Is it possible for a virus to be a descendant of a bacterium that was not through horizontal gene transfer?
How I think this could happen:
Suppose a species of bacteria lives in an environment where it gets all the resources it needs to reproduce (and let's assume that there will be no major changes in the environment). That is, if it loses a gene that synthesizes some substance (or regulates some process), it will not be at an evolutionary disadvantage. Because it can achieve this from the environment. Even depending on the gene, it can even be an evolutionary advantage, as it can become more efficient when it comes to reproducing. And I have seen that an experiment was carried out related to this question, it was shown that viruses can lose some genes in an environment where it can naturally find the protein that is associated with those genes. Thus, there is a chance that this species will become so simple, that it will end up becoming a virus.