This study investigates the dynamics of chromosome evolution in Lepidoptera (butterflies and moths) using comparative genomics. By analyzing 210 complete lepidopteran genomes, the researchers identified 32 ancestral linkage groups, termed Merian elements, which have remained largely intact over 250 million years of evolution. These elements serve as a stable foundation for understanding chromosomal rearrangements in Lepidoptera. Eight lineages have undergone extensive reorganization through fusions and fissions, while fusions are rare outside these lineages. Fusions often involve small, repeat-rich Merian elements and the sex-linked element. The study reveals constraints on genome architecture in Lepidoptera and provides insights into chromosomal rearrangements in eukaryotic genome evolution. Holocentric chromosomes, which lack a single centromere, are common in Lepidoptera and may facilitate rapid karyotypic evolution. However, most Lepidoptera species have a stable chromosome number (n = 29–31), indicating additional mechanisms constrain holocentric karyotype evolution. The study also finds that smaller chromosomes are more frequently involved in fusions, and that the Z sex chromosome is often involved in fusion events. The research highlights the importance of understanding the evolutionary forces shaping chromosome number and genome architecture in Lepidoptera, with implications for genome evolution and the role of chromosomal change in the evolution of diversity across the tree of life.This study investigates the dynamics of chromosome evolution in Lepidoptera (butterflies and moths) using comparative genomics. By analyzing 210 complete lepidopteran genomes, the researchers identified 32 ancestral linkage groups, termed Merian elements, which have remained largely intact over 250 million years of evolution. These elements serve as a stable foundation for understanding chromosomal rearrangements in Lepidoptera. Eight lineages have undergone extensive reorganization through fusions and fissions, while fusions are rare outside these lineages. Fusions often involve small, repeat-rich Merian elements and the sex-linked element. The study reveals constraints on genome architecture in Lepidoptera and provides insights into chromosomal rearrangements in eukaryotic genome evolution. Holocentric chromosomes, which lack a single centromere, are common in Lepidoptera and may facilitate rapid karyotypic evolution. However, most Lepidoptera species have a stable chromosome number (n = 29–31), indicating additional mechanisms constrain holocentric karyotype evolution. The study also finds that smaller chromosomes are more frequently involved in fusions, and that the Z sex chromosome is often involved in fusion events. The research highlights the importance of understanding the evolutionary forces shaping chromosome number and genome architecture in Lepidoptera, with implications for genome evolution and the role of chromosomal change in the evolution of diversity across the tree of life.