Multiple horizontal transfer events of a DNA transposon into turtles, fishes, and a frog

Abstract

Horizontal transfer of transposable elements (HTT) has been reported across many species and the impact of such events on genome structure and function has been well described. However, few studies have focused on reptilian genomes, especially HTT events in Testudines (turtles). Here, as a consequence of investigating the repetitive content of Malaclemys terrapin terrapin (Diamondback turtle) we found a high similarity DNA transposon, annotated in RepBase as hAT-6_XT, shared between other turtle species, ray-finned fishes, and a frog. hAT-6_XT was notably absent in reptilian taxa closely related to turtles, such as crocodiles and birds. Successful invasion of DNA transposons into new genomes requires the conservation of specific residues in the encoded transposase, and through structural analysis, these residues were identified indicating some retention of functional transposition activity. We document six recent independent HTT events of a DNA transposon in turtles, which are known to have a low genomic evolutionary rate and ancient repeats.

Keywords: DNA transposable element; Genome evolution; Horizontal transfer; Transposable element; Vertebrate.

Fig. 1

Phylogeny of horizontally transferred hAT-6_XT TEs with a sample of other hAT TEs from Repbase. Tree constructed using IQTree 2 (1000 bootstraps) based on MAFFT protein alignments trimmed using Clipkit and formatted in iTOL. Support values under 65 are displayed at nodes. Turtle species are in blue, frogs are in pink, and fishes are in green

Fig. 2

Sequence homology and phylogeny of hAT-6_XT relative to host species phylogeny. (A) Host species phylogeny (topology only) of hAT-6_XT host species. The time of divergence between some host species is shown in MYA. (B) Observed hAT-6_XT topology constructed using IQTree (1000 bootstraps) based on MAFFT protein alignments trimmed using Clipkit and plotted in iTOL. (C) BLASTN pairwise identity matrix of hAT-6_XT representative sequence alignments. Darker shading indicates a higher pairwise identity between two hAT-6_XTs. Turtle species are in blue, frogs are in pink, and fishes are in green