The X chromosome of insects predates the origin of Class Insecta
Sex chromosomes have evolved independently multiple times, but why some are conserved for more than 100 million years whereas others turnover rapidly remains an open question. Here, we examine the homology of sex chromosomes across nine orders of insects, plus the outgroup springtails. We find that the X chromosome is shared among all insect orders and springtails; the only exception is in the Lepidoptera, which has lost the X and now has a ZZ/ZW sex chromosome system. Therefore, the ancestral insect X chromosome has persisted for more than 450 million years - the oldest known sex chromosome to date. Further, we suggest that the shrinking of gene content the Dipteran X chromosome has allowed for a burst of sex-chromosome turnover that is absent from other speciose insect orders.