This is quite interesting: Lampreys selectively delete unused parts of their own genome during development. Most animals (like humans) silence unused parts of the genome by using expression markers to control which part of the genome is expressed, and carry the 'useless' DNA around. I wonder what mechanism the lamprey use to delete parts of their genome – and, more importantly, whether this mechanism can be transferred to genetically modify target vectors? If so, this may be a new corridor to research in finding ways to resolving genetic defects due to having too many copies of expressed genes (e.g. down syndrome, though there are a lot of others).
Odd vertebrate gets rid of hundreds of genes early in development
Mice silence these genes during development; sea lampreys just delete them.