Blue reflectance in tarantulas is evolutionarily conserved despite nanostructural diversity

Abstract

Slight shifts in arrangement within biological photonic nanostructures can produce large color differences, and sexual selection often leads to high color diversity in clades with structural colors. We use phylogenetic reconstruction, electron microscopy, spectrophotometry, and optical modeling to show an opposing pattern of nanostructural diversification accompanied by unusual conservation of blue color in tarantulas (Araneae: Theraphosidae). In contrast to other clades, blue coloration in phylogenetically distant tarantulas peaks within a narrow 20-nm region around 450 nm. Both quasi-ordered and multilayer nanostructures found in different tarantulas produce this blue color. Thus, even within monophyletic lineages, tarantulas have evolved strikingly similar blue coloration through divergent mechanisms. The poor color perception and lack of conspicuous display during courtship of tarantulas argue that these colors are not sexually selected. Therefore, our data contrast with sexual selection that typically produces a diverse array of colors with a single structural mechanism by showing that natural selection on structural color in tarantulas resulted in convergence on similar color through diverse structural mechanisms.

Keywords: Natural selection; Sexual selection; Structural color; multilayer; non-iridescence; quasi-order.

Fig. 1. Ancestral character analysis for blue coloration.

Blue color evolved at least eight times (▲) and was lost at least five times (▲) during evolution of Theraphosidae. The basal origin of blue is likely an artifact to the highly conservative assignment of blue as the ancestral state for any genus containing a blue species. (A to I) Photos representing the appearance of nine blue tarantula species are shown above the phylogeny. Photos courtesy of T. Patterson.

Fig. 2. Color, morphology, and nanostructure of blue hairs.

Colors observed under a microspectrophotometer [light microscopy (LM)]. Center black square (spectra-measured area), 4 × 4 μm. (A to H) Three types of hair morphology were observed under SEM: (i) smooth cylindrical hairs (A and H), (ii) irregular/bladelike protruding hairs (B, C, E, and F), and (iii) symmetric lobe-like protruding hairs (D and G). Two types of nanostructures were observed under TEM: (i) multilayer structure (D to G) and (ii) quasi-ordered structure (A to C). High-reflectance ridges are observed in (D). The center portion of the hair in (H) is blue, but not on the periphery. This is probably caused solely by the optical effect of a hollow, semitransparent fiber (that is, the hair). No conspicuous nanostructure was observed in (H). Scale bar, 2 μm. (A) Euathlus pulcherrimaklaasi. (B) Tapinauchenius violaceus. (C) Chromatopelma cyaneopubescens. (D) Lampropelma violaceopes. (E) Ephebopus cyanognathus (TEM photo courtesy of R. F. Foelix). (F) Avicularia laeta. (G) Poecilotheria metallica. (H) Grammostola rosea.

Fig. 3. Coherent scattering analyses by Fourier Analysis Tool for Biological Nano-optics.

Column I: Selected TEM micrographs that represent observed nanostructures. (A to H) Quasi-ordered structure (A to C) and multilayer structure (D to G). No structure is seen in (H). Scale bar, 250 nm. Column II: Two-dimensional (2D) Fourier power spectra of column I. Column III: Predicted reflectance spectra based on column II. (A) E. pulcherrimaklaasi. (B) T. violaceus. (C) C. cyaneopubescens. (D) L. violaceopes. (E) E. cyanognathus (photo courtesy of R. F. Foelix). (F) A. laeta. (G) P. metallica. (H) G. rosea.

Fig. 4. Reflectance spectra of tarantulas and Polyommatus butterflies.

(A) Normalized reflectance spectral measurements of tarantulas (solid lines) show conservation of peak reflectance around 450 nm. Theoretical spectra are calculated from multilayer simulations (dashed lines) and matched the measured spectra fairly well. (B) Normalized reflectance spectral measurements of Polyommatus butterflies (19) show peak reflectance broadly distributed across 400 to 500 nm. The gray area indicates the interquartile range.

Fig. 5. Distribution of blue colorations in Theraphosidae, Lepidoptera, and Aves.

(A) Scatterplots (means ± SD) of reflectance peak positions for blue Theraphosidae, Lepidoptera, and Aves. Reflectance peak distribution in Theraphosidae has a trend to be narrower than that in Lepidoptera and Aves. The c.v. is 4.65% for Theraphosidae, 8.94% for Lepidoptera, and 6.13% for Aves. (B) Scatterplot for the data points in (A) and their differences to the mean value of each group. The smaller the value, the closer it resides to the mean. However, only Lepidoptera, not Aves, show a significant difference from Theraphosidae (P = 0.039), designated by the asterisk sign, potentially because of the outlier in Theraphosidae and the small sample size. Data for the Theraphosidae group are from our own observations, whereas those shown for the Lepidoptera and Aves groups are from data available from the literature. The Lepidoptera group is composed of nine species of Polyommatus butterflies from Fig. 4 and nine other species from table S1. The Aves group is composed of 10 species from table S1, including birds from three different orders: (i) parrot (Psittaciformes), (ii) penguin (Sphenisciformes), and (iii) songbird (Passeriformes).