Chromosome evolution in Lepidoptera

Chromosomes are a central unit of genome organisation. One tenth of all described species on Earth are Lepidoptera, butterflies and moths, and these generally possess 31 holocentric chromosomes. However, a subset of lepidopteran species display dramatic variation in chromosome counts. By analysing 210 chromosomally-complete lepidopteran genomes, the largest analysis of eukaryotic chromosomal-level reference genomes to date, we show that the diverse karyotypes of extant species are derived from 32 ancestral linkage groups, which we term Merian elements. Merian elements have remained largely intact across 250 million years of evolution and diversification. Against this stable background, we identify eight independent lineages that have evaded constraint and undergone extensive reorganisation - either by numerous fissions or a combination of fusion and fission events. Outside these lineages, fusions are rare and fissions are rarer still. Fusions tend to involve small, repeat-rich Merian elements and/or the Z chromosome. Together, our results reveal the constraints on genome architecture in Lepidoptera and enable a deeper understanding of the importance of chromosomal rearrangements in shaping the evolution of eukaryotic genomes. ### Competing Interest Statement The authors have declared no competing interest.