Comparison between eastern and western monarch populations

Monarch butterflies are known for their spectacular annual migration in eastern North America, with millions of monarchs flying up 4,500 kilometers to overwintering sites in central Mexico. Monarchs also live west of the Rocky Mountains, where they travel much shorter distances to reach overwintering sites along the Pacific Coast. Over the last few decades, monarch numbers have dwindled, and it has been suggested that monarch migration is at the brink of extinction. However, genetic evidence for population decline remains lacking, and it remains unknown whether eastern and western migratory monarchs form distinct evolutionary units that require specific protection. We used a tethered flight mill to show that migratory eastern monarchs fly for longer and thus over greater distances than western monarchs. However, analyzing more than 20 million SNPs in each of 44 monarch genomes, we found no evidence for genomic differentiation between eastern and western monarchs, suggesting the existence of one panmictic migratory population. Our genomic analysis also showed identical and low levels of genetic diversity, and an identical lack of singleton alleles, indicating a shared genetic decline of eastern and western monarchs. Gene expression analysis of a subset of genes during active flight revealed differential gene expression related to non-muscular motor activity. Our results demonstrate that North American monarchs form one panmictic and declining population, and that differences in migration distance and destination are likely driven by gene expression rather than genetic differentiation. Thus, eastern and western monarchs do not form distinct evolutionary units.