Unusual horizontal transfer of a long interspersed nuclear element between distant vertebrate classes

Abstract

We have shown previously by Southern blot analysis that Bov-B long interspersed nuclear elements (LINEs) are present in different Viperidae snake species. To address the question as to whether Bov-B LINEs really have been transmitted horizontally between vertebrate classes, the analysis has been extended to a larger number of vertebrate, invertebrate, and plant species. In this paper, the evolutionary origin of Bov-B LINEs is shown unequivocally to be in Squamata. The previously proposed horizontal transfer of Bov-B LINEs in vertebrates has been confirmed by their discontinuous phylogenetic distribution in Squamata (Serpentes and two lizard infra-orders) as well as in Ruminantia, by the high level of nucleotide identity, and by their phylogenetic relationships. The horizontal transfer of Bov-B LINEs from Squamata to the ancestor of Ruminantia is evident from the genetic distances and discontinuous phylogenetic distribution. The ancestor of Colubroidea snakes is a possible donor of Bov-B LINEs to Ruminantia. The timing of horizontal transfer has been estimated from the distribution of Bov-B LINEs in Ruminantia and the fossil data of Ruminantia to be 40-50 My ago. The phylogenetic relationships of Bov-B LINEs from the various Squamata species agrees with that of the species phylogeny, suggesting that Bov-B LINEs have been maintained stably by vertical transmission since the origin of Squamata in the Mesozoic era.

Figure 1

Schematic representation of the complete of Bov-B LINE. The complete bovine Bov-B LINE is 5,464 bp in length (ref. ; accession no. Z25525); the position of truncated V. ammodytes Bov-B LINE (20, 21) is shown below. The relative locations of the REP and LIZ primers used for PCR amplification are indicated between the 3′ end of reverse transcriptase (RT) and the 3′ end of right monomer (MR). The positions of primer binding sites in regard to the complete bovine Bov-B LINE are as follows: REPas (3668–3688), REPs (4211–4229), LIZas (3811–3828), and LIZs (4091–4108).

Figure 2

Multiple alignment of Bov-B LINEs. Alignment was constructed with the program clustal w (25). The asterisks represent the nucleotides conserved between all sequences. Shared nucleotides between ancestral Squamata lineages represented by Podarcis lizard and Python and Bovidae represented by ART-2 consensus sequence (accession no. X82879) are shaded. The following Squamata sequences were used: Podarcis muralis PM7 clone, Python molurus PY1 clone, Vipera ammodytes sequence from ammodytoxin C gene (20), Walterinnesia aegyptia WA10 clone, and Natrix tessellata NT6 clone.

Figure 3

NJ-phylogenetic tree of the Bov-B LINEs. Species-consensus Bov-B LINE sequences are from nine snake and one lizard species. The following randomly chosen sequences taken from GenBank represent the Bovidae [Bos taurus (accession no. X05090), Bubalus arnee (accession no. X71731), Capra hircus (accession no. M57436), and Ovis aries (accession no. AF02566)] and Cervidae [Muntiacus muntjak (accession no. X82884)] families. The rooted NJ phylogenetic tree using the Kimura two-parameter model (32) and Podarcis as an outgroup was drawn by the treecon program (27). It represents the bootstrap consensus following 1,000 replicates; nodes with confidence values >70% are indicated. The corresponding alignment is available on request.