A flexible egg pigmentation strategy by a predatory stink bug: A response to UV radiation?

Background/Question/Methods

Oviparous animals face the problem of leaving their immobile developing eggs vulnerable to a multitude of biotic and abiotic mortality factors, the relative importance of which can vary spatially and temporally.  This has led to the evolution of a wide variety of behavioral and physiological adaptations to minimize mortality during embryonic development.  A common adaptation in several groups of animals is to apply pigment to their eggs, with proposed functions including camouflage, thermoregulation, and protection against ultraviolet (UV) radiation. However, to date, functional differences in egg pigmentation have either been observed among species or are attributable to polymorphism within species, implying that responses of egg pigmentation to changing environmental conditions occur mostly over multigenerational timescales.  Furthermore, among insects, the most diverse group of oviparous animals, the plasticity and potential adaptive significance of egg pigmentation have not been directly investigated.  Here, we show that individual females of predatory stink bug Podisus maculiventriscan lay eggs ranging from pale white to black.  Using a combination of laboratory and field experiments, we began to elucidate a possible role for this adaptation and the ecological factors that could have spurred its evolution.

Results/Conclusions

After establishing that individual P. maculiventris females are capable of laying eggs of different pigmentation levels, we found that on soybean plants, egg masses laid on the upper surface of leaves were much more pigmented than those laid on leaf undersides.  This lead us to hypothesize that the pigment functions to protect offspring against UV radiation.  Indeed, when eggs of different pigmentation levels were exposed to different doses of UVA/B radiation, embryonic developmental success decreased with increasing doses of UV radiation and increased with increasing egg pigmentation.  Integrating evidence from field experiments, we discuss how this intriguing example of behavioral and physiological co-evolution may have arisen in response to a suite of factors including mortality due to UV radiation, predation risk, and a potential physiological cost of pigment production. Our study represents the first description of an animal able to conditionally change the pigmentation of its eggs, evidently as an adaptation to spatial heterogeneity in the relative importance of abiotic and biotic mortality factors.