Two hAT transposon genes were transferred from Brassicaceae to broomrapes and are actively expressed in some recipients.

Ting Sun,Jianqiang Wu,Yuxing Xu,Yan Qin,Guiling Sun,Susanne S Renner

doi:10.1038/srep30192

Abstract

A growing body of evidence is pointing to an important role of horizontal gene transfer (HGT) in the evolution of higher plants. However, reports of HGTs of transposable elements (TEs) in plants are still scarce, and only one case is known of a class II transposon horizontally transferred between grasses. To investigate possible TE transfers in dicots, we performed transcriptome screening in the obligate root parasite Phelipanche aegyptiaca (Orobanchaceae), data-mining in the draft genome assemblies of four other Orobanchaceae, gene cloning, gene annotation in species with genomic information, and a molecular phylogenetic analysis. We discovered that the broomrape genera Phelipanche and Orobanche acquired two related nuclear genes (christened BO transposase genes), a new group of the hAT superfamily of class II transposons, from Asian Sisymbrieae or a closely related tribe of Brassicaceae, by HGT. The collinearity of the flanking genes, lack of a classic border structure, and low expression levels suggest that BO transposase genes cannot transpose in Brassicaceae, whereas they are highly expressed in P. aegyptiaca.

Highlights

I transposable elements (TEs) usually cause genome expansion since they can duplicate repeatedly, and they are considered important sources of genome size differences among species[16,17]
TEs may lose their transposition ability and are recruited as functional proteins by a process called TE domestication with or without gain of additional domains[18]. Due to their mobility and capacity of transposition, transposable elements (TEs) are involved in Horizontal gene transfer (HGT). Horizontal transfers of both classes of TEs have been reported in fungi and animals, from which 199 transferred TEs were collected by the HTT-DB database with 1/3 involving class II transposons[19]
We found that two sequences in the P. aegyptiaca transcriptomes exhibited high similarities to the A. thaliana hAT-type DNA transposons: (i) one 1121bp sequence (OrAeBC4_1751) showed 83% and 75% identities with AT3G17260 at the amino acid (AA) and nucleotide (NT) level, respectively, while being highly divergent from the most similar sequence in the Phrymaceae Mimulus guttatus; (ii) another 1656-bp fragment (OrAeBC4_65399) had 85% and 65% identities with AT3G17290 at AA and NT level, while showing ~40% identity with the most similar homolog in M. guttatus at the AA level and no significant similarity at the NT level (Fig. 1)

Summary

Introduction

I TEs usually cause genome expansion since they can duplicate repeatedly, and they are considered important sources of genome size differences among species[16,17]. TEs may lose their transposition ability and are recruited as functional proteins by a process called TE domestication with or without gain of additional domains[18] Due to their mobility and capacity of transposition, transposable elements (TEs) are involved in HGT. By searching 40 photosynthetic plant genomes, El Baidouri et al.[20] recently detected 32 class I LTR retrotransposon transfer events in 26 species, with the transposons in some cases still functional. Some of these transfer events occurred between distantly related lineages, such as palms (Arecaceae) and grapevine (Vitaceae), tomato (Solanaceae) and bean (Fabaceae), or poplar (Salicaceae) and peach (Rosaceae)[20]. We show that two novel nuclear-encoded transposon genes of class II were transferred from Brassicaceae into Orobanchaceae and are actively transcribed in both genera of Orobanchaceae, there is little evidence that their homologs in Brassicaceae are transcribed

Methods

Results

Conclusion