Abstract

The objective of this work is to investigate potential mechanisms maintaining a melanic polymorphism in the Malacosoma disstria moth. Laboratory experiments determined that the male-limited melanic phenotype is measurably darker than the simple phenotype. The melanic allele follows inheritance patterns expected for a Mendelian autosomal dominant. Melanization varies within phenotypes and is strongly dependent on dietary nitrogen availability. In female moths, melanization shows condition dependence. Melanic males tend to be smaller than simple males, allocate higher proportions of resources to melanin synthesis, and may be more susceptible to suboptimal conditions. These costs to the melanic phenotype may be balanced by thermoregulatory advantages and decreased predation pressure. Selection against the melanic phenotype may be strongest during outbreaks, but sex-limitation of the phenotype can protect the melanic allele from negative selection. The frequency of the melanic allele varies geographically; this may be related to temperature, population dynamics, or both. Multiple mechanisms appear to be balancing selection on the two colour phenotypes in M. disstria moths, resulting in a stable polymorphism that is present throughout the species’ wide distribution.