of TEs is also prone to variation. Its flexibility can induce TE transposition in the germline and, as a consequence, chromosomal rearrangements.

"Although the benefit is not immediate, transposition might have a long term advantage. Indeed, transposition bursts have numerous consequences, resulting in a renewal of genetic diversity, which is the major prerequisite for genome evolution and selection to occur."

[1] Rebollo, R., Horard, B., Hubert, B., & Vieira, C. (2010). Jumping genes and epigenetics: Towards new species Gene, 454 (1-2), 1-7 DOI: 10.1016/j.gene.2010.01.003