Horizontal transfer and the widespread presence of Galileo transposons in Drosophilidae (Insecta: Diptera)

Abstract

Galileo is a transposon notoriously involved with inversions in Drosophila buzzatii by ectopic recombination. Although widespread in Drosophila, little is known about this transposon in other lineages of Drosophilidae. Here, the abundance of the canonical Galileo and its evolutionary history in Drosophilidae genomes was estimated and reconstructed across genera within its two subfamilies. Sequences of this transposon were masked in these genomes and their transposase sequences were recovered using BLASTn. Phylogenetic analyses were employed to reconstruct their evolutionary history and compare it to that of host genomes. Galileo was found in nearly all 163 species, however, only 37 harbored nearly complete transposase sequences. In the remaining, Galileo was found highly fragmented. Copies from related species were clustered, however horizontal transfer events were detected between the melanogaster and montium groups of Drosophila, and between the latter and the Lordiphosa genus. The similarity of sequences found in the virilis and willistoni groups of Drosophila was found to be a consequence of lineage sorting. Therefore, the evolution of Galileo is primarily marked by vertical transmission and long-term inactivation, mainly through the deletion of open reading frames. The latter has the potential to lead copies of this transposon to become miniature inverted-repeat transposable elements.

Figure 1 -. Coverage graphs for six queries of Galileo against its corresponding species: (A) Dana\Galileo in Drosophila ananassae; (B) Dbuz\Galileo in D. buzzatii; (C) Dmoj\Galileo in D. mojavensis; (D) Dper\Galileo in D. persimilis; (E) Dvir\Galileo in D. virilis; and (F) Dwil\Galileo in D. willistoni. Colors correspond to the coverage scale on the right side of each graph. Axis X corresponds to base pairs positions.

Figure 2 -. (A) Ultrametric tree showing the phylogenetic relationships between species harboring nearly complete transposases, assessed through maximum likelihood. Ultrafast bootstrap (UFboot) not shown, as for all nodes UFboot = 100. (B and C) Majority-rule consensus tree showing the phylogenetic relationships between sequences of Galileo, (B) found in genomes of the montium group of Drosophila and species of the Lordiphosa genus, and (C) found in genomes of the saltans, virilis and willistoni groups of Drosophila; numbers next to each node reflect its posterior probability support. (D and E) Results of the horizontal transposon transfer (HTT) analysis in vhica, between (D) saltans, virilis and willistoni groups of Drosophila; and (E) Lordiphosa genus and melanogaster and montium groups of Drosophila. (D and E) Red squares represent statistically significant (P < 0.05) pairwise comparisons between sequences of Galileo, indicating a HTT event. Phylogenetic relationships between host genomes are shown by ultrametric trees drawn on the external sides of each graph.

Figure 3 -. Figure 3 - Number of sequences (X axis) masked as Galileo elements by RepeatMasker for the top 15 species (Y axis) with the highest number of sequences. Species highlighted in bold are endemic to the Neotropical region.