A 180-year-old ‘law’ in zoology has found its best support so far in a study of floral colour, which not only documents darker plants growing closer to the equator, but also supports the idea that the colour stems from ultraviolet protection.
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As long ago as 1833 the German ornithologist Constantin Lambert Gloger noted that birds from warmer regions (usually interpreted as the tropics) tend to be darker than related species from cooler areas1. Since then ‘Gloger’s rule’ has been observed in several bird and mammal species including humans. Writing in Nature Plants Koski and Ashman breathe new life into this old idea by investigating the phenomenon in plants2. Observing 35 populations of the silverweed cinquefoil, Argentina anserine, across a range of latitudes in both hemispheres they saw a clear trend towards greater dark pigmentation in populations from lower latitudes. This pattern is not visible to the casual human observer, being due to pigments that absorb ultraviolet light.
Gloger’s rule has had a venerable history, rife with conjectures as to its cause. One explanation is that it arises as an adaptation against feather degradation by bacteria in moister climates3. The pigment responsible for much of the variation in darkness, eumelanin, improves feathers’ resistance to both physical and chemical damage. Other zoologists have noted that various mammal groups also adhere to Gloger’s rule, at least using latitude as a proxy for climate4,5, but have tended to favour different explanations.
The direct overhead solar illumination of the tropics also produces more intense dorsoventral shadowing. This is thought to favour stronger countershading (darker back, lighter belly) to counteract the self-shadowing and aid camouflage, as supported by data from a comparative study of ungulates6. Conversely, the preferred explanation for the variation in skin colour in humans is a trade-off between the benefits of protection from UV damage and need for UV for vitamin D3 synthesis7,8. In the tropics the balance is towards UV protection and dark skin, at higher latitudes the weaker sun favours reduced pigmentation to allow vitamin D3 synthesis9. Because Koski and Ashman observed variation in UV patterning they could directly test the UV protection hypothesis as it applies to flowers2.
A ‘bullseye’ pattern is common in many flowers, often with the petal bases absorbing UV and the petal tips strongly reflective (Fig. 1). Although attracting and guiding pollinating insects are likely functions, a role has been suggested in the protection of pollen from UV light reflected onto the anthers by the disk of petals. Koski and Ashman measured the size of the UV bullseye in their populations of A. anserine. Across four latitudinal transects, bullseye size increased towards the equator. Crucially for the proposed function, the level of UVB irradiance at each location was the best predictor of bullseye size, followed by temperature but not rainfall.
Although similar to the naked eye (left) the upper bloom shows a larger area of dark pigmentation under UV light (right). The flowers come from the extreme north (top) and south (bottom) of New Zealand. COURTESY OF MATTHEW KOSKI
But does a larger bullseye really protect the flower from UV? Koski and Ashman took A. anserine plants into the lab and exposed half the flowers of each plant to ecologically realistic levels of UVB irradiation, with the other half unexposed. In the absence of UV, pollen germination was actually higher in flowers with smaller bullseyes, however in the presence of UVB higher pollen viability correlated with larger bullseyes, but not too large. In a second lab experiment, with artificial flowers surrounding the anthers from real plants, flowers with large bullseyes had pollen germination levels similar to those in the absence of UV, while those from flowers with small bullseyes had 28% lower germination rates. These two lab experiments demonstrate that UV-absorbing petal tissue does enhance pollen viability, consistent with the latitudinal variation being the result of selection for UV protection.
That protecting pollen from UV damage is a cause of floral colour variation is an important conclusion, although it does not rule out other factors from being as (or more) significant, including the effects of UV on other plant functions. Nevertheless, it is somewhat ironic that, after 180 years of research on Gloger’s rule by zoologists, some of the strongest evidence for the driver of that latitudinal pattern comes from research on plants. It is of course possible that Gloger’s rule has no single explanation and that different groups are responding to different selective agents, all correlated with latitude. After all, Gloger’s original observation was of a relationship between pigmentation and warmth as much as latitude. Also, although for both A. anserine and humans UVB irradiance is a stronger correlate of pigmentation than humidity or latitude per se, in humans it may be UVB-induced folate photolysis that is the major selective agent8. What is clear, is that nearly 200 years on, the observations of Constantin Lambert Gloger are still providing inspiration for innovative research.