Wulsch, S.; Öztoprak, H.; Guiglielmoni, N.; Jeffries, D. L.; Bast, J.

Sexual reproduction, while often associated with separate sexes, is an ancient and widespread feature of multicellular eukaryotes. While a diversity of sex determination mechanisms exist, for many organisms, which of these mechanisms is used remains unknown. Exploring sex determination mechanisms in Acariformes, among the oldest chelicerate clades, is intriguing due to its potential to unveil conserved sex determination systems. This insight can have implications for understanding sex chromosome evolution and its broader impact on higher taxa.To identify the mechanism of sex determination in Acari, i.e., oribatid mites, we generated a high-quality chromosome-level genome assembly of Hermannia gibba (Koch, 1839) by combining PacBio HiFi and Hi-C sequencing. Coverage and allele-frequency analyses on pools of male and female individuals suggest a female-heterogametic ZW sex-determination system with little degeneration of the W chromosome. To date, this represents the only documented case of a ZW system in Acariformes. Further comparative studies in H. gibba will reveal how old the ZW system is and whether it exhibits conservation or polymorphism.