Pterin-pigmented nanospheres create the colours of the polymorphic damselfly Ischnura elegans

Abstract

Animal colours commonly act as signals for mates or predators. In many damselfly species, both sexes go through a developmental colour change as adults, and females often show colour polymorphism, which may have a function in mate choice, avoidance of mating harassment and camouflage. In the blue-tailed damselfly, Ischnura elegans, young males are bright green and turn blue as they reach maturity. Females are red (rufescens) or violet (violacea) as immatures and, when mature, either mimic the blue colour of the males (androchrome), or acquire an inconspicuous olive-green (infuscans) or olive-brown (obsoleta). The genetic basis of these differences is still unknown. Here, we quantify the colour development of all morphs of I. elegans and investigate colour formation by combining anatomical data and reflectance spectra with optical finite-difference time-domain simulations. While the coloration primarily arises from a disordered assembly of nanospheres in the epidermis, morph-dependent changes result from adjustments in the composition of pterin pigments within the nanospheres, and from associated shifts in optical density. Other pigments fine-tune hue and brilliance by absorbing stray light. These mechanisms produce an impressive palette of colours and offer guidance for genetic studies on the evolution of colour polymorphism and visual communication.

Ontogenetic colour change signals sexual maturity in a non‐territorial damselfly

“…In territorial odonates, such as Crocothemis sevilla, Sympetrum darwinianum and Sympetrum frequens, ontogenetic colour change generates male dichromatism where pre-reproductive juvenile males are inconspicuous yellow and attain conspicuous red colouration upon sexual maturity (Futahashi, Kurita, Mano, & Fukatsu, 2012). Similar developmental colour changes have also been documented in non-territorial species (Henze, Lind, Wilts, & Kelber, 2019;Hinnekint, 1987). Mating frequencies in the field and an indirect correlation between colour change and spermatogenesis suggest that colour change signals sexual maturity in non-territorial damselflies (Hinnekint, 1987).…”

“…Ischnura heterosticta, I. genei, I. elegans and I. senegalensis females change colour during ontogenesis; juvenile females carry fewer eggs and mate less frequently than adult females (Hinnekint, 1987;Huang & Reinhard, 2012;Sanmartín-Villar & Cordero-Rivera, 2016;Takahashi & Watanabe, 2011). In I. elegans, the males also change colour from green to blue, which correlates with spermatogenesis (Henze et al, 2019;Hinnekint, 1987). Outside the odonates, both male and female wood frogs (Rana sylvatica) become more conspicuous upon sexual maturity, and remain conspicuous throughout the breeding season (Lambert, Carlson, Smylie, & Swierk, 2017).…”

Temperature drives pre‐reproductive selection and shapes the biogeography of a female polymorphism

“…The morph differences in the rate of sexual development and colour maturation (Fig. a) might partly reflect the costs of pigment production such as pterins and maybe also melanin (Roff & Fairbairn ; Henze et al ), since the male mimicking females do not deposit as much pigment on their abdomen tips as the gynochrome morphs (Fig. ).…”

Optical costs and benefits of disorder in biological photonic crystals

“…In the late 1990s and early 2000s, structural colours produced by coherent scattering were attributed to the quasi-ordered structures of many natural organisms, including bird caruncles, 62,63 bird feathers, 13,14,[117][118][119][120][121][122][123][124][125] mammalian derma, 12 drag-onies and damselies. 126,127 These structures have diverse morphologies that include twisted channels and amorphous packing arrangements of spheres or bres. In the case of bird feathers, they are believed to form by self-assembly, through phase separation of b-keratin from cytoplasm in medullary cells.…”