How beneficial are defensive symbionts?

(This post is late. Sorry, Folks! In my defense… snail dissection. So. much. snail. dissection.)

In my aphid cartoons thus far, Sal the Aphid has been bragging about how she has H. defensa. But just how awesome is it to harbor defensive bacteria? Like we said last time, when aphids get attacked by parasitoid wasps, aphids with H. defensa are more likely to survive than aphids without H. defensa. That seems like a pretty big bonus provided by the symbionts. We might expect natural selection to then favor aphid lineages with symbionts, leading to domination by lineages with H. defensa. But that isn’t what we see. Instead, only some aphids have H. defensa – the symbionts are maintained at intermediate frequencies. So, if defensive symbionts are so great, then why don’t all aphids have them?

In some earlier work, researchers found that H. defensa may not always be beneficial for aphids. For instance, when no parasitoids are present, the frequency of H. defensa in aphid populations declines, suggesting that H. defensa may be costly to maintain (Oliver et al. 2008). So, Vorburger et al. (2013) set out to determine whether the cost of harboring H. defensa is constitutive, induced, or both. That is, is H. defensa always costly, regardless of parasitoid presence (constitutive cost), does the cost come after a parasitoids attack and H. defensa kill the parasitoid larvae (induced cost), or both?

To test this question, Vorburger et al. (2013) exposed aphids with and without H. defensa to attacks by parasitoid wasps. The first 2/3 of the aphids that the wasps attacked were put in an “attacked” treatment group, and the aphids that the wasps did not attack were put in an “unattacked” treatment group. And then Vorburger et al. (2013) kept track of aphid survival and reproductive output.

Like we said last time, when aphids had H. defensa, they were more likely to survive a wasp attack. But when aphids weren’t attacked by wasps, the aphids with H. defensa had reduced fitness in comparison to aphids without H. defensa. That’s the constitutive cost we mentioned before. For aphids without H. defensa, attacked aphids had lower lifetime reproduction than unattacked aphids. That makes sense, of course. But get this: for aphids with H. defensa, attacked aphids had higher lifetime reproduction than unattacked aphids. That’s the opposite of an induced cost! It’s an induced benefit!

So, what caused the “induced benefit”? Well, Vorburger et al. (2013) aren’t sure. But they have one amazing hypothesis. They suggest that maybe when a wasp injects all that venom into an aphid, it kills off a bunch of the H. defensa. That is, the induced benefit is to reduce the constitutive cost of harboring H. defensa by killing off some of those costly symbionts. In that case, H. defensa isn’t sounding so nice afterall, is it? 

I rest my case: parasites and parasitoids are crazy awesome.

Oh, and the paper is open access! Check it out!

AphidSitcomSoreLoser

Reference:

Oliver, K.M., J. Campos, N.A. Moran, and M.S. Hunter. 2008. Population dynamics of defensive symbionts in aphids. Proc. R. Soc. B Biol. Sci. 275:293–299.

Vorburger, C., P. Ganesanandamoorthy, and M. Kwiatkowski1. 2013. Comparing constitutive and induced costs of symbiont conferred resistance to parasitoids in aphids. Ecology and Evolution 3(3):706-13.

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