BuT2 is a member of the third major group of hAT transposons and is involved in horizontal transfer events in the genus Drosophila

Abstract

The hAT superfamily comprises a large and diverse array of DNA transposons found in all supergroups of eukaryotes. Here we characterized the Drosophila buzzatii BuT2 element and found that it harbors a five-exon gene encoding a 643-aa putatively functional transposase. A phylogeny built with 85 hAT transposases yielded, in addition to the two major groups already described, Ac and Buster, a third one comprising 20 sequences that includes BuT2, Tip100, hAT-4_BM, and RP-hAT1. This third group is here named Tip. In addition, we studied the phylogenetic distribution and evolution of BuT2 by in silico searches and molecular approaches. Our data revealed BuT2 was, most often, vertically transmitted during the evolution of genus Drosophila being lost independently in several species. Nevertheless, we propose the occurrence of three horizontal transfer events to explain its distribution and conservation among species. Another aspect of BuT2 evolution and life cycle is the presence of short related sequences, which contain similar 5' and 3' regions, including the terminal inverted repeats. These sequences that can be considered as miniature inverted repeat transposable elements probably originated by internal deletion of complete copies and show evidences of recent mobilization.

Keywords: Drosophila; MITE; hAT; horizontal transfer; transposase.

Fig. 1.—

Schematic representation of BuT2 nucleotide and predicted protein. A: Organization of D. buzzatii BuT2 coding sequences, TSS, transcription start site; Exons 1–5; PolA, polyadenylation signal. Arrows indicate the primer annealing regions. B: Organization of D. buzzatii BuT2 predicted amino acid sequence with the domains found.

Fig. 2.—

Unrooted ML phylogenetic tree of hAT elements amino acid transposase sequences. Node supports are bootstrap values (1,000 replications). The three proposed hAT families, Ac, Buster, and Tip are shown.

Fig. 3.—

Phylogenetic relationships of BuT2 copies and associated MITE sequences. A: BA of the BuT2 short sequences obtained using the BuT2_F and BuT2_R primers (Bf1) and by in silico PCR (scf). B: BA of BuT2 copies obtained with the primers BuT2C_F and BuT2CR (Bf2) and by in silico searches (scf). Node supports are shown by posterior probability (only above 50%). Drosophila buzzatii BuT2 canonical sequence is shown in boldface. The species are the following: Dequ, D. equinoxialis; Dcap, D. capricorni; Dmoj, D. mojavensis; Dbuz, D. buzzatii; Dwil, D. willistoni; Dneb, D. nebulosa; Dpau, D. paulistorum; Dpap, D. pallidipennis; Dsuc, D. sucinea; Dfic, D. ficusphila; Dbip, D. bipectinata; Dsal, D. saltans; Dstu, D. sturtevanti; Dpro, D. prosaltans; Dkik, D. kikkawai; Deug, D. eugracilis.

Fig. 4.—

Comparative analysis of the divergence found among the BuT2 sequences and the nuclear genes Adh and Amd. A χ² test was used to verify whether the BuT2 observed divergence is significantly different from the expected based on the Adh and Amd genes. Results of χ² test: ***P < 0.001; **P < 0.01.

Fig. 5.—

Scheme of phylogenetic relationships of different Drosophilidae species groups employed in this study based on several works (Lewis et al. 2005; Robe et al. 2005; Kopp 2006; Robe, Cordeiro, et al. 2010; Robe, Loreto, et al. 2010; Oliveira et al. 2012). Numerous species that do not have BuT2 were omitted and only the number of species tested is shown. Blue and green bars near species represent the distribution of BuT2 complete copies and MITEs, respectively. The bars at nodes point out the potential presence of those sequences in the main ancestors. Orange crosses represent possible lost events of BuT2, and red arrows indicate probable cases of HT from some species of willistoni or saltans groups to D. pallidipennis, D. mojavensis, and D. buzzatii.