Lepidoptera Z-chromosome evolution. Lepidoptera

The canonical model of sex chromosome evolution posits that sex chromosomes originate either when an autosome acquires a sex-determining gene, or when it fuses to an existing sex chromosome. This common autosomal ancestry of XY and ZW pairs is well established in vertebrates, and presumed to be the mode of evolution of most new sex chromosomes.Moths (Lepidoptera) usually have a pair of differentiated ZW sex chromosomes. However, the most basal lineages, as well as the sister order Trichoptera, are Z0 (females lack a W-chromosome). The W-chromosome is therefore thought to have been acquired secondarily, either through: 1. A fusion between an autosome and the Z-chromosome; 2. A non-canonical mechanism (e.g. through the recruitment of a B-chromosome). Our comparative genomics analysis of basal and derived moths supports the secondary acquisition of the W-chromosome and argues against such a fusion, suggesting a non-canonical origin of the Lepidoptera W-chromosome.