The question is, which kind of environment pressure and context lead this ants to this adaptation. Perhaps a high mortality rate in their own species happened at some point? Maybe something like an STD?

The other thing to note is that integration of "foreign" genetic material also happens inside of individuals themselves, e.g. the famous cases of gut bacteria or mitochondrial DNA. One general puts a lot of emphasis on inter-species competition, e.g. predators and preys, but there's a lot of cooperation and symbiosis happening at all levels too.

> The question is, which kind of environment pressure and context lead this ants to this adaptation

Given how unusual this seems to be, maybe it's better to ask how it happened rather than why it happened? If this was environmental pressure, then why don't we see the same thing happening elsewhere (was this really such a unique environment)? Maybe it's better regarded as an artifact of messy evolution - a quirk rather than a feature?

It'd seem surprising if this persists for long (evolutionary timescale) given that the mother "species" is expending resources to support a different "species", unless there is some kind of symbiosis here - a mutual benefit.