The study investigates the dynamics of chromosome evolution in Lepidoptera, a group of insects that includes butterflies and moths. Chromosomes are central to genome organization and function, and their stability is crucial for processes like recombination and segregation. The research analyzes 210 chromosomally complete lepidopteran genomes and identifies 32 ancestral linkage groups, termed Merian elements, which have remained largely intact over 250 million years of evolution. However, eight lineages have undergone extensive reorganization through fusions or fissions. Fusions are rare, and fissions are even rarer, with fusions often involving 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. The findings highlight the importance of chromosome length and haploidy in constraining genome rearrangement, with smaller chromosomes being more frequently involved in fusions. The study also explores the evolutionary dynamics of Merian elements, the distribution of fusion and fission events, and the consequences of these events on genome structure and function.The study investigates the dynamics of chromosome evolution in Lepidoptera, a group of insects that includes butterflies and moths. Chromosomes are central to genome organization and function, and their stability is crucial for processes like recombination and segregation. The research analyzes 210 chromosomally complete lepidopteran genomes and identifies 32 ancestral linkage groups, termed Merian elements, which have remained largely intact over 250 million years of evolution. However, eight lineages have undergone extensive reorganization through fusions or fissions. Fusions are rare, and fissions are even rarer, with fusions often involving 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. The findings highlight the importance of chromosome length and haploidy in constraining genome rearrangement, with smaller chromosomes being more frequently involved in fusions. The study also explores the evolutionary dynamics of Merian elements, the distribution of fusion and fission events, and the consequences of these events on genome structure and function.