Genome-scale analysis of positionally relocated genes

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Abstract
During evolution, genome reorganization includes large-scale events such as inversions, translocations, and segmental or even whole-genome duplications, as well as fine-scale events such as the relocation of individual genes. This latter category, which we will refer to as positionally relocated genes (PRGs), is the subject of this report. Assessment of the magnitude of such PRGs and of possible contributing mechanisms is aided by a comparative analysis of related genomes, where conserved chromosomal organization can aid in identifying genes that have acquired a new location in a lineage of these genomes. Here we utilize two methods to comprehensively identify relocated protein-coding genes in the recently sequenced genomes of 12 species of genusDrosophila. We use exceptions to the general rule of maintenance of chromosome arm (Muller element) association for mostDrosophilagenes to identify one major class of PRGs. We also identify a partially overlapping set of PRGs among “embedded genes,” located within the extents of other surrounding genes. We provide evidence that PRG movements have at least two different origins: Some events occur via retrotransposition of processed RNAs and others via a DNA-based transposition mechanism. Overall, we identify several hundred PRGs that arose within a lineage of the genusDrosophilaphylogeny and provide suggestive evidence that a few thousand such events have occurred within the radiation of the insect order Diptera, thereby illustrating the magnitude of the contribution of PRG movement to chromosomal reorganization during evolution.