Of broccoli, butterflies and Arabidopsis
Today, I’m venturing into the world of Arabidopsis, a plant I usually leave to the geneticists. More specifically, into it and its relatives’ evolutionary past.
DNA sequences can be used to estimate how long ago species separated. Once they separate, they stop interbreeding, and their DNA sequences start to evolve separately. So the more differences there are in their DNA, the longer it is since they split. Different bits of DNA change at different speeds: from essential genes evolving very slowly, to so called ‘junk DNA’, which doesn’t code for anything, so can change much faster.
All the data needs calibrating, though. We’ve got to know how fast particular bits of DNA change, before we can use them to date anything. The key is fossils. Say you’ve got two groups of living species, one of which has a novel and distinctive feature that the other group lacks (and presumably never had). If you find a fossil that looks like your species, and you can see it has that key feature, then the two groups must have split by the time of the fossil. I won’t go into the various ways you might find the age of the fossil, but there’s info on Wikipedia.
The paper I’ve read today did this calibration for the cabbage family, Brassicaceae (which includes Arabidopsis, along with a number of vegetables including cabbage, broccoli, turnips and oilseed rape), and some related families, like that of capers, the little buds used in Mediterranean cooking. They used five fossils which have previously been found of these plants. Perhaps the most important was a fossil seed pod from a plant called Thlaspi primaevum, which lived about 30 million years ago. Under a scanning electron microscope, they could see a characteristic pattern of ridges that it shares with living Thlaspi species, meaning that they must have split off from a closely related genus at least that long ago.
Using these fossil dates, and a more flexible model of evolution, they estimated the age of various splits between species. They put the split between the model plant Arabidopsis thaliana, and its close relatives like A. lyrata back to about 13 million years ago, three times previous estimates, and reckon that those species separated from vegetables like cabbage (Brassica) about 43 million years ago, about twice previous estimates. There are big margins of error around those “optimal reconstructions”, though.
So what does that tell us? Apart from having to re-examine how quickly various genes evolved, it places the earliest split of the ‘Brassicales’, when the ancestors of the cabbage and the caper bush separated, back in the Cretaceous, before the mass extinction that killed the dinosaurs. That suggests that those plants, after surviving the mass extinction, began to diversify to take advantages of new opportunities afterwards. Their survival and diversification could also be connected with genome duplications, where the plants ended up with two copies of all their genes, which can open up new possibilities to evolution.
The authors also suggest that the timing matches up with speciation in a family of butterflies called the Pierids, which includes the cabbage white and the orange tip. Their caterpillars can munch on these plants, as gardeners know all too well, because they can break down the toxic chemicals which protect them (the same chemicals that give mustard its sharp flavour). That’s interesting, because it suggests that the plants and the butterflies might have diversified together. Perhaps the butterflies adapted to the diversification of their host plants, or perhaps each group drove speciation in the other.
Finally, it puts the origins of Arabidopsis thaliana, the favourite of geneticists, back in the Miocene, a somewhat warmer period than the Pliocene, when previous estimates had it splitting. That might change our ideas about how it evolved.
Beilstein, M., Nagalingum, N., Clements, M., Manchester, S., & Mathews, S. (2010). Dated molecular phylogenies indicate a Miocene origin for Arabidopsis thaliana Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0909766107