Phylogenetic relatedness rather than aquatic habitat fosters horizontal transfer of transposable elements in animals
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Horizontal transfer of transposable elements (HTT) is an important driver of genome evolution, yet the factors conditioning this phenomenon remain poorly characterized. Here, we screened 247 animal genomes from four phyla (annelids, arthropods, mollusks, chordates), spanning 19 independent transitions between aquatic and terrestrial lifestyles, to evaluate the suspected positive effects of aquatic habitat and of phylogenetic relatedness on HTT. Among the 5,952 independent HTT events recovered, the vast majority (>85%) involve DNA transposons, of which Mariner-like and hAT-like elements have the highest rates of horizontal transfer, and of intra-genomic amplification. Using a novel approach that circumvents putative biases linked to phylogenetic inertia and taxon sampling, we found that HTT rates positively correlate with similarity in habitat type but were not significantly higher in aquatic than in terrestrial animals. However, modelling the number of HTT events as a function of divergence time in a Bayesian framework revealed a clear positive effect of phylogenetic relatedness on HTT rates in most of the animal species studied (162 out of 247). The effect is very pronounced: a typical species is expected to show 10 times more transfers with a species it diverged from 125 million years (My) ago than with a species it diverged from 375 My ago. Overall, our study underscores the pervasiveness of HTT throughout animals and the impact of evolutionary relatedness on its dynamics.
Significance statement
Genetic material can be transmitted between organisms through other means than reproduction, in a process called horizontal transfer. The mechanisms and factors underlying this phenomenon in animals remain unclear, although it often involves transposable elements (TEs). TEs are DNA segments capable of jumping within genomes, but also occasionally between individuals. Here, we show evidence for nearly 6,000 transfers of TEs among animals, based on genomic comparisons among 247 species of annelids, arthropods, chordates and mollusks. Contrarily to expectations, we found no excess in the rates of transfers in aquatic versus terrestrial animals. By contrast, most analyzed species appeared engaged in many more horizontal transfers with close than with distant relatives, highlighting the strong impact of phylogenetic relatedness on horizontal transfers of TEs.
