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waterflea helmets - lamarckian, or epigenetic?

Water fleas - Daphnia - are rather cute little freshwater arthropods:

Daphnia ambigua Courtesy of An Image-Based Key To The Zooplankton Of The Northeast (USA)

In some circumstances (water temperature, presence of predators), rather than having that sharp little point on their heads (top of the picture, above the eyespot) some Daphnia will have a longer, spikier 'helmet'. And this is where it gets interesting: it depends on the mother. If a 'helmet-less' female Daphnia is in an environment where there are also predators, her offspring will sport helmets. If not, they won't. This observation has been described in one article as an example of Lamarckian inheritance.

But it's not. It's an example of epigenetics in action. And PZ does his usual excellent job of explaining what's going on, & why the original story was so far off the mark. As he says, "Genes don't execute rigid, predetermined programs of development — they are responsive to the environment and can express radically different patterns in different contexts." And this is what's happening with the waterfleas (where the contexts are presence & absence of predators). Great stuff.

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2 Comments

I thought that the Newsweek article simply said that one aspect of Lamarckism, the idea that the environment of the mother could change the appearance of the offspring, had been proven to be true. It's nice that you've given the water-flea-helmut-phenomenon a name of its own, but if I were Mr. Lamarck, I'd feel somewhat vindicated.

Well, it doesn't match up with the common perception of Lamarck's thinking, that offspring inherited traits their parents acquired as a response to the environment, since the parents lack helmets & their offspring have them (and vice versa - it's not a case of gradual acquisition).
As PZ Myers said, the fleas have a certain amount of genetic plasticity - in their case, females under stress from predators will switch on different genes in their offspring compared to mothers who aren't stressed. The original Newsweek article presented this as something new that would shock evolutionary biology, but that's certainly not the case: that environmental influences can have multi-generational effects, and that developmental programs can cue off of the history of the germ line, is not a new idea, especially among developmental biologists. http://scienceblogs.com/pharyngula/2009/01/sharon_begley_how_could_you.php

The author of the Newsweek article also didn't seem to recognise what had been going on in evolutionary terms. She was presenting the sudden appearance of helmeted fleas as an example of Lamarckian evolutionary change - whereas in fact it's an example of epigenetics. And the underlying genetic regulatory mechanisms are almost certainly the work of generations of mutation and selection, with the current outcome that changes in the environment can cause a phenotypic - but not a genetic - shift in a single generation.

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