Structural background of intraspecific color polymorphism and the driver of geographic patterns in a shining leaf...

The phenomenon of color polymorphism has been extensively documented in a range of animal species. A series of hypotheses have been proposed to explain potential functions of color variations in diverse habitats. However, the generation of color is an intricate physical, chemical and biological process. In this instance, the attempts to explain the distribution patterns and their potential causes lacking structural background of color formation, are likely to be misguiding.

Here we studied the distribution pattern of color phenotypes in the beetle Popillia mutans (Insecta: Coleoptera: Rutelinae). Three phenotypes (blue, green and red) are distributed in a not mutually exclusive manner, with the blue phenotype tending to be more prevalent in the cooler northern area, seemingly following Bogert's rule, and the others mainly in the warmer southern area. Subsequent analysis demonstrated that this type of distribution correlates with the environmental factor of temperature. Based on the optical, mechanical and chemical characteristics of the cuticle, we found that this species represents a special case in which melanin layering causes structural coloration, whereas pigmentation plays a primary role in red phenotype and physical coloration is the dominant factor in blue and green phenotypes. However, the structural alterations within the cuticle have no influence on its mechanical properties, different from previously suggested. We have also shown that the blue phenotype exhibits a slightly faster heating rate than the other phenotypes facilitating their adaptation to lower-temperature regions.

Our results elucidate the structural background of color and identify the possible natural selection factor from an evolutionary standpoint. This aids in understanding species formation, as well as prospective dynamic distribution of the phenotypes under the pressure of climate change.