Color variation in insects is common, often driven by the advantages of mimicry and camouflage for predator avoidance. A textbook example is industrial melanism in the British peppered moth (Biston betularia), where a rapid phenotypic shift from grey to melanic forms occurred, driven by a camouflage advantage on soot-darkened surfaces during the Industrial Revolution. In Lepidoptera (moths and butterflies), while massive researches have focused on butterflies - often the more colorful members – the color variations of moths, which comprises 90% of the described species, have received much less attentions.
A recent collaborative study, published in National Science Review, investigates the genetics and evolution of melanism in the tea geometrid (Ectropis grisescens), a moth species widely distributed in China's tea gardens. Like the peppered moths, adult tea geometrids exhibit both melanic and grey forms in nature. However, the ecological scenario in tea gardens, where tea plants typically grow densely with trunks fully sheltered by leaves and twigs, minimizes the potential advantage of color variation for crypsis, if any. This unique setting allowed the authors to trace the parallel evolution of melanism under different geographical, ecological, and selective scenarios, particularly in the absence of predominant selection.
The research team applied genetic and genomic approaches to map the melanism locus to the 'cortex' locus, a well-known 'hotspot' region that controls industrial melanism in British peppered moths and wing color patterns in many butterflies. Through gene editing experiments, the study functionally verified that the hidden non-coding RNAs within this locus - which had been previously identified in butterflies - serve as the primary effectors of pigmentation in moths as well. The team found distinct mutations around this locus responsible for the independently derived melanism in various tea geometrid populations across China. Field surveys conducted at multiple locations revealed that melanic morphs of the tea geometrid are generally maintained at relatively low frequencies in the wild, especially in northern regions. This pattern may be linked to the reproductive disadvantage observed in the melanic morph, which becomes more pronounced under cooler temperatures.
In conclusion, this study reveals that melanism in moths operates under a highly constrained mechanism but also reveals novel environmental factors other than visual predation that influence its evolution. The findings propose a paradigm: a highly variable genetic hotspot can fuel the repeated evolution of both adaptive and non-adaptive phenotypes, playing a key role in shaping biodiversity.
