Evolution: over and over again, but with a twist

Garter snake (Thamnophis sirtalis)

Some of the topics and studies I’ve written about (fish and PCBs, salamanders and road run-off, for example) share an interesting phenomenon which is the evolution of resistance to the same toxicant but in disparate populations. This seems to occur even when evolution occurs fairly rapidly (or perhaps especially when evolution is rapid?!).  While the actual mechanism of tolerance is not always known (yet) some studies suggest that while the ends are the same, the means or mechanisms might be only somewhat the same.  Evolution of the same trait (resistance to PCBs in tomcod and killifish; or wings in birds and bats) either in different species or perhaps distinct populations of the same species when they occur can involve very different genes, or, as a new study by Chris Feldman and others shows may be constrained to “a relatively few genetic hotspots and fix in a repeated and predictable pattern.”

Feldman and colleagues’ study explores resistance to highly toxic tetrodotoxin in snakes. The toxin, one of the more potent naturally produced toxins known, is best known to human predators perhaps in the form of puffer fish or fugu, but it also protects newts and a few varieties of frogs and toads from predation.  The toxin interferes with a highly conserved and very basic sodium channel in nerves and muscles by binding to a pore and interfering with sodium ion movement – a potentially lethal turn of events. Which means that resistance at the level of the sodium channel, alterations in how it might interact with both toxic and internal chemicals (in this case sodium) would mean messing with  essential processes and possibly intolerable consequences.  Systems like this, with little leeway for alteration and which share underlying commonalities in their evolutionary history seem to be prime candidates for convergent evolution under pressure. And this seems to be the case when it comes to sodium channels, tetrodotoxin, snakes and even puffer fish.

Conclude Feldman and colleagues “…the pool of mutations that can reduce TTX (tetrodotoxin) affinity and at the same time preserve critical sodium channel functions is limited.  Notably, there are remarkable patterns of molecular convergence among pufferfish…and between pufferfish and snakes…”

For more see: Constraint shapes convergence in tetrodotoxin-resistant sodium channels of snakes by Chris R. Feldman et al. (2012) PNAS 109:4556-4561

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