Global invasion history of the world’s most abundant pest butterfly: a citizen science population genomics study

A major goal of invasion and climate change biology research is to understand the ecological and evolutionary responses of organisms to anthropogenic disturbance, especially over large spatial and temporal scales. One significant, and sometimes unattainable, challenge of these studies is garnering sufficient numbers of relevant specimens, especially for species spread across multiple continents. We developed a citizen science project, “Pieris Project”, to successfully amass thousands of specimens of the invasive agricultural pest Pieris rapae, the small cabbage white butterfly, from 32 countries worldwide. We then generated and analyzed genomic (ddRAD) and mitochondrial DNA sequence data for these samples to reconstruct and compare different global invasion history scenarios. Our results bolster historical accounts of the global spread and timing of P. rapae introductions. The spread of P. rapae over the last ∼160 years followed a linear series of at least four founding events, with each introduced population serving as the source for the next. We provide the first molecular evidence supporting the hypothesis that the ongoing divergence of the European and Asian subspecies of P. rapae (∼1,200 yrBP) coincides with the domestication of brassicaceous crops. Finally, the international success of the Pieris Project allowed us to nearly double the geographic scope of our sampling (i.e., add >1,000 specimens from 13 countries), demonstrating the power of the public to aid scientists in collections-based research addressing important questions in ecology and evolutionary biology.Non-technical summaryWe provide genetic evidence that the success of the small cabbage white butterfly—its rise to one of the most widespread and abundant butterflies on the planet— was largely facilitated by human activities, through the domestication of its food plants and the accidental movement of the butterfly by means of trade and human movement (migration). Through an international citizen science project—Pieris Project—people from around the world helped to unravel the global invasion history of this agricultural pest butterfly by collecting samples for DNA analysis. The success of this citizen science project demonstrates the power of the public to aid in collections-based research that address important questions related to ecology and evolutionary biology.