Complicated expansion trajectories of insertion sequences and potential association with horizontal transfer of Wolbachia DNA

Y. H. Miao, D. W. Huang and J. H. Xiao,  Zoological Research,  44:273-275. 2023.

Insertion sequences (ISs) are the simplest structural transposable elements (TEs) in prokaryotes, consisting only of a transposase coding sequence and its bilateral short terminal inverted repeats. Due to their gradually streamlined genomic construction, TEs rarely exist in the genomes of obligate endosymbionts. However, TE content, especially ISs, is abundant in the genome of Wolbachia bacteria, obligate endosymbionts widespread in arthropods and nematodes. Although IS indels are reported to affect genome structure and gene function in Wolbachia, the distribution patterns, sources, and transfer trajectories of ISs remain poorly understood. Furthermore, whether IS transposition is associated with dynamic horizontal transfer of Wolbachia DNA is still unclear. Based on distribution patterns in supergroup A Wolbachia strains, ISs accounted for 11% of the genome of the Wolbachia strain wWpum, one of the highest IS genome coverages reported for Wolbachia to date. Three types of ISs showed rapid expansion in wWpum, possibly due to horizontal transfer from other Wolbachia strain supergroups or more distant prokaryotes. We also found the first evidence that ISs can carry flanking Wolbachia sequences for transposition, resulting in the horizontal transfer of Wolbachia DNA into the eukaryotic genome, thus implying a potential association between ISs and horizontal gene transfer from endosymbionts to eukaryotes.

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