Sun Selection

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When we think about the evolution of flower morphology, we instantly jump to conversations about pollinators. This is for good reason. If it were not for these third party players, much of the sexual reproduction in the plant world would grind to a halt. However, one aspect of floral evolution that I almost never consider is light availability. Plants live in a wide variety of different habitats and thus experience vastly different levels of light. 

The way in which organisms respond to light can vary but a major driver of this is latitude. Latitudinal variation in pigmentation, for example, was first documented in birds by a man named Constantin Wilhelm Lambert Gloger. In 1833 he noted that birds from more humid environments around the equator tend to be darker in color than birds living in more northern, arid environments. Without getting too far off topic, this observation led others to notice this trend in mammals as well. Mammals from higher latitudes tend to be less pigmented than those from equatorial regions.

The simple answer to this has to do with UV radiation. There is more UV radiation making its way to the surface at the equator than at higher latitudes. Simple physics really. Over time, this manifests in selective pressures on the organisms that live there. This phenomenon has been coined "Gloger's Rule." Whereas this has been documented in a wide variety of animal life, explanations for geographic variations in floral pigment are lacking. 

A recent paper published in Nature Plants changes this. Using a widespread species in the rose family, Argentina anserina, researchers documented changes in the floral pigmentation patterns in the UV spectrum as a function of latitude. As many of you know, many pollinators see well beyond what we humans can see, deep into the UV spectrum. Flowers utilize a wide array of patterns only visible as UV wavelengths to attract them. What the researchers found was that in Argentina anserina, the size of the UV patterning on the flowers increases in plants growing closer to the equator. This enhances plant-pollinator interactions where pollinators are more prevalent. In higher latitudes, there simply isn't as much UV radiation to utilize. 

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Photo Credit: Matthew H. Koski & Tia-Lynn Ashman

Further Reading:

http://www.nature.com/articles/nplants20147