In higher organisms such as vertebrates, it is generally believed that lateral transfer of genetic information does not readily occur, with the exception of retroviral infection. However, horizontal transfer (HT) of protein coding repetitive elements is the simplest way to explain the patchy distribution of BovB, a long interspersed element (LINE) about 3.2 kb long, that has been found in ruminants, marsupials, squamates, monotremes, and African mammals. BovB sequences are a major component of some of these genomes. Here we show that HT of BovB is significantly more widespread than believed, and we demonstrate the existence of two plausible arthropod vectors, specifically reptile ticks. A phylogenetic tree built from BovB sequences from species in all of these groups does not conform to expected evolutionary relationships of the species, and our analysis indicates that at least nine HT events are required to explain the observed topology. Our results provide compelling evidence for HT of genetic material that has transformed vertebrate genomes.transposon | interspersed repeat R epetitive DNA is abundant in metazoan genomes and is largely composed of transposable elements (TEs). Retrotransposons are a class of TEs that are able to "copy and paste" themselves within the genome via an RNA intermediate (1). Long interspersed element (LINE) retrotransposons encode an endonuclease that nicks the DNA and allows the reverse transcriptase encoded by the element to copy the RNA produced from the TE back into DNA during repair of the nick, integrating the LINE into a new genomic position (2). However, unlike retroviruses, LINEs and other TEs do not encode an envelope protein and are hence unable to disperse horizontally without a vector between species.Horizontal transfer (HT) of TEs is largely inferred by similarity of DNA sequence; however, where the mechanism of HT has been demonstrated, a vector such as a parasite or virus was involved. For example, both P elements, between species of Drosophila (3), and the Space Invader DNA transposon, between tetrapods (4, 5), are transmitted by arthropod parasites (5, 6). The Sauria short interspersed element (SINE), has been shown to have transferred into a West African rodent poxvirus from the snake, Echis ocellatus, also supporting viruses as mechanisms for retrotransposon HT (7). HT of retrotransposons is significant because conservative estimates of their prevalence indicate that they make up between a third and a half of typical vertebrate genomes. Thus, demonstration of widespread HT for retrotransposons has significant implications for our understanding of genome structure and evolution. In this report we describe a comprehensive analysis of HT of BovB, a LINE about 3.2 kb long, which has previously been described in ruminants, marsupials, squamates, monotremes, and African mammals (8-11). Results and DiscussionTo determine the sequence conservation of BovB across taxa and examine the evidence for HT, we identified BovB sequences in all publicly available genomes and in sev...

Horizontal transfer (HT) is the transmission of genetic material between non-mating species, a phenomenon thought to occur rarely in multicellular eukaryotes. However, many transposable elements (TEs) are not only capable of HT, but have frequently jumped between widely divergent species. Here we review and integrate reported cases of HT in retrotransposons of the BovB family, and DNA transposons, over a broad range of animals spanning all continents. Our conclusions challenge the paradigm that HT in vertebrates is restricted to infective long terminal repeat (LTR) retrotransposons or retroviruses. This raises the possibility that other non-LTR retrotransposons, such as L1 or CR1 elements, believed to be only vertically transmitted, can horizontally transfer between species. Growing evidence indicates that the process of HT is much more general across different TEs and species than previously believed, and that it likely shapes eukaryotic genomes and catalyses genome evolution.