Fly STD’s, and the help of Bacteria
Here we demonstrate that a maternally transmitted bacterium, Spiroplasma, protects Drosophila neotestacea against the sterilizing effects of a parasitic nematode, both in the laboratory and the field. This nematode parasitizes D.neotestacea at high frequencies in natural populations, and, until recently, almost all infections resulted in complete sterility. Several lines of evidence suggest that Spiroplasmais spreading in North American populations of D. neotestacea and that a major adaptive change to a symbiont-based mode of defense is under way.
Here’s the deal – flies have bacteria, and flies have worms. There’s a parasitic worm infection that causes sterility in female fruit-flies (the worm hijacks the reproductive tract, causing worm eggs to incubate in the fly rather than fly eggs). But one of the fly’s commensals (Spiroplasma) learned how to block this sterilizing effect, which is vital since the bacterium is transfered from mother fly to off-spring. In other words, no fly babies, no more Spiroplasma.
Even cooler, this “protective symbiosis” seems to have evolved in the relatively recent past. In the 80’s this particular worm wasn’t a big problem, but over the last 30 years, it’s come to infect a large portion of the fly population. The sterilizing effect puts a strong selective pressure on both the fly and the Sprioplasma (which can only be transmitted if the fly has babies), but the bacteria can evolve faster and so they’ve born the brunt of the protection. It’s natural selection in action!
Update: There’s a pretty good write-up of this paper in the Guardian if you want more info.