An episodic burst of massive genomic rearrangements and the origin of non-marine annelids.

The genomic basis of cladogenesis and adaptive evolutionary change has intrigued biologists for decades. Here we show that the tectonics of genome evolution in clitellates, a clade composed of most freshwater and all terrestrial species of the phylum Annelida, is characterized by extensive genome-wide scrambling that resulted in a massive loss of macrosynteny between marine annelids and clitellates. These massive rearrangements included the formation of putative neocentromeres with newly acquired transposable elements and preceded a further period of genome-wide reshaping events, potentially triggered by the loss of genes involved in genome stability and homoeostasis of cell division. Notably, whereas these rearrangements broke short-range interactions observed between Hox genes in marine annelids, they were reformed as long-range interactions in clitellates. Our findings reveal extensive genomic reshaping in clitellates at both the linear (2D) and three-dimensional (3D) levels, suggesting that unlike in other animal lineages where synteny conservation constrains structural evolution, clitellates exhibit a remarkable tolerance for chromosomal rearrangements. Our study thus suggests that the genomic landscape of Clitellata resulted from a rare burst of genomic changes that ended a long period of stability that persists across large phylogenetic distances.