The Biologist Is In: Wild Carrot Flowers

Tuesday, August 18, 2015

Wild Carrot Flowers

Wild Carrot (Queen Anne's Lace, Daucus carota) is a common roadside weed/wildflower. The typical flower structure is a flat umbel with hundreds of tiny flowers. The florets at the edge have larger petals on their outer sides. Some of the plants produce a small number of dark purple florets (usually just one) in the center of the umbel, but it isn't clear why it does so. The purple florets are considered sterile (though this photo suggests not always), so there has to be some selective advantage to having them that maintains their presence in the population. Some suggest the dark floret acts as an insect mimic to draw in flying pollinators, but the research is ambiguous at best.

While was taking photographs of one plant, an ant climbing across the surface of one umbel paused to investigate the dark center floret. This started me wondering if the plant might have improved pollination from ants crawling across it with the dark purple center to attract them. More generally, the dark floret could act as a nectar-guide to draw bees and other effective pollinators to the umbel. The impact on insect visitation (and presumably on fertilization) of dark marks has been shown for other plants, so it is reasonable to extrapolate the same adaptive role may be at play in the wild carrot.

Once the dark central floret drew my attention, I looked for variations in the trait within the small population I was walking along. The most pronounced difference I noted was in the height and size of the central floret. In most plants the central floret was held at the same level of all the others. In some few it was held below and in one, a larger central floret was held well above the other florets in the umbel. I thought this was an attractive variation, so I harvested a near-mature seed-head from the plant. I might grow a plant or two in a controlled location some time -- when I'm not trying to get seed from edible carrots (they readily hybridize), that is.

I also noted some variation in color of the central floret. The typical example is a very dark purple, but I also found a few plants with a much lighter pink central floret. I don't really like the aesthetics of the photo at right, but it was the best I got when trying to place the two different colored central florets next to each other for comparison.

The diversity of variations of the central floret in this small population suggest that the trait isn't under strong selection (at least in this small population). If a trait is under strong positive selection, mutations which interfere with it will rapidly be lost. To me, this suggests the dark central floret is a historical anachronism. It was once under strong positive selection, which is why it is so common, but it is not now under such selection (again, at least in the small population I examined). This could mean that in whatever small corner of the world where the plant came from, there is a specialized pollinating insect that likes the dark central floret. When the plant started traveling with humans, it found other pollinators that didn't care if it had the central floret and the trait began to diverge as mutations accumulated. If this hypothesis has any sort of validity, then we would predict that where carrots originally evolved (Afghanistan, Turkey) there would be less variation in this central floret trait.

Once I started looking online for carrot flower color variations, I found a few photos people have taken of wild flowers in shades of pink and purple (in references below). I found a nifty blog post discussing how to dye Queen Anne's Lace using food-coloring. I even found a variety of Queen Anne's Lace being sold commercially that has darkly-colored flowers.

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