Symbiont gain should be called "symbiogenesis" (though, in fact, that term is often used to refer to something else). Much of the symbiology literature addresses this topic.
There's an interesting book about symbiont loss: The Ghosts Of Evolution Nonsensical Fruit, Missing Partners, And Other Ecological Anachronisms by Connie C. Barlow.
In a previous article I offered a classification scheme for mutations. It is interesting to think how the various types of mutation apply to symbiotic genetics and population genetics.
Symbiotic genetics is particularly relevant to students of cultural evolution - since cultural evolution is heavily based on symbiology.