Genomic time-series data may reveal the true magnitude of horizontal exchange between species.

Species largely evolve in isolation, with little exchange of genetic material between them. Occasionally, however, mechanism like horizontal transfer disrupt this isolation and transfers genetic information between species. Transposable elements (TEs), short DNA sequences that can multiply within genomes, are frequently involved in such exchanges. The frequency of horizontal transposon transfer (HTT) between species remains a fundamental open question in biology, as classical sequence comparison approaches may substantially underestimate the true extent of HTT. In this review, we highlight that genomic time-series (GTS), i.e., genomes of specimens sampled across different years in the past, enable an alternative approach that may provide a more unbiased estimate of the true magnitude of HTT between species. Analysis of GTS in fruit flies suggests that HTT may be rampant, with 12 events found over the past 200 years. Species may thus exchange genetic material far more frequently than previously thought. We discuss the pros and cons of different approaches for detecting HTT, show how GTS can be generated for diverse species, and highlight various strategies for identifying HTT events. GTS may generally constitute a valuable resource for addressing a broad range of evolutionary questions.