Evolution of Mutator transposable elements across eukaryotic diversity

Abstract

Background: Mutator-like elements (MULEs) are a significant superfamily of DNA transposons on account of their: (i) great transpositional activity and propensity for insertion in or near gene sequences, (ii) their consequent high mutagenic capacity, and, (iii) their tendency to acquire host gene fragments. Consequently, MULEs are important genetic tools and represent a key study system for research into host-transposon interactions. Yet, while several studies have focused on the impacts of MULEs on crop and fungus genomes, their evolution remains comparatively poorly explored. Results: We perform comprehensive bioinformatic and phylogenetic analyses to address currently available MULE diversity and reconstruct evolution for the group. For this, we mine MULEs from online databases, and combine these search results with available transposase sequences retrieved from online databases andpreviously published studies. Our analyses uncover two entirely new MULE invertebrate-specific clades that contain elements almost entirely restricted to arthropod hosts, considerably expanding the set of MULEs known from this host group, and suggesting that many additional MULEs may await discovery from further arthropod genomes. In several cases, close relationships occur between MULEs recovered from distantly related host organisms, suggesting that horizontal transfer events may have played an important role in the evolution of the group. However, it is apparent that MULEs from plants remain relatively separate from MULEs identified from other host groups. MULE structure varies considerably across phylogeny, and TIR length is shown to vary greatly both within and between MULE groups. Our phylogeny suggests that MULE diversity is clustered in well-supported groups, typically according to host taxonomy. With reference to this, we make suggestions on how MULE diversity can be partitioned to provide a robust taxonomic framework. Conclusions: Our study represents a considerable move forwardadvance in the understanding of MULE diversity, host range and evolution, and providesing a taxonomic framework for the classification of additional further MULE elements that await discovery. Our findingsIt also raises a number of questions in relating to MULE biology, suggesting that this group will provide a rich avenue for future study.