Hum, yep summertime already. But hey, I intended to write this post in spring, though field season decided otherwise. It’s never too late anyway, since Spring Beauty is about a specific kind of beauty: Claytonia virginica, a wonderful spring species. This little plant belongs to the Portulacaceae. As an ephemeral, it is blooming early in spring, and mostly in damp forest of the Northern part of USA and South Canada.
I don’t know if you noticed, but the first time I encountered this species, I just thought: “incredible, here is a monocot with a flower that looks like a eudicot”. I was wrong of course: it’s in fact a eudicot with leaves that look like monocot. I finally remembered my teachers in botany telling students we were that leaves were not really such a strong characteristic, even if useful most of the time. I guess it’s nevertheless so generally true that it’s tempting to make it a rule. I’ll remember to give this as a good counter-example next time I’ll teach some introductory botany.
Here you can see it’s pollinated by halictid bees. Well, it’s also visited by lots of other hymenoptera and flies. I didn’t count precisely but the fruit set seems fairly reasonnable, at least in the population I was walking through (a huge population in woods next to Pymatuning Lake, Ohio). This is not really surprising, because Spring Beauty produces a fair amount of nectar given the relatively small size of its flowers (even if it is considered moderately rewarding) . Thus a huge population does indeed represent a very interesting source of reward for pollinators, at an early time when not many species are blooming.
What’s more amazing is that the species is polymorphic with regard to flower colour. Plain white or pale pinkish flowers are common, but you may also see a scale toward more redish petals. Usually, biologists are fond of such diversity, because it’s asking for an explanation (there you go: design experiments with cool species for your study! That’s why empirical studies are so funny). Why would there be different colours in this species?
Well, so, the answer was worked out a few years ago . The amazing thing is that there is apparent selection by pollinators to increase redness in this plant: increased visitation of plants with redish flowers leading to higher reproductive success. On the other hand, there’s an indirect selection by herbivores and pathogens to decrease redness of flowers, though this opposing selection was fluctuating among years in this study.
I guess it may not be directly linked to flower colour, though we still can think of a possible causal hypothesis: investment into flower pigmentation may cost you some energy (floral pigments are complex chemicals to produce afterall), a resource that you may need for other purposes like resisting pathogens or developping protections against herbivores. This would be an interesting trade-off between different ways to improve fitness: either attracting pollinator better, or better protect yourself against enemies… A good reason why we would still observe diversity of flower colour in this species.
On the other hand, it has also been shown nevertheless that there’s no strong association between traits in this plant, so that selection may be relatively free to apply without constraints . Since there is no strong correlation between the characteristics undergoing opposite selection, we may expect some day colour to be much less diverse in this species, in case counterselection of redness in flowers is really indirect (and not as we hypothesized here, that there is a cost associated with producing pigments on resources that could be spent in a better defense against enemies).
Would be interesting to know…
-Dailey, T. B. and Scott, P. E. 2006. Spring nectar sources for solitary bees and flies in a landscape of deciduous forest and agricultural fields: production, variability, and consumption. Journal of the Torrey Botanical Society, 133(4): 535-547.
-Frey, F. M. 2004. Opposing natural selection from herbivores and pathogens may maintain floral-color variation in Claytonia virginica (Portulacaceae). Evolution, 58(11): 2446-2437.
- Frey, F. M. 2007. Phenotypic integration and the potential for independent color evolution in a polymorphic spring ephemeral. American Journal of Botany, 94(3): 437-444.