Extreme competence and extreme incompetence

The February reading group paper was “Extreme Competence: Keystone Hosts of Infections”. If you’ve been following the blog for awhile, you probably know that this is a topic near and dear to my heart; I’ve often mused about superspreaders, superreceivers (here and here), and other types of “super hosts”. In fact, I think about this so often that I’ve started to get a bit bored with wondering why some individuals in a host population or some host species are really good at passing on their parasites. As Martin et al. (2019) point out, the superspreader idea is pretty sexy and superspreaders might be especially conspicuous, so it seems like everyone is looking for them and talking about them. But not me. My new quest is to figure out what makes a host “bad”.

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In my hunt for bad boys (most of which cannot be discussed in a public venue), I worked on an idea that wasn’t brought up in the Martin et al. (2019) paper: other symbionts can make hosts super bad for parasites. That’s right, folks. Without a substantial Twitter discussion to guide this post, y’all are being subjected to a story from my dissertation. BRACE YOURSELVES.

On almost every continent on this planet, there are freshwater snails, and it seems like all of those snail species are at least sometimes infested by little ectosymbiotic oligochaete worms, called Chaetogaster limnaei. Chaetogaster are fascinating for several reasons, but their claim to fame in the literature is their diet: they’ll eat anything that fits in their mouths, including trematode parasites. From a snail’s perspective, this is awesome, because they gain at least some protection from being infected by trematode eggs, miracidia, and cercariae. In fact, after an absolutely abhorrent amount of pipetting – which caused by left thumb to grow a muscle as big as an egg – I found that the more Chaetogaster a snail had, the less likely the snail was to get infected by free-swimming trematode larvae. (And because Chaetogaster rapidly asexually reproduce, the more free-swimming trematode larvae a snail is exposed to, the more Chaetogaster it suddenly has, meaning that snails in risky waters get increased parasite defenses!)

Snails with many Chaetogaster are not a good target for trematodes, in the same way that a hotdog stand surrounded by hungry lions isn’t a good target restaurant for buying your lunch. But this probably isn’t particularly inconvenient for trematodes, because just just like most hotdog stands aren’t surrounded by hungry lions, most snails don’t have many Chaetogaster. As I’ve blogged about before, Chaetogaster are aggregately distributed amongst snail hosts, such that most snails have 0 or 1 worms, and just a few snails have many worms. Therefore, just a few snails are what we might call “super bad hosts”.

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As Martin et al. (2019) point out, we know that 20% of hosts are typically responsible for 80% of parasite transmission. That 20% contains the superspreaders that we’re all so excited about. But my dissertation work shows something else: ~20% of hosts are “super bad hosts” that might be acting as superdiluters, with Chaetogaster literally sucking up the trematode population. This is interesting because Chaetogaster is just one defensive symbiont species in a world full of hosts covered in symbionts that eat parasites. Defensive symbionts are probably affecting host competence in many, many systems, so these interactions might be a good place to look as we start carefully quantifying variation in host competence within populations.

Finally, since we’re talking about symbiosis today, I’ll leave you with some advice: this year’s March Mammal Madness has an ENTIRE LINEUP of symbioses, including ants + aphids and Bornean bats + pitcher plants. Obviously, one of these will be the 2019 champion. Go fill out your brackets accordingly!

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