I'm your Venus

It’s always good to get a Bananarama song into the title!

The journal New Phytologist has a collection of papers on plant evolution but one that stands out is a piece on the origin of the Venus Fly Trap. This plant fascinated Charles Darwin so much that he once wrote of one of its relatives ‘I care more for Drosera than the origin of species… it is a wonderful plant.’

Plant carnivory evolved at least 6 times independently. Seed fragments of snap-traps like Dionaea date to the late Cretaceous (85-75 million years ago). It is also a good example of how different disciplines come together to tell a story. Molecular analysis has revealed the clues to help piece together the picture, showing how the family trees work out, but then the question is how the progression was made?

The data shows that snap-trap plants evolved from sticky trap-plants (see above for examples of each). Although these differ greatly in morphology and action, therir structures, physiology and modes of action share many common features. Pre-adaptations for all their features exist in their closest relatives. The evolutionary pressure to change seems to be that capturing larger prey brings disproportionately large rewards to traps that can act swiftly and strongly enough to hold on to them.

The genus Drosera is full of plants with sticky traps. Their tacky hairs catch an insect and the more it struggles the more hairs it comes into contact with (many of the hairs even bend towards their stimulus). Before long it is stuck, waiting only to be digested.

But the hairs of snap-traps on the other hand respond by initiating the closing mechanism. As well as that to adapt to being a snap-trap plant you need nectar glands, spaced teeth, rapid closing, crosshatched veins and ability to seal the prey. Though these represent a considerable challenge there are more clues to go on.

The snap-traps teeth and trigger hairs are homologous to those sticky tentacles (although they have lost their mucilage). Indeed in some sticky species the outer hairs don’t actually have any glue, instead they are used to flick prey inside. Seeing as the ‘snap’ is better than the ‘stick’ in wet conditions perhaps that was another selective pressure.

The authors of this paper model the various benefits for each of the changes and illustrate how they would each have helped catch larger prey, concluding that ‘all these traits serve both to retain larger prey within sticky traps and as a pre-adaptations to evolve snap-traps’.

The ‘snap’ and the ensuing digestion uses a lot of energy - which is fine when you’re capturing large prey, so its interesting that the trap's hairs are spaced so that small prey can escape and let the trap re-set before any more energy is wasted.

It is a very interesting paper, though much is necessarily theoretical.