Effects of pupal temperature on wing melanization in the butterfly Pieris napi

Abstract

Melanization is the production and accumulation of melanins, distinctively dark pigment molecules. Melanins have many properties, and serve various functions in organisms, including thermoregulation, protection against environmental stress and communication between individuals. Melanization is subject to plasticity; the degree of melanization can vary between individuals depending on experienced conditions, such as temperature or humidity.

A study on the effects of urbanization on the butterfly Pieris napi brought forth the observation that diapaused (overwintered) individuals had more melanized wings than their non-diapaused conspecifics. The cause of the increase in melanization remained unknown, whether diapause itself or something else such as the cold winter conditions experienced during diapause were behind the increase in wing melanization. This led me to study the effects of temperatures experienced during the pupal stage on wing melanization in non-diapausing individuals.

Non-diapausing pupae were kept in environmental chambers set to different temperatures. Wings of the adults were measured for the degree of melanization on the ventral side. I found that individuals that spent their pupal stage in lower temperatures had more melanized wings than their conspecifics that experienced higher temperatures. The temperature experienced as a pupa also affected wing size and ground coloration, with the wings being smaller in individuals that experienced lower temperatures in the pupal stage, and the base color being an unusually bright yellow in individuals that experienced higher temperatures in the pupal stage. From these results, I infer that temperatures experienced during the pupal stage in non-diapausing Pieris napi individuals can affect wing melanization.

While this experiment provides a proximate explanation for wing melanization, this being the temperatures experienced during the pupal stage, the ultimate reason as to why lower temperatures induce increased melanization remains unclear. A plausible explanation could be that individuals that experience lower temperatures during the pupal stage anticipate lower temperatures during the adult stage, and increased melanization in the adult allows the individual to heat up more effectively in the sun to achieve an internal body temperature that enables flight. The increase in wing melanization in response to a lower pupal temperature may also be a side effect of something else, such as an investment in a stronger immune system in anticipation of an increased risk of infection or being parasitized during the pupal stage, which leads to increased melanization throughout and results in darker wing coloration.