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Pollinators as selectors for evolution

Following from yesterday, do pollinators act as selectors for evolution? A pollinator shift explains floral divergence in an orchid species complex in South Africa by Peter and Johnson tests this idea.

The orchid in question is Eulophia parviflora. This is a deceptive orchid found in Africa, and deceptive means it doesn’t offer a reward to pollinators, it merely looks like it does. The aim is to entice insects in when they look for food and hit them with their pollinaria to carry to other orchids. To do this they need to look and smell convincing, but they also need to make things as easy as possible for the pollinators. The orchid’s problem is that there are so many insects that it could build its flowers in all sorts of ways.

This is indeed what happens.

Eulophia parviflora forms
Floral morphology of Eulophia parviflora. Left, Short-spurred morph. Right Long-spurred morph. Image © Peter and Johnson.

Peter and Johnson identified two forms of Eulophia parviflora. In the image above, the one on the left is the short-spur morph. This grows tall from the ground with plenty of flowers. The one on the right is the long-spur version. This opens when the stalk is barely out of the ground. They look different and they smell different, but they’re both E. parviflora. So what is it that makes the same plant grow long or short spurs?

The answer seems to be the pollinators. The short-spur plants are pollinated by the beetle Cyrtothyrea marginalis who can get in close to the orchid. The long-spur orchid is pollinated the bee Amegilla fallax. However, simply watching and seeing that the plant has two forms pollinated by two different creatures isn’t enough. There might be some other cause, like local climate that explains the spurs and the presence or lack of an insect. So Peter and Johnson have done some experiments.

Are bees deliberately picking long-spurs in flowers? If they are then that would show the bees are selecting flowers and helping drive the morphological change. The experiment is simple. Reduce the size of the spurs in some of the long-spur flowers. If the spurs matter, then the bees will pick the long-spur plants and ignore the short spur plants. Sure enough, the bees went for the long-spur flowers.

Another experiment was to see how the scent attracted insects. They tried it with both beetles and bees, but found the bees weren’t cooperating, so there were just results from beetles. The experiment is simple and elegant. You have a Y shape. At the top of each arm of the Y you have a fan pushing out the scent of a flower. Put a beetle at the bottom and where does it go to? In this case, it picks the scent of the short-spur plant.

In fact the paper notes the experiment wasn’t quite as simple as I made out. It wasn’t just the scent that attracted beetles, they’d also pick a tunnel depending on the position of the sun, so they found they had to calibrate the tunnels properly before they could sensibly test the beetles.

Peter and Johnson also show that the two forms of the plant are not just diverging in shape but also in time. It makes sense to flower when the pollinators are about. The short-spur flower doesn’t start till after the winter frosts in October (remember South Africa is in the southern hemisphere). This is when the beetles emerge. In contrast the long-spur flower can get going sooner in July when A. fallax starts getting active.

The isolation in time for exchanging pollen, and the specificity of the pollinators means that the pollinators seem to be definitely acting as selectors for the plants. Peter and Johnson say that the two forms might already be considered two sister species given the genetic differences.

You can pick up the paper from Annals of Botany.

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