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Also of interest: the virus is in the family Partitiviridae, has four genes (one on each "chromosome"), and each "chromosome" is packaged into a separate viral particle.

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I'm unclear as to why the virus is considered symbiotic if it is causing so many male eggs to not hatch. How does that serve the virus? And how does that serve the fly?

Charles Darwin ruined us all with the implication that evolution is driven by goals. it isn't. the most successful survive, that's it.

the fact that this disproportionately effects the male offspring sure would seem to imply an evolutionary disadvantage, so we can probably assume they have another major advantage elsewhere that overshadows this seeming flaw.

I think that a lot of the framing is being lifted from the bacterial endosymbionts of insects, which can also cause male killing. The bacteria is only transferred through the egg, so killing males doesn't harm the bacteria (and may benefit if the females have more resources due to the lack of males).

(these bacteria can also help the insect gain various other benefits)

But as a virus, this explanation doesn't make as much sense (since it should be easier to transfer in the sperm than a whole bacteria), and to my knowledge no benefits have yet been observed. But it's borrowing from the same framework, so I think that's why they called it a symbiont.

edit: Also technically speaking, symbiote refers to any long-term close interaction. Mutualistic symbiosis is the classic codependent relationship, but you also have commensal and parasitic symbiotic relationships as well.

I suspect it's because a higher ratio of females to males wastes less resources on unnecessary males. If there is 1 female to every 1 male, a population of 10 can have 5 sets of offspring every mating season. If there are 4 females to every 1 male, a population of 10 can have 8 sets of offspring every mating season.

yeah, I suspect you're onto something here. The numbers flesh out one way or another in the end I'm sure.