Horizontal gene transfer via retroviruses that change an organism's germ line DNA exist, even in humans, and account for a minority but not insignificant portion of the genomes of many, if not most, species on Earth.
Would it be viable to have an organism in which horizontal gene transfer was a normal and expected part of the life cycle of a multi-stage organism?
For example, at stage one, the organism would have just a bare minimum genome. Some stage one organism that survived to adulthood might then receive a retrovirus causing it to become a stage two organism (perhaps facilitated by state two organisms themselves with a sexually transmitted or blood transfused retrovirus). Some stage two organisms, in turn, might receive another retrovirus causing it to be a stage three organism (perhaps facilitated in a similar way by stage three organisms).
To be clear, I know that this is not what is going on in multi-stage organism that are familiar to us like frogs and butterflies. I'm simply asking if there is any reason that this couldn't be biologically viable.
My motive for considering this is that it might reduce mutation rates in DNA associated with later stages since fewer organism would have the later stage, in an environment that was highly prone to mutation such as one with mutation encouraging chemicals and/or radiation present at high levels), but ultimately that is just background. (Also, it is just a cool idea.)