Sunflower genes for UV and pollination

Native to North America, sunflowers are instantly recognisable. But, to pollinators, some are more eye-catching than others. And that’s because they have hidden pigment patterns that make the flowers look different to animals that can see in the ultraviolet part of the spectrum, like bees. It turns out that those patterns are down to a molecule produced by a single gene. And as well as making plants more or less attractive to insects, can also protect the seeds from UV and affect how the plant handles water and keeps itself cool. Making it comes at a metabolic cost though, so the plants out there in nature have to balance resistance to aridity and UV exposure against attractiveness to bees and other pollinators, as Chris Smith hears from Marco Todesco…

Marco - In this particular case we looked at flower colour, which might not be the first thing that comes to mind with sunflowers, because they all appear yellow to our eyes, but we knew that there are ultraviolet patterns in their flowers that we can't see, but that pollinators like bees can see. What we found is that there's a tremendous amount of diversity. We set out to figure out how they're able to have this diversity, what is the genetic basis, what are the genes that make this sunflower look so different from one another to bees, and why are they are so diverse?

Chris - When one looks at the relationship between the patterning and the behaviour of pollinators, does it make a difference? Do plants that have different patterns get different responses from the insects that pollinate them?

Marco - Yes, that's actually the case. We found all kinds of variations, UV absorbent in the middle and UV reflecting on the side and it's known that this is helpful to attract pollinators. We found plants that have completely UV absorbent flowers and completely UV reflecting flowers. We found out that pollinators tend to prefer plants that have intermediate patterns, a bit UV reflecting and a bit UV absorbing.

Chris - And were these just randomly distributed as part of a mixed population right across the geography you studied, or did you see that certain places have certain characteristics of these patterns?

Marco - That's the thing that really stood out. So, the first question was, if one pattern is better than the other for pollinators, why do you have this diversity? And then we started to look a bit more into this, and we found that the variation is not randomly distributed across North America, some patterns are more common in some areas and some in others. We tried to look at what might be driving this and we found that sunflower populations that came from very dry places had larger UV absorbing patterns.

Chris - And how do you tie that to the fact that they've become more common in those areas?

Marco - We try to look at what may be the reason as to why these different populations have different patterns of UV. We found that there is one gene that is controlling most of this diversity and this gene is responsible for the production of a class of chemicals which are called flavanol glycosides that are UV pigment - they create these patterns, but they're also important for the plant to resist different kinds of environmental stresses. One of the things that these flavanol glycosides do is help plants control how much water they lose from different tissues. Plants that had more of these UV flavanols, in dry environments, were able to retain more water from their flowers. That's, of course, something that is very important if there is not a lot of water to go around. That's probably one of the reasons why larger UV patterns, a higher level of accumulation of these chemicals, is selected in dry places.

Chris - Presumably there's a metabolic cost to doing that, which is why you get the diversity in other areas of the country where you don't have the same drive to defend yourself against desiccation?

Marco - Yes. That's a very common pattern that is seen in ecology, right? Things that are favourable in one environment might not be favourable in another one. There's probably a metabolic cost, which is why somewhere else won't have these smaller patterns.

Chris - It's interesting, isn't it, that one gene makes the pattern change and the pattern protects the plant at the same time against water loss, but also affects how attractive it is to pollinators? It just goes to show how rich the web of connections in any given ecosystem is in terms of how organisms all interreact and interrelate to each other and how they behave.

Marco - Yeah, absolutely. I mean, it shows that things are never quite straightforward, right? And that evolution can drift in any way. So, if it can get two things done with a single gene or with a single trait, it'll certainly try to do that.