As background, this post was spurred by a former student demanding I make a post containing my own speculations instead of randomly strung-together facts. Given how batshit insane my pet hypotheses tend to be (e.g. sponges have a nervous system, HERESY!), I had to think hard to get something non-controversial – this blog is meant to be a ressource, not a personal sounding board. I think the hypothesis at the end fits the bill. Rip me to shreds if I have it all wrong please (I’m used to it. Seriously, sponges with a nervous system? Not even the zoologists of the 19th century came up with something as crazy as that. And we’re talking about people who thought annelid gills gave rise to the vertebrate penis!). Anyway…
The Pieridae are undoubtably monophyletic (see the purple branch in Wahlberg et al., 2005), and the result of the most rigorous systematic study of them is pictured above (Braby et al., 2006), showing that at least the whites are more dervied than the sulphurs by virtue of their pupal morphology.
Pierids are worthy of attention for several reasons. They are one of the model systems for research into the developmental basis of butterfly wing-pattern generation, an exciting field of research that plays into the whole origin of evolutionary novelty through developmental changes aspect of evo-devo. This is worth a post of its own, so I’m not going to summarise it here.
Another reason is for one of my pet topics, coevolution of plants and insects. The cladogram above is enhanced with two additional data sources: one is a molecular clock-based timeline of the taxon divergences (the absolute dates should be taken with a grain of salt, but the relative dates should be okay) and data on host plant associations. The latter is what is of primary interest: these results show the exact same pattern we would expect if coevolution were a real phenomenon: phylogenetic clustering of taxa. In other words, most Coliadinae feed on Fabales (yellow branches), reflecting the ancestral condition. However, the Pierinae experienced a major host-shift, moving to glucosinolate-containing plants (e.g. cabbages; green branches). They derived from the ancestral condition and when the last common ancestor moved to the new host plant, they experienced a radiation as they had little competition in using their new ressource. And within both the pierines and coliadines, individual taxa moved to non-glucosinolate-containing plants (mistletoe; blue branches), and again experienced radiations, as exemplified by the Aporiina.
Given that they are relatively well-resolved, at least at the family level (let’s face it, 45/1100 species is not a good taxon sampling by any standard), and they show several diversifications within their diversifications (the Aporiina within the Pierinae, for example), they are a valuable research group for studying more complex coevolutionary patterns and processes.
Pierids are also cool from a visual perspective. For example, they are especially adapted for resolving blue colours, since they have a specific pigment for blue and another one for violet (Arikawa et al., 2005). I may be talking out of my ass because this hasn’t, to my knowledge, been studied yet, but it appears to me to be related to sexual communication.
Whites may appear white to us, but that’s only because their wing scales contain structures that scatter light (see the “beads” above above, Stavenga et al. (2004)), with the wing scales of the male containing pigments (pteridins) that absorb any reflected UV light (Stavenga et al., 2004). The females don’t have these pigments. So it is easy for a butterfly to tell a male and female apart: just look at the one that’s brighter (= reflecting UV light).
In the sulphurs, we get a similar story, but with different colours. Their pteridins absorb UV and blue light – that’s why they appear to us as yellowish. But in males, similar structures are formed by the scales, and they end up reflecting UV light (Ghiradella et al., 1972). Combine this with the overall yellowish colour, and a butterfly would see the male as purplish.
So it can be imagined, by me at least, that the pierid propensity for seeing blue and purple colours comes from this – given that the sulphurs are the basalmost pierids, this pigment can be seen as being present in the hypothetical last common ancestor of the Pieridae, where it was used to recognise gender, retained its function as such in the sulphurs, and in the whites it is used generally.
Of course, this may have been proposed before. That butterflies, especially pierids, can see UV light has been known for a long time, as have the different UV patterns produced by the wings of pierids (see Table 1 in Silberglied (1979) – that’s some old stuff).
This is something that can be investigated using the well set-up systematic framework we have. And if I were personally interested in pierid butterflies, I’d do it. Also, if I had funding.
Arikawa K, Wakakuwa M, Qiu X, Kurasawa M & Stavenga DG. 2005. Sexual dimorphism of short-wavelength photoreceptors in the small white butterﬂy, Pieris rapae crucivora. Journal of Neuroscience 25, 5935-5942.
Braby MF, Vila R & Pierce NE. 2006. Molecular phylogeny and systematics of the Pieridae (Lepidoptera: Papilionoidea): higher classification and biogeography. Zoological Journal of the Linnean Society 147, 239-275.
Ghiradella H, Aneshansley D, Eisner T, Silberglied R & Hinton HE. 1972. Ultraviolet reﬂection of a male butterﬂy: interference color caused by thin-layer elaboration of wing scales. Science 178, 1214-1217.
Silberglied RE. 1979. Communication in the ultraviolet. Annual Review of Ecology and Systematics 10, 373-398.
Stavenga DG, Stowe S, Siebke K, Zeil J & Arikawa K. 2004. Butterﬂy wing colours: scale beads make white pierid wings brighter. Proc. R. Soc. B 271, 1577-1584.
Wahlberg N, Braby MF, Brower AVZ, de Jong R, Lee M-M, Nylin S, Pierce NE, Sperling FAH, Vila R, Warren AD & Zakharov E. 2005. Synergistic effects of combining morphological and molecular data in resolving the phylogeny of butterflies and skippers. Proc. R. Soc. B 272, 1577-1586.
Wheat CW, Vogel H, Wittstock U, Braby MF, Underwood D & Mitchell-Olds T. 2007. The genetic basis of a plant-insect coevolutionary key innovation. PNAS 104, 20427-20431.
Research Blogging necessities :)
Ghiradella, H., Aneshansley, D., Eisner, T., Silberglied, R., & Hinton, H. (1972). Ultraviolet Reflection of a Male Butterfly: Interference Color Caused by Thin-Layer Elaboration of Wing Scales Science, 178 (4066), 1214-1217 DOI: 10.1126/science.178.4066.1214