Questions: Transposable Elements and Evolution

Question 1

Which of the following is the best example of TE exaptation — the co-option of a transposable element sequence for a new host function?

• A. A LINE element inserting into an exon and disrupting the encoded protein
• B. The RAG1 and RAG2 recombinase enzymes, which drive V(D)J recombination in the vertebrate immune system and evolved from an ancient transposase
• C. DNA methylation silencing active TEs throughout the genome
• D. A retrotransposon increasing from 10 to 10,000 copies across generations

Answer: (B)

Exaptation is the evolutionary co-option of an existing structure or sequence for a new function different from its original one. The RAG1/RAG2 system is a canonical example: what was once a transposase — an enzyme that mobilized a DNA transposon — was co-opted to perform the site-specific recombination that generates antibody and T-cell receptor diversity in jawed vertebrates. This illustrates the key insight that TEs are not merely parasites but a major source of evolutionary novelty. Options A and C represent host-TE conflict; option D illustrates TE replication, not exaptation.

Question 2

A researcher observes that a mobile element in a plant genome has increased from approximately 50 copies to over 5,000 copies over the course of 50 generations. Which mechanism most likely explains this copy number increase?

• A. Cut-and-paste transposition by a Class II DNA transposon, where the element excises and reinserts elsewhere
• B. Copy-and-paste retrotransposition: the element is transcribed to RNA, reverse-transcribed to DNA, and the new copy integrates while the original remains
• C. Ectopic recombination between existing copies amplifying them through tandem duplication
• D. The transposase enzyme replicating independently of the host genome

Answer: (B)

Class I retrotransposons use a copy-and-paste mechanism: the element is transcribed into RNA, then reverse-transcribed back into DNA by a reverse transcriptase encoded by the element itself, and the new DNA copy integrates at a new genomic location — leaving the original intact. This inherently increases copy number with each successful transposition event, explaining how retrotransposons can reach hundreds of thousands of copies (e.g., human Alu elements: ~1 million copies). Class II DNA transposons (cut-and-paste) do not inherently increase copy number because the original is excised; the only way they gain copies is if the donor site is replicated before transposition.

Question 3

Class II DNA transposons increase their copy number with each transposition event because the original element remains in place while a new copy is inserted at a different location.

• T. True
• F. False

Answer: False

This description actually applies to Class I retrotransposons (copy-and-paste). Class II DNA transposons use a cut-and-paste mechanism: the transposase enzyme excises the element from its current location and inserts it elsewhere. The original copy does not remain — it is moved, not copied. Copy number stays the same (or can actually decrease if excision is imprecise). DNA transposons can occasionally achieve net copy number gains if transposition happens during S phase after the donor site has been replicated but before the target site has been, but this is not inherent to the mechanism.

Question 4

Although most TE copies in mammalian genomes are inactive or silenced, TEs have been a significant source of regulatory sequences, novel proteins, and evolutionary innovations over deep evolutionary time.

• T. True
• F. False

Answer: True

Despite the fact that most TE copies are mutated into silence or actively suppressed by DNA methylation and small RNA pathways, the sheer abundance of TE sequences (~45% of the human genome, >80% of maize) means that even a tiny fraction of exaptation events produces enormous numbers of new regulatory elements and functional sequences. Documented examples include RAG1/RAG2 (adaptive immunity), syncytin proteins (placental development from retroviral envelope genes), and thousands of enhancers and promoters derived from TE sequences. TEs are a primary driver of regulatory evolution and genomic novelty.

Question 5

Why does the host genome evolve mechanisms to suppress TE activity, and how can TEs nonetheless contribute to evolutionary novelty despite this suppression?

Model answer: TEs are suppressed because they are primarily harmful in the short term: insertions into genes disrupt function, and ectopic recombination between dispersed TE copies causes chromosomal rearrangements (deletions, inversions, duplications). The host genome and its TEs exist in an evolutionary arms race, with TEs 'seeking' to replicate and the host evolving DNA methylation, piRNA pathways, and other silencing mechanisms to keep them in check. Despite this suppression, TEs contribute to novelty over deep evolutionary time in two ways: first, occasional insertions in regulatory regions can fortuitously improve fitness and be preserved by natural selection; second, TE-derived sequences can be exapted — their coding or regulatory potential repurposed entirely — as occurred with the RAG recombinase and syncytin. The critical insight is that a mechanism that is harmful on average can still be a net source of innovation across millions of years of evolution.

The host-TE relationship is often called 'genomic parasitism,' but this framing is incomplete. TEs are more accurately understood as an ongoing source of mutational variation — mostly neutral or deleterious, occasionally beneficial — at a scale that single-nucleotide mutations cannot match.