Bishop et al. have a new paper in JXB, Elevated temperature drives a shift from selfing to outcrossing in the insect-pollinated legume, faba bean (Vicia faba). I didn’t spot it till @ReadingUrbPolls tweeted the graphic below by @jakecologist, which gives a brief summary – though like the bees it studies, the paper has a sting in its tail.
The problem Bishop et al. see is that climate change is going to have an effect on pollination. A small rise in temperature doesn’t sound so harmful, but this is an average increase. What pushes up the average are extreme events, which the authors point out will get stronger, longer and more frequent. If we want to know what crops are going to be up against in coming years we need to know more about plant responses to extreme weather, especially heat waves. How are the plants going to pollinate?
Bishop et al. point out there’s been some research into this. They point to Koike et al.’s recent paper in Plant Production Science and the slightly older paper in Annals of Botany on wild petunias by Munguía-Rosas et al. These papers show that plants can self-pollinate in closed flowers, which should be more shaded. If there’s self-pollination, then plants are coping with the heat by ignoring the world’s problems and doing their own thing. What Bishop et al. have found is that the opposite is true for the broad bean, Vicia faba (also faba bean or fava bean).
They way they tracked outcrossing of pollen was elegant. They used two varieties of bean, Wizard and Buzz. They have the same kind of flowers and same environmental needs but they differ in one key way. You need to look into their eyes, or hilum.
Wizard has a white hilum, but the genes that causes this is recessive, so Wizard must have two white genes. Buzz is the opposite. It has a black hilum. What’s important is that the black gene is dominant, and Buzz has two black alleles. So if any Wizard outcrossed with a Buzz plant, then the Wizard plant would produce beans with a black hilum. It does mean that if a Wizard outcrosses with another Wizard, you still have white beans, so it’s not a perfect way to measure outcrossing, but it’s easy to measure and good enough for the purposes of the experiment.
So they took the Wizard plants and grew them. Some were under control conditions while other sweltered in temperatures that were 10°C higher. They were then taken out either to flight cages or a couple of experimental plots and released the bees to see what happened. They also ran experiments with the pollinators excluded to check outcrossing didn’t happen by other means, and you can see the result below.
What is going on?
In all cases, there was more outcrossing of pollen, though the authors note that the results at Sonning weren’t significant. What appears to be happening is that heat is damaging pollen. so that self-pollination becomes more difficult. In this situation, the incoming pollen has a reproductive advantage being able to fertilise the ovule faster than its stressed competitors. With healthy pollen coming in, there’s more stirring of the gene pool so there’s more opportunity to develop resistance to heat and to restore yields. This sounds like a good news story for climate change, effectively promoting crop development, but there are two other factors, something call a ‘lose-lose’ scenario.
Firstly when plants get heat stress, they’re stressed. A stressed plant is a less productive plant. The outcrossing shows there are ways to recover, but that still doesn’t mean stressing your crops is a good idea. The authors note that when yields drop the common response is to intensify production, and here’s the sting.
Intensifying production can have disastrous consequences for pollinators, just as climate conditions mean you’re more reliant upon them than before. If the plants are suffering heat stress then it’s unlikely your pollinators are entirely happy. Reducing nesting opportunities for insects when you need them working is clearly a Very Bad Idea.
The conclusion that Bishop et al. come to is that protection for pollinator services have to be a key part of any response to dropping yields after extreme weather events. So while it’s true that the sun could be making plant have better sex, it’s going to require a lot of work to make sure it keeps happening.
In the experimental setting, the Buzz pollen donors were not stressed, so there was healthier pollen available for the heat-stressed Wizard plants. Not so in the real world, when all plants will be heat-stressed at the same time…