Background: Transposable elements are mobile DNA sequences, colloquially known as jumping genes because of their ability to replicate to new genomic locations. TEs can jump between organisms or species when given a vector of transfer, such as a tick or a virus, in a process known as horizontal transfer. Here, we propose that LINE-1(L1) and Bovine-B (BovB), the two most abundant transposable element families in mammals, were initially introduced as foreign DNA via ancient horizontal transfer events.Results: Using analyses of over 759 plant, fungal and animal genomes, we identify multiple possible L1 horizontal transfer events in eukaryotic species, primarily involving Tx-like L1s in marine eukaryotes. We also extend the BovB paradigm by increasing the number of estimated transfer events compared to previous studies, finding new parasite vectors of transfer such as bed bug, leech, and locust, and BovB occurrences in new lineages such as bat and frog. Given that these transposable elements have colonized more than half of the genome sequence in today's mammals, our results support a role for horizontal transfer in causing long-term genomic change in new host organisms.Conclusions: We describe extensive horizontal transfer of BovB retrotransposons and provide the first evidence that L1 elements can also undergo horizontal transfer. With the advancement of genome sequencing technologies and bioinformatics tools, we anticipate our study to be a valuable resource for inferring horizontal transfer from large-scale genomic data.Dataset: The deposited dataset contains the identified TE sequences (L1 and BovB) from all genomes and the putative horizontal transfer clusters described in the text. See Additional File 1 (Tables S1-6) and Additional File 2 (Figures S1-55) for detailed descriptions of the sequences and clusters.

Background: Transposable elements are mobile DNA sequences, colloquially known as jumping genes because of their ability to replicate to new genomic locations. TEs can jump between organisms or species when given a vector of transfer, such as a tick or a virus, in a process known as horizontal transfer. Here, we propose that LINE-1(L1) and Bovine-B (BovB), the two most abundant transposable element families in mammals, were initially introduced as foreign DNA via ancient horizontal transfer events.Results: Using analyses of over 759 plant, fungal and animal genomes, we identify multiple possible L1 horizontal transfer events in eukaryotic species, primarily involving Tx-like L1s in marine eukaryotes. We also extend the BovB paradigm by increasing the number of estimated transfer events compared to previous studies, finding new parasite vectors of transfer such as bed bug, leech, and locust, and BovB occurrences in new lineages such as bat and frog. Given that these transposable elements have colonized more than half of the genome sequence in today's mammals, our results support a role for horizontal transfer in causing long-term genomic change in new host organisms.Conclusions: We describe extensive horizontal transfer of BovB retrotransposons and provide the first evidence that L1 elements can also undergo horizontal transfer. With the advancement of genome sequencing technologies and bioinformatics tools, we anticipate our study to be a valuable resource for inferring horizontal transfer from large-scale genomic data.Dataset: The deposited dataset contains the identified TE sequences (L1 and BovB) from all genomes and the putative horizontal transfer clusters described in the text. See Additional File 1 (Tables S1-6) and Additional File 2 (Figures S1-55) for detailed descriptions of the sequences and clusters.